BMW Airhead motorcycle ignition (2023)

What they had done is to re-design the unit so that NO current would flow UNTIL the engine was rotated, such as with the starter or kickstarter. That avoided the ignition-on, not starting, overheating problem, entirely. (Snowbum still wonders why BMW never did this in the very first module design, as it seems obvious). The turquoise painted version was capable of handling the higher current required by a " souped-up" much improved ignition coil, and incorporated the new more reliable sensing of timing pulses, etc. Snowbum and Oak disagreed about this interpretation, as Snowbum did tests on some early modules, and knows that some modules shut down if the engine does not rotate within a second or three. CONFUSED?

e) BMW also had problems with ignition coil failures (the earlier gray-colored-plastic single coil with two towers from the early 80's onward). The black Bosch single tower coils (installed in pairs) of the 81 thru 84 models are OK and work perfectly. The gray plastic shell twin tower coils would crack & develop open or shorted windings. It often showed up as a problem in rainy conditions; but also often in any conditions. BMW decided to finally fix that problem with an improved coil. There were two generations of later single coils (with the twin towers). The first replacement was no longer a gray color. Some folks used the Oilhead coils, which were not difficult to mount properly. BMW eventually installed the 1991-1995 type coil, often called the Red Coil. It had a rather low primary resistance, ~0.5 ohm, sometimes described as 0.6 or 0.7 ohm. The secondary was approximately 12.8K ohms. These also provided a more powerful spark, with a lowering of primary resistance to permit a higher input current. The coil can be retrofitted to earlier models but would require the latest in ICU (module) design due to the higher current need. It is necessary to use the 477 suffix ICU part number with turquoise paint code, or use the very latest ICU (with the integral riveted heat sink--part 12-14-2-325-284) which OBSOLETES ALL PRIOR VERSIONS. BMW warns, rightly-so, that you must NOT use the updated coil on ANY of the older ICU's unless the ICU is coded with a turquoise paint dot. If you do use the new high power coil on an incapable older ICU, it will cause overheating of the ICU and very likely cause it to eventually fail.

There are only TWO module assemblies now available from BMW; 12-14-2-325-284; and 12-14-2-325-550. The differences are the mounting. The latest ICU with riveted-to-heat-sink construction, for use on the right side of the backbone, should be factory equipped on all Airheads, 1988 & later.It is fully updated & will handle the new high current hot spark single coil with dual output 12-13-1-244-426. That coil should be on all 1991 thru 1995 models. You may need some of the mounting hardware to retrofit the latest ICU & coil, depending on year and model. The Telefunken 477, turquoise, is THE module.

Some words of caution involved with use of aftermarket ICU's (modules): BMW took a long time to get their act together on ICU & coil updates. Some aftermarket ICU'swill work ...or; may not. They all have the same terminal coding which apparently is standard ...but that is where the similarity may end. The internal designs have been upgraded a number of times. If you use an aftermarket module, that is your decision. The cost savings may be considerable. If it works OK and LASTS;..... you won!

Note my comments regarding the modules with turquoise printing ending in 477.

There is plenty of confusion about coils. Some of literature is wrong ...or misleading. One of the various problems is that coils may not be marked, coils may have BMW numbers on them, coils may have Bosch numbers on them. People also do sometimes purchase coils that are not original factory equipment.

BMW coil 12-13-1-351-584, is Bosch 0 221 100 022, used on the /5, a points model. No longer sold by BMW.
BMW coil 12-13-1-243-452, is Bosch 0 221 101 003, used on the POINTS MODELS, in the /6 and /7 era. It replaces the above coil for those years and the /5 too.Both of these coils are 6 volt coils that have primary windings of about 1.5 ohms each, and two are used in series connection for the primaries, paying attention to the polarity markings (+ and -, and sometimes 15 and 1). It is possible that some literature will say that the two above coils are the same. For practical purposes, they do interchange. These early coils had secondary windings of ~6.5 to 7 K ohms.

Do not confuse points coils with certain 1981+ bikes that had two 6 volt coils & were used with electronic ignition. These had primary windings of ~0.67 to 0.77 ohm.They have a lightning bolt symbol. If you used these lower primary resistance lightning bolt coils with points, the points would not last long. Example is the BMW coil 12-13-1-244-142 which is Bosch 0 221 100 028. Some call it the Lightning Bolt coil. Some literature will refer to 12-13-1-243-142, but there is no such real number. Literature may refer to it as the 1981+ coil with 0. 5 or 0.7 ohms as Bosch number as 0 221 100 313. You will find all this confusing! The main thing to know is that you must not use fractional ohm coils with points. You must not use the lightning bolt 6v coils with a points system....unless you have a specially modified booster/amplifier.

The electronic ignition models began with model year 1981. Because the model year begins in the prior year's September production, some literature might show or imply a points coil being used for a 1981 model year bike, or a 6 volt electronic ignition coil being used for a points bike, for 1980. Other literature, including some on-line-fiche, may, more properly, show 09/1980 as the changeover date.

Electronic ignition models:The original troublesome gray-colored twin tower single coil tended to crack; fail intermittently; usually eventually totally failing, but initially usually failing when damp/wet. The troublesome coils were used on various models including the R45, R65, R80GS, R80ST. The OLD coil was BMW 12-13-1-243-910. That coil is for pre-1991 engines and had a 0.9 ohm primary and 12.8K secondary; control units (Modules) with white or pink lettering are used with it. The very latest modules can be used with any of the coils. The -426 coil is similar to the 12-13-1-243-910.

Be very cautious if mounting the Bosch "blue" coils ...they can interfere with the fuel tank.

