Menu Close

Care and Feeding of the BMW Ignition Trigger Unit – Twins 1981-1996

David A. Braun –
(July 1997, updated Feb 2004)


Date: Jul 1997
From: Flash – DoD #412
Subject: Ignition Trigger Repair Bulletin

Care and Feeding of the BMW Ignition Trigger Unit – Twins 1981-?
by David A. Braun – Flash – DoD #412 – BMWMOA #18854
Copyright 1997
Updated February 2004

(Note: I had a couple of 1981 R80G/S’s. The trigger unit on these bikes is used on plenty of other BMWs. If someone wants to follow up with a listing of models to which this information applies, I feel confident that many riders would be most appreciative.)

They say that the ignition trigger “has no user serviceable parts” and that in the event it fails it “must be replaced as a unit.” In a word, bullshit.

There are a couple of things which might go wrong with your trigger which you can fix instead of replacing it. The first is that it becomes “sticky.” That is, if you check through the timing hole with a timing light you do not see a smooth progression from the slow mark to the fast mark and/or vice versa. The second is failure of the hall effect transistor.

(Note: the following paragraph of trouble shooting methodology is straight out of the factory manual.)

If you have NO spark, a dead pickup transistor is easy to diagnose. You can verify that your ignition AMPLIFIER is operational. If the amp is good, then the trigger unit MUST be bad.

  1. Unplug the connector between the trigger unit and amplifier. (See instructions for disassembling connector below.)
  2. Stick a pin in the AMPLIFIER’S connector, center pin. That’s the connector coming from “up”.
  3. Remove ONE spark plug from the head, stick it back into the ignition wire, be sure the ground portion of the plug is in good contact with the head and you can SEE the spark gap.
  4. Everytime you ground the pin sticking out of the amp’s connector (with the ignition operational and on) you will get a spark AT THE SPARK PLUGS if the amplifier is good. In that case the problem is probably your trigger unit. If you do not see that spark, then your problem is NOT likely to be your trigger unit. Fix that problem first, then deal with the trigger unit IF necessary.

If your trigger unit has failed, the hall transistor can be replaced with one from a 79-85 VW, 78-83 Audi, or 79-82 Porsch. Take the old part with you when you go to the import car parts place, or cheaper yet, junk yard. A local parts place quoted me $95 for a new one in ’96 and their jobber’s part number was 30-1032. I THINK the Bosch number is 1237011052.

This little tidbit came in from Frank Warner down under (fwarner@swifty.tip.CSIRO.AU):

“The hall effect device can be had from electronic suppliers. My notes say You can replace the Hall effect device with a Siemens unit part number HKZ101 (this is available from Jacar in Australia part no ZD-1900 for about $A42) Connections are Red +12 volt, Green signal, Black earth. Should be less than $40 US and brand new, if you can find a supplier in your part of the world. Being new the rivits are ready to be rolled over on the base plate of the triger unit.”

Whether you have a sticky trigger or a failed transistor, you’ll need to disassemble and reassemble the unit in the following fashion…

I received email from who was good enough to send me several photos. The photos are here: If you try to open the zipfile here and it is empty, download it to your machine and then unzip it. Refer to the photos in the file as referenced in the text as you read the description below. He took some of the shots with the can still in the bike. But I certainly do not recommend trying to disassemble it that way. In any case, MANY thanks to Goran for sending in a valuable addition to this post.

Removing the unit has a little trick to it. Not the unit itself, but the connector. (See photo 01-connector-b.jpg.) The connector is held in a positive lock with a thin wire spring clip. You can take a small flat screwdriver and carefully push the open ENDS of the clip to make it slip out of its groove. That will allow the plug to be pulled apart. As soon as you get the plug apart, snap the clip back into its groove. That way it won’t get lost. For reassembly, there is no need to fiddle with the clip. The connector simply plugs together. Two allen screws hold the unit to the timing chain cover.

It would be handy to have a 35 or 36mm x 3mm neoprene o-ring handy for when you reassemble the thing. If you buy one from a bearing/seal house, you’ll pay a lot less than you’ll pay at a BMW dealer. It is just an o-ring for gosh sakes.

For the purpose of this discussion, “top” refers to the end of the can with the lid and the two philips head screws and “bottom” refers to the end which mates with the cam. The unit itself is pretty hardy. But there are two caveats involved in the disassembly/reassembly procedure. First, use a scratch-awl and mark things to simplify reassembly. There are plenty of asymmetries, and marks remove the puzzle aspect of reassembly. Second, DO NOT STRESS THE SPRINGS. I am sure that somewhere, someone has replacement springs available (possibly some sewing machine repair place), but I have no clue where to get them.

Required tools to make the job reasonably easy:

  • Screwdrivers with GOOD +/- tips
  • Impact driver with good +/- tips
  • Jeweler’s screwdriver set
  • Bench vise
  • BF Hammer
  • Assortment of punches
  • Snap ring pliers
  • Scratch awl

Use the impact driver to break free the two philips on top of the can and the three flat screws around the circumference of the can. (See photo 02cantop-b.jpg.)

After marking the top and can, remove the philips screws from the top and remove the top. You are now looking at a bearing retainer which holds the bearing in which the outboard end of the shaft runs. Mark the retainer to indicate how it relates to the “slot” in which the plastic strain relief for the trigger wires resides. Remove the two flat screws and then remove the retainer. (See photos 03no-top.jpg and 05wire.jpg.)

You are now looking at a big internal snap-ring. (See photo 04snapring.jpg.) You should carefully mark where the ends of this ring reside and make a mental note that it goes in the bottom groove. You don’t need snap-ring pliers for this. You can re-install the screws and easily pop it out with your fingers or a screwdriver.

