Airhead ignition
/2 and earlier
Magneto. Popular upgrade is the MZ-B or the Pentacom plate. These systems are covered in great detail on Duane Ausherman's site so there's no need to cover it here.
/5 through '78
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| Bosch 6V coils |
Points and condensor, dual coils. The camshaft has a long 'nose' on which the advance mechanism mounts. The points plate is fastened to the timing chain cover, and is rotated to adjust the timing.
When the key is turned on, power is applied to the coils, which ground through the points. When the points open, the coil sparks. Coils for this system should have about
Correct points setting involves both the opening moment and the fully-open gap. Because the points are opened by a cam, the gap determines how long they are open and how long they are closed. The closed time can be thought of as the 'charging time' for the coils, where too much will overheat them and too little will result in a weak spark.
Total coil resistance should be about 3 Ohms for points ignition, which usually means two 1.5 Ohm coils like the ones shown here. The short ones are Bosch 0 221 100 028 and the new taller one is Bosch 0 221 124 001. An alternative is the dual output Dyna green coil.
'79 and '80
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| This side of the canister faces the engine. The offset key engages the end of the camshaft. |
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| Installed view of canister. |
Points-in-can, same dual coils as before. The condensor is mounted on the outside of the canister. Technically this system is just like the previous one, but the points are enclosed in a fully sealed canister. The camshaft has a blunt nose with an offset slot across it, and the canister has a matching part to engage that slot. The timing chain cover is different, as the mounting for the canister differs from the points plate mounting.
The points can is completely rebuildable. The points and condensor can be replaced individually, and the advance mechanism can be cleaned. Because it stays so clean, it's a pretty robust system.
'81-on
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| Cracked gray coil |
Electronic ignition, initially with two single-output 6V coils (.7 Ohms each) and then with a single dual-output coil (1.5 Ohms). New models (like the G/S and ST) got the dual-output coil in '81 and the others got it in '85.
The canister is identical to the previous one but now it contains two hall-effect sensors with a centrifugal advance. The signals go to the ignition control module (ICM), which is mounted on the right side of the frame backbone. That module controls the dwell time and grounds the coil. It's a good idea to periodically remove the module and apply some heat-sink paste between the module and the cooling plate.
Early dual-output coils (BMW 12 13 1 243 910, Bosch 0 221 500 200, known as gray coils) are prone to failure by cracking. Some fail without any visible cracks whatsoever. It's a good idea to replace these coils wherever they appear. The later black coil, or Dyna brown, or many other coils with a 1.5 Ohm primary winding can be used.
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| Updated ICM |
Around 1991 the black coil (BMW 12 13 1 244 426, Bosch 0 221 500 203) was introduced. The ICM was simultaneously upgraded so that it cut off coil power after 1.2 seconds of inactivity, to prevent overheating damage to the coil. These ICMs have turquoise lettering (shown here) rather than white or pink. Using the new ICM with an older coil is fine, but BMW recommended against using the new coil with an older ICM lacking the time cut-off.
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| Newest ICM |
Even more recently, the ICM was upgraded again, now with new part number 12 14 2 325 284 and integral heat sink..
Interestingly, BMW never got away from the mechanical timing advance. Besides the early coil problem, sticking advance mechanisms are the only significant weakness of this system.
Replacement ICMs are available from the aftermarket (Motorrad Elektrik and Rocky Point Cycle), but I don't know which version they are equivalent to. One such replacement is the Transpo BM300.
For these systems, the Accel 140404 two-output coil is commonly used for dual-plug bikes; it is 0.7 Ohms. Two are used; each replacing a 6V coil.
Aftermarket
Boyer-Bransden
Boyer's MkIII (now Micro Digital) and Micro-Power systems replace the entire timing system, implementing an electronic pickup and digital control over timing and dwell. The system mounts in place of the points plate and advance mechanism on '70-'80 systems (inside the canister on '79 and '80 models). Micro Digital works with existing coils, while Micro-Power uses Boyer's own coils and is easily fitted to dual plug heads. Contact Rocky Point for more information.
Dyna
The Dyna III ignition is an electronic upgrade, using an electronic pickup mounted on the cam nose. Advance is still handled by the stock advance unit.
Dyna also has a booster module, used to reduce the current load on the points. This is especially popular for dual-plug applications where two coils are charging from one signal.
Omega
The Omega system is a pickup and processor unit mounted at the alternator, so it gets the signal directly from the crankshaft position. The points can even stay in place as a backup system. 16 timing curves from which to choose, uses existing coils.
Silent-Hektik
Like the Omega, this is a crank-mounted system. A trigger wheel is mounted to the alternator rotor, and the pickup unit is on the stator. The system also includes a processor unit (with 16 different curves) and their own dual-output coil. The processor can power two of these coils for dual-plug heads.
Resources
Motorrad Elektrik - supplier of many Airhead electrical parts, including Dyna coils and ignition systems.
Stephen Bottcher - supplier of Omega ignitions. Good FAQ page, also.
Rocky Point Cycle - supplier of Boyer-Bransden ignitions for BMW.
Boyer-Bransden, good technical background information.
Silent-Hektik, typically counterintuitive German website.
Dyna (aka Dynatek).
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