Coils can fail catastrophically, or bit by bit; or, even have a sudden slight failure that remains. IMO, for most failures, failure is bit by bit until you seriously notice problems. Coils can act 'weak' if they have an internal wire breakage. A wire breakage is detectable with a simple ohmmeter test. Coils with a break in the secondary windings can still seem to work OK ....although may give strange engine power output effects, particularly with large amounts of throttle, which cause higher cylinder pressures and thus more coil power is required to jump the spark plug gap. The reason that coils can work (maybe); or get weaker with secondary windings having an actual break in the wires, is that the same voltage firing your spark plugs is also jumping across the broken coil wire! You might never notice, until it gets bad enough, typically by jumping to more wires nearby, or widening its broken wire gap.

Ignition coils with an internal short circuit (usually that happens between adjacent turns, or between two close layers of windings) can seem to work OK, but, as with some open's, the spark is weaker; this can show up as what might initially be thought of as a lean-running engine problem ...or, just shows up ...such as with large throttle amounts reduced performance. In almost every instance, a shorted winding happens on the secondary winding, as just a turn or two problem ...or between two adjacent winding layers. These change the coil electrical resistance value by a really miniscule amount, so is not detectable by using an ohmmeter. One could compare a good coil versus a bad coil, using the A.C. transformer method or with an oscilloscope, but you are unlikely to have the needed setup ...and knowledge. Additionally, it might only show up when the coil is under actual stress of operating the engine. If a coil secondary OPENS & remains open, that certainly IS detectable by an ohmmeter. Except for weak spark or MANY turns of shorting, or an actual wire being OPEN, there is NO common simple method of coil testing until things get quite bad....EXCEPT that if you have experience, and own an oscilloscope and appropriate pickup coil or other connection means, you CAN determine that a coil is bad. With battery powered oscilloscopes being available for 'reasonable' cost these days, it is possible to even do testing out on the road (don't look at an oscilloscope where others are traveling, lest you have an accident). No, I have NOT (I may, sometime?) shown any sketches of what the good waveforms versus bad waveforms, will be.

When an ignition coil of any type fails, it often happens little by little. Coil failures are perhaps 99% more likely on the secondary high voltage winding. Due to the high voltages involved, this, usually, slowly leads to additional such failures, & as they accumulate the coil output DEcreases.

The Airhead coils have a large capacity for our modest to medium-high compression 2 cylinder engines that hardly top 7,000 rpm, so some things can deteriorate a LOT before you notice a problem. Depending on the type of failure, you may have poor starting; this can happen both with a cold engine & a hot engine; you could have very poor ignition at higher rpm (and/or large throttle amounts) ...with stumbling, even backfiring sometimes.

Damage to coils is often done by owners, or even by some shops, by removing the spark plug caps from the spark plugs while the ignition is powered. This usually happens during carburetor synchronization. IT IS A BIG NO-NO to pull off the cap. The damage, if it occurs, may not show up immediately can suddenly show up years later after enough heat/cold cycling. Damage typically occurs with the engine running ...but damage can occur as the ignition is turned on, caps off. The spark plug caps need to be grounded, via the INternal cap contact; usually done by having the spark plug attached, and the spark plug metal body is grounded to a cylinder fin or the cylinder head. Lifting the caps with the key-ON for such as carburetor synchronization is a big NO-NO on Airheads. It was acceptable on the old magneto models, pre-1970, which means BMW bikes before the /5 series, as they had a 'safety gap' structure. BMW themselves erroneously carried over/2 era advice on lifting spark plug caps into the Airheads era & you may see such in some owner's booklets. DO NOT DO IT! IMO, it should NEVER be done on ANY engine. You might see such erroneous advice from BMW re-printed, again and again, in such as owners booklets and Clymers and Haynes repair manuals!!

All sorts of failures have been seen numerous times in the various coils ....with a few types of failures specific to certain coil models. As I noted earlier, the early 12 volt twin-tower coils were infamous for the gray plastic case cracking. These coils, sometimes not so nicely called Crack-O-Matics, were first seen on the original R45, R65, R80ST and R80G/S motorcycles. The case cracking could break wires in the secondary winding, often giving a weak spark, which could continue to deteriorate. The crack would usually allow moisture to get inside, completely or almost completely eliminating the spark ....and the coil might not allow starting the motorcycle, or it might allow the engine to run very poorly. In some instances, drying the coil THOROUGHLY, & filling the crack with a non-metallic epoxy sealant or glue, could allow a 'wet' tour to continue until the coil could be replaced. Oilhead coils or coils from another brand were sometimes used as a replacement for these old twin-tower coils.

Contrary to comments by some, it IS possible for two-tower 12 volt coils to fail in such a way that ONE cylinder works pretty well, and the other cylinder misfires or hardly fires at all. Reversing the coil tower coils will reverse the effect, that is, the cylinders are now reversed in which one is not working properly. This effect has been seen in failed stock two tower coils as well as failed Dyna coils. Usually an ohmmeter test either shows an open circuit for the secondary winding, but not always. Usually grounding is not the problem. The best test is a substitute. If the bad cylinder and good cylinder reverse, it is almost always the coil that is the problem; but there have been instances of module problems, as previously explained.

GREEN CORROSION in coil towers:

(Video) WEDGETAIL IGNITIONS AUSTRALIA instructions for upgrades to all post 1980 BMW motorcycles PART 1.

Any coil tower, single or dual types, can get green corrosion green colored corrosion in it. Remove the spark plug wire, look deep inside the tower. This is usuallyfrom small amounts of moisture getting inside the coil tower, perhaps from a poor rubber boot. Sometimes it happens from wires not fully seated, which allows sparking ...and oxygen and sparking are not nice to electrical connections. The green is a chemical reaction from the copper in the brass connectors. Once in a while green corrosion can allow the coil tower internal metal & the spark plug wire metal end to act like a lousy diode & you could have all sorts of effects, including massive radio noise.