Look down into the can and notice the three “tabs” into which the three flat screws from outside the can go. Mark one tab and the can to simplify reassembly. Remove the three screws.

There is a little plastic tit which sticks out at the bottom of the plastic strain relief stripe. Pry or pull this out. It is a button which will come all the way out. Once it is out, the strain relief will slip in the slot with very little effort.

On the bottom of the advance unit, you’ll note that the coupler key has an offset to it. Mark the bottom of the shaft so you can put it back together correctly. (Although, if you think about it, it MIGHT not really matter at all.) There is a “spring” wound around in a groove in the coupler. Carefully pry one end of this loose from the groove and work it off the coupler. Now you can see the pin which holds the coupler to the shaft. That pin is IN there.

A bench vise which opens far enough to contain the can is a Good Thing. Open the vise and note that you can put the tangs of the coupler atop one jaw. As you carefully close the vise, note that the top of the shaft sticks out above the top of the can. You want the shaft to rest on top of the other jaw of the vise, with the vise clamping the can ONLY. You want the vise merely snug, not torqued. (See photo 06vise-b.jpg. But Goran put his can 90 degrees out from the way I had mine. I had both tangs on one jaw of the vise and the “top” of the can up against the other jaw. Maybe my vise is lots bigger than his. I don’t know. But he reported having no trouble removing the ping using his method. That makes me think you could just do it on the edge of a workbench with a metal plate on it if you had one person to hold the can and the other to whack the pin. Goran said that a wooden vise just will not do. He tried, gave up and bought a metal vise. Then the pin came right out without any trouble.) Using your punch and BF Hammer, take a couple of whacks and knock the pin loose. Take the rig out of the vise. (Photo 07pinpunchb.jpg is a shot of a pin punch sticking through the hole from where the pin was removed.)

Everything which holds the assembly to the can is now loose. Wiggle, fiddle, pull, and poke until you manage to slip the guts out. PAY ATTENTION. There are stacks of washers on either side of the bottom of the can where the shaft goes through. Note the orientation and order of both stacks. (See photo 10parts-layout.jpg.)

Use a jeweler’s screwdriver to carefully disengage the outboard end of the flyweight springs over the tops of their posts. (Photo 08w-springs.jpg shows a spring on its post. You’re looking down the throat of a coil-spring that is below the red wire. Photo 09wo-springs.jpg is a much better shot, with the springs removed. On the right, two posts are visible where the spring was formerly mounted.) You can now safely pull the flyweight assembly off the bottom.

If your problem is a sticking advance, remove the e-clips one at a time and carefully remove the flyweights. Clean everything up nicely, paying particular attention to all shafts and bushings. The flyweights ride on a piece of teflon, so do NOT lubricate the surface on which they ride. I like to use synthetic grease, sparingly, for items like the shaft and bushings. Some folks prefer Lubriplate(tm). Suit yourself. Reassemble in reverse order. Install the new o-ring before you install the unit back on the motor.

If your problem is a dead pickup, you’ve got some more to do… The trigger magnet is retained by an e-clip and a snap-ring. Remove these. And draw yourself a picture so that you put the whole mess back together correctly. Note that a pin locates the proper orientation of the magnet with respect to the shaft. Judiciously utilizing your bench vise and punch set, punch the shaft down out of the trigger magnet. Do NOT lose the tiny locator pin.

Replacing the hall transistor will require you to drill out the rivets in the old one. You can install the new one with STEEL pop-rivets or with nuts and bolts. Be sure you pay attention to which spacer goes where. And it is probably a good idea to scribe a line all around the old transistor unit before you remove it. (Part-crossing information is listed earlier in this bulletin.)

You’re going to have to splice the wires from the new unit to the old harness. There is plenty of slack to accomplish this and still fit all the stuff inside the can without fouling anything if you are careful. Use heat-shrink tubing rather than electrical tape to insulate your solder joints.

As they say, “reassemble in reverse order.” Pay attention to your marks, make sure everything is clean and the correct things lubricated. You just saved about $250.

Finally, an answer arrived to the plea: “We are still waiting for some kind soul to tell us to which models applies. Did these cans go on ALL the airheads from ’81 on? Are these things still used on the oilheads? Do K-bikes use them?”

From: “Milton” (
Subject: Hall effect sensor
Date: Mon, 31 Dec 2001 13:43:08 +1100

I just located your site re: the hall sensors and thought I might add my thanks and a little more…

Yes, all the R 2 valves up to 1996 use the same sensor although some are slightly different in the rivets. Some have a base plate which can be removed with 2 small screws underneath to make access easier while re riviting and some have the lower plate as part of the top plate where the (rivet holes are) which I have not been able to dissassemble without dammaging at this point. You can still re rivet the new sensor to these by drilling from underneath 2 access holes aprox 9mm to insert a riviting tool. All the early K series use the same sensors. Of course there are 2 of these per bike . They dont use a canister but an easy access base plate and the job is heaps easier although re timing is a bit more fiddily. Late models have a different plate but use the same sensor.

R series 4 valvers use hall sensors also without a canister being very similar to k series with a backing plate. I cant remember if these use the same sensor or not but would be very surprised if they are different. In essence one sensor fits all! Hope this info is usefull to you.

Milton Matuschka
Tasmania, Australia

Subject: Hall effect compatibilities
Date: Thu, 18 Apr 2002 11:23:01 +1000

Hi again,

I’ve checked bmw part numbers on the airhead trigger canisters – these are all the same for R80 and R100 ‘s.. must be some compromisers here but keeps the parts inventory down.

Further R11 oil heads are confirmed as using the same hall effect unit. So this makes a common part.

Regards Frank Warner
motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S
“A radar detector is a tax loophole that you have to buy.” – D. Fry
David A. Braun – – DoD # 412

Leave a Reply