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BMW Motorcycle Electronic Ignition Module Maintenance

Here's something you're supposed to do about once every millenium or so on your trusty BMW Airhead - apply heat sink compound to the underside of the electronic ignition module. 

Thanks to Matt Parkhouse and Bob "Snowbum" Fleischer for the inspiration for this article.  Visit the Airheads website for more information and join the Airheads!

Several esteemed Master Airhead Wrenchers recommend performing this simple but important maintenance task about once every 10 years or so.  The post-1981 BMW motorcycles have an electronic ignition module or system, and the module sits on top of a "heat sink". 

A heat sink is a piece of hardware that is used to transfer heat away from a critical component.  It is used most often with electronic or microelectronic equipment.  Every modern microprocessor, for example, uses a heat sink to help keep it cool.

If your bike runs fine when cold, but quits after it gets hot, and you can't find any other cause, it may be that the electronic ignition module is running too hot.  Here's a simple maintenance task that you may want to perform next time you're under the tank.


Radio Shack sells a small (6.5 gram) tube of Heat Sink Compound for $1.99.  This is enough to do about 30 Airheads. It's part number 276-1372; their website lists it as "heat sink grease", but the label on the package clearly reads "heat sink compound".
BMW motorcycle electronic ignition module. Here's the electronic ignition module on an '84 R100RS.  My '86 R65 has one in a similar position, right under the fuel tank.

The module is attached with two 7mm bolts.  After removing the bolts, you may find that you have to tap or gently pry the module up off of the heat sink, as the old heat sink paste will probably be pretty dried up.

BMW motorcycle electronic ignition module and heat sink. Here's the ignition module in my hand, and the heat sink.  Note that this unit apparently had heat sink compound applied at the factory (unless someone did the maintenance, which I highly doubt).  

I've heard that some early BMW motorcycle models that used the electronic ignition did not have any heat sink compound applied at the factory, so you may want to check out your bike to see.

Cleaning the electronic ignition module. I cleaned the bottom of the electronic ignition module and the heat sink with alcohol.  Then I cleaned it up a bit more with some fine 3M Scotchbrite.
Applying heat sink paste to the electronic ignition module. Here are the two pieces with the heat sink paste applied.  It's some type of inert compound that's non-toxic; I believe it's made from silicone and zinc oxide.  

Rub a thin coating on each mating part and reattach the 7mm bolts.  Once the fuel tank is off, the whole job should take less than 5 minutes.

Be careful not to over-tighten the bolts; they only need to be snugged up a bit.

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