Many years ago, power diodes were made from a form of copper oxide coated plates ....the same thing was also done with selenium coatings. Besides diode-type vacuum tubes [such as mercury vapor tubes], those coated plates were just about the only power diodes available long long ago, and were used with early alternators and in battery chargers, and commonly [in tiny versions] in radios; some were still being used into the seventies.

Problems can be temperature sensitive. Cleaning the tower internal brass part to clean/shiny will fix the problem. Do be sure the spark plug wire & end clip all fit each other and the tower properly, and the protective rubber boot is in good condition. You can put a bit of silicone dielectric grease on the wire end clip as you insert it, it will help protect against the atmospheric effects, but industry practice is to not allow grease on the electrical ends, but it is OK by me, IF ...IF ...the END FITTING and coil tower inner metal, are both CLEAN and SHINY, and the wire end fitting fits very tightly. It takes a LOT of pressure to fully-seat the stock good condition brass end fitting that fits over the wire and goes into a coil tower. Industry practice is to use silicone grease only on the inside of the rubber cover over the coil towers and where the rubber insert spark plug caps fit onto the spark plug. I like to have the spark plug wire ends/clip be shiny, the coil tower innards shiny, and I DO use a trace of silicone grease.

Coil towers rarely get 'carbon-paths' if the rubber boot is intact. Carbon-paths look like thin black carbon lines that conduct high voltage from the inside of the tower to the outside, and usually extend to the metal case or a terminal. In a few rare instances the tower rubber boot deteriorates and begins to conduct electricity; almost always viewable in a dark area with the engine running. Carbon paths and/or deteriorated boots are usually easily fixed. Clean the carbon path completely; if severe you might have to 'dig into' it, scraping with sharp tools, I use metal dental picks. Leave no trace of the carbon path remaining. I usually seal such scratched/etc. areas using clear epoxy or shellac or lacquer or other type of insulating product, even clear epoxy. Replace the boot, coating the inside of it with a quite thin coating of silicone grease (dielectric grease).

All coils were generally quite reliable (excepting the gray bodied twin tower ones) if not abused by such as open secondary connections. Coil failures are seen. Sometimes from overheating the coil with engine off, ignition on, points closed; other times it is just old age from constant heating and cooling over all the miles and years.

Primary windings in coils are usually reliable because of the larger gauge wire and low voltage. A short circuit will usually not show up on an ohmmeter test; an open circuit will. Both defects are rare. Tests are usually done by substitution of a known good coil, as that saves a lot of special testing labor...but there are some quickie tests that are reasonably good.

A not-so-rare problem was seen long ago, when BMW improperly crimped the end connectors of the short jumper wire that inter-connects the two single tower coils. The ignition could be intermittent, in strange ways. Just fix the wire or replace it. Testing for the problem is easy, use an ohmmeter and pull/wiggle a bit on the wires near the connectors.

The metal-case coils have the innards potted in tar, and are usually quite reliable. All coils heat up from both engine heat & primary current flow. It is possible for coils to electrically overheat on points models. This is much less likely on the electronics models. For the points motorcycles, as much as 70 watts of internal heating, besides the heat coming upwards from the motor, is possible in each of the stock 6 volt coils, although that power is not averaged, as there is a dwell angle involved which varies with the particular ATU model. The average power is probably 1/3rd. That is still a considerable amount of heat. The twin tower coils also can get hot ...but less likely as the electronics modules shut down if the engine is not rotating.

A coil overheating problem happened to the /5 era police (Authorities) models, because they had metal radio frequency covers over the already metal-cased coils ...that kept more heat inside. BMW made a change to the dwell angle on the ATU which reduces the average value of current during engine operation, thus less heat. BMW did this for all models soon after. It is poor practice to turn on the ignition on points models & forget to start the engine, while you have a 'conversation' with someone. Nerdy note: For full-out racing engines still using the points on the end of the cam method for ignition (pre-1979), use of the early /5 era long dwell points cam can be helpful, together with 1000 ohm spark plug caps.

In extremely hot weather, with stop & go, such as in congested city traffic, the coils can get stressed from rising engine heat; many cycles of such days can sometimes result in coil failure from such aging. In less severe use, aging just happens slower. Heat/cold cycling causes expansion & contraction of the coil innards & eventually can cause wire breakage. The secondary wire size is very thin, a necessity to enable enough turns, so it is susceptible to breaking from coil expansion and contraction.

Coils are not easy to fully test. Usually an ohmmeter test is reliable, particularly if the coil secondary is open. A reasonably thorough test means doing the test with the coil both hot and cold.The very best test is to do all the normally done tests with ohmmeter, etc., and also then additionally consider that coils must produce the electricity to jump the spark plug gap during compression stroke pressures, which is far more stressful on the coil than if the spark plug gap is not under compression pressure. You can use an oscilloscope, but few own them. A test that is fairly good, can be done without an oscilloscope. Coils, mounted in the bike, can be tested for spark appearance (preferably in a dark area) using a very much wider gap than normally used for the spark plug, the plug very securely grounded. The wider gap is a substitute for the lack of compression pressure. I have used 'surface gap' plugs, as used on some outboard motors; but normally I just use an old Airhead spark plug, see the photo. BMW has specifications for spark distance testing; and, while I do show them later, you should think first, because it is dangerous, & especially because BMW does not specify the shape of the items the spark is to jump from (it is not spark plugs for them). Tests, which are easy to do wrongly, will injure the coils. For those interested in modifying ordinary spark plugs from your Airhead, for spark gap tests, simply cut off the grounding electrode, leaving it with sharp edges, but hardly proud of the spark plug body. You must ground those spark plug metal bodies. The use of modified spark plugs is a good test. More a bit further down.

BMW Airhead motorcycle ignition (1)

There were real coil and spark plug testers equipment in old-time repair shops. These testers were sometimes a combination coil & spark plug tester; but both types were often slangly referred-to as 'bomb testers' (yes, really), & used a chamber that could be pressurized by shop air. The chamber had a tiny window in order to observe the spark. However, above the photo I describe spark jumping on the modified spark plug, and this is adequate for your coil testing, combined with use of an ohmmeter first. Be sure the spark plug is grounded to the cylinder, use a bungee or? The spark plug cap must be securely on the spark plug. Observe in a darkened area while engine is being cranked. A good blue spark is required.

If one cylinder is acting up, and the other seems OK or nearly so, SWAP the ignition secondary winding leads; that is, swap the tower leads (even on 12 volt twin tower coils), and if the other cylinder now acts up, replace the coil.

Use of spark plug adapters that are sold for ignition testing, including for coil testing, such as the Colortune, are absolutely not recommended by me (and OAK). They may damage your head threads. They are NOT useful for testing the ignition system, no matter the manufacturer's claims!

The first updated twin tower coil was black bodied.

Coil Substitutions, in brief:

An Oilhead coil could be used. The Oilhead coil was 12-13-1-341-978. The plughousing primary side is 61-13-1-459-515; the connector for the housing is 61-13-1-459-516. The Secondary side (ignition cable to the coil) is 12-12-1-289-837. Connector for the coil is 12-12-1-705-656 ...needed because the cable can't be screwed into the plug. Add a rubber cap at the coil. Bosch does have a part 0 356 250 033-000 plug & it can have the cable screwed-in.

You could use two each 6 volt Bosch coils, which can used on electronic module Airhead motorcycles from 1981. To be really correct, you need the proper coils ...but, just substituting the Oilhead or 6V Bosch coils will work OK, if not 100% optimally. The 6V Bosch coils with the lightning bolt on the side has the correct electrical characteristics. The part number for the coils through 1980 is 12-13-1-244-142. If substituting for a single two-tower coil, mount one coil where the original twin-output coil was. Use that coil for the left spark plug. Mount the other coil towards the rear, under the relay bracketry, and use it for the right spark plug. Interconnect terminals 15 & 1 between the two coils. The remaining terminal 15 goes to the original green wire; the remaining terminal 1 goes to the remaining black wire. It is a good idea for the coils to be solidly mounted such that a good electrical contact is made to the metal body. These Lightning Bolt coils are generally very reliable.


Some models had a reddish-colored dual tower coil from around 1991, and they seem bulletproof. The red coil has a primary of 0.50 ohm, often said to be 0.70 ohms. Do not use early electronic ignition modules with these coils .....see information well above & maybe further on. Do not use these coils with points ignition, unless you have a high power points amplifier/booster; and, in such service they draw more power from the battery.

The original gray two-tower 12 volt coil was 1.15 to 1.35 ohms.

It is possible that some of the numbers I used in this article are not in the BMW North America ordering system, but they exist ....and your BMW dealer can get them.

More on Failure modes, etc. A simplified version of how coils, points, electronics modules, etc., actually work.

There are several possible failure modes, but the most common causes are two. One is that the case plastic (or tar inside of metal case coils) and the copper and iron core parts inside do not expand and contract with heat changes at the same rate. That stresses the extremely thin wires of thousands of turns, used on the secondary winding. Being of copper, those wires also expand and contract with temperature. The wire needs a good compromise in its own characteristics too. The other common cause is lifting the spark plug cap with the ignition power on ....that causes an extremely high voltage to be developed in the coil, until it either jumps around inside, or is otherwise grounded. The jumping around can be between turns, and the result can be an insulation break-down.

One or more places in the secondary windings can break, and this can keep happening, over time. These wire breaks cause high voltage sparks jumping, which further degrade performance. The main degradation is often short circuits between nearby windings, but sometimes the windings are far separated, and that always greatly reduces spark energy. That acts, and is, like a single or modest multiple turns shorted loop surrounding the entire coil. Thus, a double whammy ...the open due to the break, and the shorted turn, acting as a shorted secondary winding in itself. That shorted turn absorbs energy the coil was designed to use to produce the high voltage for igniting the fuel-air mixture inside the cylinder head. There is a particularly well-known instance, where, on some models, BMW installed a single coil with twin towers, the coil case was easily identified because it was all-plastic & gray in color. It would test fine when new, but was poorly designed and poorly manufactured. It is tricky to encase the coil electric parts, control the cooling, etc., and the plastic type formulation is also critical.

The coil(s) are charged from the battery when the POINTS are CLOSED. The points are closed TWICE, for each points cam rotation. The points are also opened twice, each of which opening causes the coil to discharge. The period of time the points are closed, with reference to a single 360 degree rotation, is called the dwell. The dwell is expressed as degrees compared to a full rotation, and dwell must be large enough for the coil to fully charge for every ignition event to take place using a full charge. As RPM increases, it is obvious that the time for charging decreases. That can lower the coil voltage output once the RPM is high enough. Coils have numerous electrical characteristics, but one important consideration is that the coil output must always be high enough for a good coil output at any RPM that the engine is likely to be able to attain, and the ignition must still work well if the battery is not fully-charged, or the battery voltage is decreased by age or cold temperature. Allowance must be made for all factors, including stiff oil and reduced clearances on a cold engine ...etc.

Note: Points closure is, in effect, duplicated by the ignition control module, in the 1981+ electronics ignition modules. The dwell angle is different on those models, but all the theory is the same. When the module turns off, the coil discharges. The module is driven by a magnetically sensitive transistor, opposite which is a butterfly metal piece, so there is two poles of the butterfly rotating past the transistor.

When the points close (or module turns on), the coil, having been discharged by the previous spark event, now has ~12 volts applied. There are complex equations to calculate the coil current at any point of time during the charging period of time, as it is a curved exponential function. In points models there is a capacitor, called a condenser, which most think is used only to suppress points erosion from sparking, but it has a considerable complex effect on high voltage generation. I will NOT get into any of this, being way beyond what is needed in this posting.

Simplified for our example here, let us suppose the coil primary winding is ~3 ohms, and the battery is presenting 12.5 volts to the coil, and the points are closed. Ohms law says that 4.17 amperes is flowing, and the power in the coil is 52 watts. The actual averaged heating is less, because the points do not stay closed, and in fact are open for two periods during that cam's rotation (2 cam lobes). One can calculate by using the dwell angle and some fancy math; but, the results still are that a considerable amount of heat is developed in the coil windings, and this is besides engine heat rising up to the coil. The coil can get rather hot inside. Heating (and cooling cycling) is not good for the coil. Thus, the coil designer wants low heating, low heat-cold expansion effects, sufficient energy and voltage output, and good heat dissipation. Many many decades of ignition coil design experience can still, as we've seen, result in problems from actual manufacturing practices.

If the points cam is too sharply pointy, the points may not operate correctly by failure to 'follow' the cam shape...they can bounce, or otherwise not stay closed long enough, and may open too much. Compromises must be made for shape....but are rather easily accommodated for modest RPM engines with small numbers of cylinders. But, the dwell must not be excessive, or the heat generated in the coil will be excessive. This happened on the Authorities (Police) bikes in the /5 era. The basics were the same, but for radio interference reasons, the coils had metal shields around them. That shield enabled the coils to RETAIN more heat than in the non-authorities models. Coils began to fail. The system has always been rather over-designed for Airheads, so BMW elected to change the points cam to one with less DWELL angle. This worked well, so BMW elected to use the same points cam for all the bikes in the second (of several) points cams era. There are other considerations, such as coil output must be considered, and how fast it appears from zero to peak value before the spark occurs at the spark plug. These considerations will generally keep the designer of the coil from lowering the primary current too much.

Dwell angle has varied further as time went on and ignition output needs changed, particularly with the 1981+ models that required a more powerful spark to reliably ignite lean mixtures used to help reduce emissions (smog). BMW went to a lower ohm primary that increased input to the coil, to further improve the high voltage output characteristics. So, there are various compromises in coil design. The very last of the modules and coils were probably the very best that BMW and Bosch could do, without going to a capacitive discharge system with its complications. Certainly, the last module and coil were more than adequate for the stock engine; and, for that matter, certainly plenty good enough for a quite hopped-up engine too.

There are 'meters' with Dwell function measurements available, usually these are Automotive type Multimeters. They need not be expensive. Most do not read properly for two cylinder engines, but one simply applies a single multiplying factor, and uses the 4 cylinder position of the switch. Some folks set points gap by means of a Dwell meter. This can be quite accurate, but is not needed with our Airheads. IMO, doing so can even create problems if the points are too narrow from adjustment by dwell angle, and poorly made points can exacerbate the situation. I can argue these points.

The specifics about the ATU's, by BMW and Bosch model number are, of course, shown in this website, for the curious. More about ignition theory itself, and how the ever-so-simple-appearing Kettering ignition REALLY works, is also on site.

If you are not completely brain-dead from reading all the previous; is more information; ....before I get into things in depth!

1. Suppressor (resistor) spark plug caps were metal sheathed from 1970-1976, 1000 (1K) or 1200 ohms nominally, depending on which piece of literature you are looking at. The tolerance isofficially +-20%; so if they measure 800 to 1500 ohms, that's OK. From 1977 all caps were 'nominal' 5000 ohms. ALL points bikes can use 1000 or 5000 ohm caps. NGK caps, with the added rubber protector covers that they usually come, are preferable to the metal-covered BMW/Beru types strictly for long term protection against spark jumping ...but the BMW/BERU types are OK ....if you clean the insides now and then ...and they do look nice. The stock electronic ignition bikes (1981+) MUST have 5000 ohm, often just called 5K caps. If the actual value was much lower than 2500, the ignition can be long-term damaged. The 5000 ohms is 'nominal' in specification. NGK numbers for 1000 ohm caps are LB010F; for 5000 ohm are LB05F. They make 10,000 ohm caps too (LB10....), so be sure to actually measure them, you might be confused by the part numbers for 1K and 10K and you definitely want good resistors. BMW uses spark plugs with stud threaded tops, needing matching caps. Some Airhead motorcycles had metal caps molded to the cable,& most were 5000 ohms.There is an article on this website with extensive information on spark plugs and all the NGK caps information, etc.:

2. Where so marked, coil terminal 15 & 1 and/or + and -, all have meaning. With two coils models one coil is jumpered from terminal 1 on one coil to terminal 15 on the other coil. The remaining terminal 15 goes to the ignition switch; the remaining terminal 1 goes to the points (and tachometer if electronic type) or ignition module. This maintains proper high voltage polarity at the spark plugs, as well as having a complex effect from reverse voltage in the system, that need not be explained here.

NOTE: There have been instances of misfiring on bikes as shipped from the factory with miss-wired coils. This has been seen only on the R45 & R65 (AFAIK).Check your motorcycle.Example, a 1979 R65: The rear coil should have a green/blue wire (12 volt + power) going to terminal 15. On that same rear coil there is a black/yellow wire going to terminal 1, which connects to the front coil terminal 15. The front coil should have two black wires going to terminal 1; one is to the points, the other to the tachometer.

3. QUICK TESTING of coils & ignition caps:

(Video) 1983 BMW R80ST Install Alternator, Diode Board & Electronic Ignition Sensor

a. The quickest and simple test is to remove the spark plug cap from both the left & the right cylinder spark plugs. Insert a longish ohmmeter test lead into one cap, & the otherlongish test lead into the other cap, both are inserted into the caps at the same time. You then measure the total resistance of both spark plug caps and the coil(s). The reading must not be infinite or in the hundreds of thousands or higher ....that signifies an open coil, open cap, bad wiring, or some-such thing. The reading should be, with the pre-1977 1000 ohm caps, ~15000 to 20000 ohms. For 1977 & later, 5000 ohm caps, both points & electronics models, but only for those models with two separate single tower coils, the reading will be ~17000 to ~30000 ohms. 1981 and later must use 5,000 ohm caps. For those with one twin-tower coil, & must-have 5000 ohm caps, the reading will be 22000 to 35000 ohms. You may find some of these readings somewhat off if you can not tell what the cap resistances are visually, as they are still inserted into the coils for this test.

1981 & later Airheads have electronic ignition. The ignition caps absolutely must be nominally 5000 ohms each. Do not use resistor spark plugs!!! Do not eliminate the 5000 ohm cap resistors by using a zero or l000 ohm cap.

b. A somewhat better test (although none of the above, nor this one, nor the following tests, will show up some types of shorts in the coils), is to first unplug the spark plug wires from the coil towers. Measure one of the spark plug caps with the wire attached to it, from the contact inside the spark plug cap to the other end of the wire, the folded metal end. Then measure the other one. 4K to 8K is acceptable for the electronic ignition models; and about 800 ohms to 8K for the points models. 1K rated caps are the best for all points models performance, although those were only installed on the early motorcycles, as has been noted. You can also measure a coil tower to any side terminal (single tower coils only); then measure the other coil similarly. Be sure the metal at the bottom inside of the tower is clean and shiny. About 4K to 5.5K is OK for either coil. For the single twin tower coil, measure between the two towers. About 7.5K to 9K; and infinity between a tower and primary terminal (same for towers to ground).

The spark plug wires brass pressure contacts should fit tightly into the coil towers & both wire contact and the tower inside metal need to be clean and shiny ....if not, fix these things. I recommend, once things are shiny and clean, that a faint smear of silicone dielectric grease be put on the folded contact that goes into the coil. Others disagree; industry practice is to use the grease only on the inside of the rubber boot. I believe industry practice is wrong, because you, correct? ...will first see to it that your clean/shiny wire folded brass fitting ends fit tightly! ...into clean/shiny towers. If so, the grease will eliminate corrosion .....for MANY years, if not decades. Insert the wire, which will need some decent force (I use pliers) to seat it fully; it will almost snap into place with enough force.

c. Measuring the PRIMARY winding side of any coil is more difficult, as for an accurate reading you must short the ohmmeter leads together rather well, then subtract that reading from the primary reading, which also must be a good connection for the meter leads, which must not be moved in their meter sockets during the testing, lest you have irregular readings. The reading should be quite low resistance, which is always more difficult to measure properly. Primaries very rarely open or short. You can simply check for a low resistance. I won't even list the exact values you might get, but for the twin-tower coil it will be around a half an ohm to around one ohm; & ~ 1-1.5 ohms for the two separate 6 volt coil models. Obviously the values must not be a direct short, nor open.

d. The best test is probably by substitution with a known good coil, to see if whatever the ignition problem was, it is now OK (or, not).

e. The second best test (possible first best?), & this test is quite good, is that you first ohmmeter the coil as previously described, and then test the coil both cold & hot with a modified spark plug. My method of doing this was described earlier in this article.

BMW published a spark gap test for the 6 volt coils prior to introducing, in 1981, the electronics models. While I think the test is relatively decent for the electronics models, I have not seen a published test from BMW, perhaps due to the possible damage to the system if the test is not done correctly. The old test is for a minimum 8 mm spark at 300 sparks per minute at 3 volts to the coil; with the spark lengthening to 13.5 mm minimum, at an unspecified primary voltage, at 3600 sparks per minute. BMW's specification was the same for all the different ATU dwell versions including the early 78° & later 120° with the 1979-1980 points-in-canister model. You need a special setup to do those tests, which are stressful on the coil, and IMO, more stress than what you should do. You will notice that the test is done at 3 volts of battery power & uses 8 mm AND 13.5 mm test gaps. 13.5 mm is a fairly severe test. You may notice that BMW's figures on the lengthening are reversed. If you want to make up testing jigs & do these tests, fine by me. I recommend you don't. Additional problems with the BMW specs were that the gap test did not, in my literature, specify the two parts needed for the gap, & pointy gaps need less voltage. BMW probably (?) meant that the tests were to be done with one of their 'tools', & it had a free-air gap, etc. If using a 12 volt coil (BMW only used 12 volt coils of the type with two towers), then apply 6 volts, and use only one spark plug, the shell to one tower, the spark plug stud to the other coil tower. Actually BMW was wrong about lengthening with RPM, so, this entire paragraph is for reference purposes only, in case you see the BMW information someplace...IT IS ALL WRONG!

4. BMW Airhead points bikes have two cam lobes on the Automatic Timing Unit (ATU). On the models up through the 1978 model year, the points & timing unit are mounted at the nose of the camshaft & not in a canister. The forward part of the camshaft, that nose area, of non-canister models, can be found bent at times. You will not see that with your eyes. There is a way to fix it, with a small brass hammer, but that is not the main point of what I want to emphasize here. The bent part may be only a fraction of a thousandth of an inch to a few thousandths. As the ATU rotates, the points will not have the exact same gap nor timing point ...for both ATU timing lobes. This will result in a dual-image when using a strobe lamp on the flywheel. The result is vibration, often at one or more narrow ranges of RPM. This type of problem is often wrongly interpreted as carburetors being out of perfect synchronization, or, the engine is mechanically not balanced, or has some other mechanical problem

The Dyna dual-pickup electronic ignition will eliminate much/most double-timing, and there are other ignitions available for the camshaft nose, and there are crankshaft mounted ignitions available. The camshaft tip is, as noted, fixable. Another cause for this sort of timing change is advance unit wear &/or irregularity in the grinding of the cam lobes. BMW at one time actually recommended 'stoning' those lobes to equality. Don't bother!

A quite poor ATU and/or cam tip needs fixing, somehow. An expensive way, is to purchase a crankshaft triggered ignition. These can provide very good ignition, and eliminate ignition problems from a bent tip and also worn timing chest items...but do not fix the retardation of the valves from worn timing chest items. A worn timing chain, chain guides, & wear on one or both timing sprockets, will also cause double images on the stock standard ignitions. Quite noisy chains at on and off idle rpm with the throttle are a giveaway, but the chain need not be noisy. The cam tip "tapping with a brass hammer fix" must be done carefully, not knocking off the threaded cam tip. Use of a dial indicator on the smooth side area surface of the cam tip will tell the story ....and if you have fixed the problem ...or made it worse. Do not try the brass hammer fix unless you know what you are doing, have a dial indicator, & know how to use it.

5. The electronic ignition came in 1981 and the dwell angle was 104°. For the points models, BMW has used three different dwell angles: 78°, 110°, and 120° (of crankshaft rotation), with 120° being used in the 1979-1980 canister points models. Much more information is located in the following:
ATU parts interchange physically for all pre-1979, so you might have almost any year/version of ATU or ATU parts in your motorcycle. I've posted the various Bosch numbers printed on the outer metal, versus the timing and dwell. The springs have also varied.
Here's the .gif of most of that information:
What is not shown is the full Bosch number; nor is the rpm for beginning the advance (which can be slightly below 800 rpm on the early /5, so if the /5 is idled above that, readings can be strange, if you were not informed. Now you are.

6. The rubber seal strip located in the PRE-1979 timing chest, in a groove surrounding the points cavity, has two part numbers in the parts books. These come in lengths you cut to fit. Install it after cleaning them with acetone or MEK, & clean the cavity for it quite well too. Put small droplets of cyano-acrylic glue (CrazyGlue, etc.) in the cavity groove, before installing. Push a few times into position if need-be, cut with an Xacto knife to fit the ends rather tightly; & let sit overnight. CLEAN with acetone or MEK again & smear a faint amount of silicone dielectric grease on the surface, before replacing the outer cover. Be careful to engage the outer aluminum cover into the points cavity rubber grommet slot.
11-14-1-265-394 is supposedly 3.2 mm, used until 1974.
11-14-1-262-644 is supposedly 4.2 mm, used after 1974.
If you were to order either, you probably will get just -394 size. Maybe. Also, some fiche may show a substitute for either number, 11-14-2-300-295, which is not a good number.
I have plenty of various O-ring materials around, you may not....and...Tom Cutter said you can use a Classic K bike oil filter cover O-ring, which is 11-13-1-460-425, even a used one (I do suggest you clean the groove and O-ring material quite thoroughly, before you try to glue it to the engine)!

7. You may occasionally need to know the spacing between markings on the flywheel (or clutch carrier). You may need to know the distance per degree on the circumference. The diameter for the larger engines are all the same and there is 2 mm between each degree. For the R45/R65, the diameter is smaller, so it is 1.5 mm between each degree.

Accel m/c products; 10601 Memphis Ave #12; Cleveland, OH 44144; (216) 688-8303; part of Mr. Gasket
Accel points amplifier, wiring schematic: see page 38 here:
The sketch is for dual plugging coils, but OK for single plug coils.

Alpha (Emerald Island) ignition (canister):
Rick, at Motorrad Elektrik, told me that the brown Dyna coil (1.5 ohm primary) works fine with the Alpha ignition system. He told me the latest Alpha version also works OK with the blue 0.7 ohm coil. I am speaking here of one single coil, dual output towers, not dual-plugged.

I don't know, yet, about long term use/reliability using the Alpha2 ignition system with the stock last version BMW 0.5 ohm coil, but my limited testing showed it being OK on my own 1995 R100RT, which has the Alpha ignition. See next paragraph.

Ted Porter ( who stocks the Emerald Island electronic ignition (Rick, above, calls his, which is the identical item from Emerald Island, the Alpha 2), has a lot of experience with this ignition, & he reports NO problems with any of the stock coils including 1.5 ohm coils (two in series, 1970-1980 coils), & the later low ohm stock coils, & the Dyna DC9-1 0.7 ohm coils in series, and also the 1.5 ohm DC2-1 single coil.

NOTE: The EI (Alpha, etc.) electronic ignition uses its own version of the module, and uses electronic advance, not a mechanical ATU. There are, to my understanding, two standard sold versions, one for single plug, one for dual plugging....and they have been changed/updated/modified/whatever, over time. I am not overly happy that the specified number of degrees of advance (well, the spread) is not accurate, by 2 or 3 or so degrees. Maybe that will get fixed.

Technical information on most models is available at:
You may find some of the information confusing I will give you clarifying information here.

(Video) Install Euro Moto Electrics EDL-IGNS Electronic Ignition for 450W BMW Charging System on BMW Airhead

For the 70-80 points Airheads, Dyna sells an Ignition Booster ....and it can also be used for a dual-spark-plug modified Airhead, with correct selection of the two primary coil resistances, to keep within the booster current limitations. Pay attention to the coil(s) resistance for non-dual-plugged installations too, of course!

Dyna dual-separate-pickups electronics ignition allows separate timing for the two cylinders, which, in some instances, provides a smoother engine.

Many years ago, the original Dyna D35-2 system was not a wasted-spark system. They used DC1-1 green 3 ohm coils on the dual-plugged conversions & the sensor plate had three wires. Each cylinder could be set separately with a timing light. For a dual-plugged engine, since this was not a wasted spark setup, each coil fed one cylinder; that is, each coil fed the top and bottom spark plug on one particular cylinder). Dyna has not offered this D35-2 in many years. Now, what is available is the D35-1, wasted-spark model.

The D35-1 has two wires at the sensor plate; usually installed with 1.5 ohm Dyna coils ...which are brown or orangey looking, & are series connected. Timing is set with an ohmmeter or test light, as you rotate the engine by hand, so watch the magnet location, to know what side sensor you are adjusting. Yes, you could use a dual-plugging setup.

Watch out for bad Dyna sensor insulation; that is, broken or corroded wires at the sensor plate where they exit the epoxy, & for coils shorting a primary to ground when hot.

Use NGK 1000 ohm spark plug caps.

Dyna sells many different coils. Do not use the Green dual-tower ""6 volt in some literature"" coil with the Bosch electronic ignitions that came on the BMW Airheads from 1981. These Dyna coils are 3 ohms, and would be marginal in that application.

The popular Dyna III model D35-1 was for the 70-78 Bosch coils; or, Dyna coils.
The wiring for the Dyna box:
White, left cylinder.
Red, right cylinder.
Black is ground.
The other red goes to +12 at ignition switch & to the left coil (+). The coils are interconnected by a jumper, same as on original BMW, same numbers at coil primaries, if using those coils. The right coil (-) is the one with the brown wire from the Dyna box. Unconfirmed: Dyna III uses UGS3040T Hall devices.

DYNA III, type -1 and type -2 Electronic Ignition Trouble-shooting:
If only one cylinder is working on a -1 Dyna III it probably is a bad sensor. This can be corrosion, so remove the assembly and inspect it; it can be the sensor itself, bad wire, etc. If both cylinders are NW, it is most likely a bad electronic module, bad coil, or in the wiring.

Disconnect the sensor wires and touch the module end of the input wires to ground. Do not touch the 12 volt (red) sensor wire to ground. You should get a spark at the appropriate spark plugs each time the inputs are grounded. If not, it indicates a bad electronic module, a bad coil, or a problem somewhere in the wiring not associated with the sensor plate. Check the Dyna website to confirm my testing methods.

Dyna coils are color identified, although you could use an ohmmeter for confirmation:

Blue coil is 0.7 ohms on primary winding. This is a dual-tower coil. Used on 92-95 airheads; secondary is 11.5K in some literature and 10.9K in other literature. Includes bracket. Can be used with any Airhead with the last of the modules that are rated OK for 0.5 or 0.7 ohm primary winding (see much earlier in this article, as this can cover many other years, and you need to have the proper BMW module).
Gray coil is 2.2 ohms and 14K secondary.
Green coil is 3.0 ohms, dual tower. 3.0 ohm primary winding, 13.25K secondary winding.
Black coil is 5.0 ohms, secondary unconfirmed....17K? 14K?
Brown coil is dual tower, 1.5 ohms primary, 14K ohms secondary (13.5K ohms secondary in some literature), for 81-92 stock Airheads, with proper mounting bracket. Read about the BMW modules!

DC9-1 0.7 ohms;
DC2-1 1.5 ohms.

Boyer versus Lucas Rita Ignition:

Boyer-Bransden (aka "Boyer"):
The old black box Boyer, the Mk3, can be used with or without resistor spark plug caps.

The Microdigital (red box) is sensitive to RF interference, so 5000 ohm spark plug caps are required. The unit won't be injured if you don't use these, but the microprocessor could ...or will ...go crazy and upset the timing.

URL for troubleshooting the Boyer MK 3: That site has a schematic diagram of some innards, operation, wiring, & spark curves. Boyer's own troubleshooting guide.

The early Boyer ignitions have been known to cause rough running problems, & the bike being hard to start. This is usually from low battery voltage, typically a voltage under 10 or so, and/or SLOW cranking speeds, both of which are disliked by the old Boyer. The early Boyer models also have an ignition curve that does not match the Airhead engine requirements. In particular, the early Boyer had an ignition curve that was slow rising & kept rising, there was no practical rising limit ....the ignition would continue to advance with increasing RPM. Many are in use, however, on Airheads, with 'no problems noticed'.

I have the schematic diagram of the innards of the old Boyer units, with all parts identified, numbers, etc, & I also have the timing advance curve on the old (& the later, below) Boyer units, & I have it on other ignitions, such as the Lucas "Rita". For the nerdy, or whomever really needs this sort of information, they can contact me.

Some Boyer-Brandsden's can have a peculiar problem, being not easy to shut off the engine with the ignition or kill switch. A cure may be available by disconnecting the red fused wire that goes from the Boyer to the battery & the left (+15) coil terminal. Tape it & do not use it. Connect both green wires to that coil terminal.

(Video) Upgrading Airhead Bean Can to Digital Ignition 1988 BMW R100RT

The Boyer-Bransden ignition has been updated. The Boyer was always an ignition to consider when the tip of the pre-1979 camshaft was broken off, although that cam nose is usually repairable. Now, with the new Boyer model with the updated ignition curve (also available in a modified version for dual-plugging installations) (and supposedly now works OK with a slightly weak battery, low voltage that is, during starting), there is less reason to not recommend it.

Below is the Boyer-Bransden Microdigital Electronic Ignition advance curve, courtesy of and obtained from Stan Smith when he owned rockypointcycle.


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4. Will my BMW R80 finally start? (Starting Diagnosis #2) - Ep. 5
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6. 1977 BMW R100RS Testing The Ignition Switch
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