Boxerworks.com

I have a friend who has the subject bike in his shop and cannot figure out what is the root cause of the problem. He is one sharp cookie having been a Euro bike dealer for 45 years. I have tried to help him since it is electrical in nature and although I have theories, I'm not actually working on the bike myself. My background was controls for power plants so a motorcycle should be a piece of cake. But I have to say that the wiring diagram looks more like something for a cruise missile and BMW has found a way to "hide" some of the info.

The problem is as follows. Bike starts up and runs fine. But as the revs come up and the charging circuit kicks in, it immediately kills the engine. If you disconnect the charging circuit and run the engine off the battery, the problem goes away.

My review of the circuits involved in the charging system include a box with what appears to be a bi-polar transistor drawn inside a box. There is no circuitry shown inside that box. There are legs of various circuits, mostly charging circuit, going to that box. But what happens inside the box is a mystery. As I understand it, a bi-polar transistor is a switching device. I suspect it is involved in what turns the alternator light on and off, or plays a part in keeping the +12VDC from infiltrating other circuits. My theory is something has failed and when the +12VDC from the alternator comes into play, it is killing the EI or the coils directly.

Problem is, Lance is doing the work and he has gone thru just about everything in the system and came up with similar theories to mine and tested them out. So far, he cannot find the cause for this problem.

I'm hoping this is a problem others have run into and can shed some light on the cause for the issue.

Thanks and regards,
Rob

Snakeoil wrote:
My review of the circuits involved in the charging system include a box with what appears to be a bi-polar transistor drawn inside a box. There is no circuitry shown inside that box. There are legs of various circuits, mostly charging circuit, going to that box. But what happens inside the box is a mystery. As I understand it, a bi-polar transistor is a switching device. I suspect it is involved in what turns the alternator light on and off, or plays a part in keeping the +12VDC from infiltrating other circuits. My theory is something has failed and when the +12VDC from the alternator comes into play, it is killing the EI or the coils directly.

Sounds like you are describing the voltage regulator. It is indeed just a switch that turns the current to the rotor windings on and off very rapidly so as to maintain the stator output voltage at the required nominal 14 volts.

In the absence of a spare voltage regulator you could do a brief test by removing the voltage regulator and shorting out the D+ and DF terminals which will allow the charging system to operate with full current on the rotor windings. The voltage will rise rapidly well above 14 volts so best not to sustain it for more than a few seconds and keep the revs down. Should be long enough though to see if it was the voltage regulator that was causing the problem.

Of course it could be that the voltage regulator is already acting as a dead short so it might be an idea to monitor the system voltage at the moment the engine dies to see if that gives us a clue as to what is going on.

Thanks, that's great input. I got a similar response from another list, only with what one might call a bit of a condescending attitude.

Knowing what it does, helps in determining if it is in play here or not.

Thanks and regards,
Rob

Snakeoil wrote:Thanks, that's great input. I got a similar response from another list, only with what one might call a bit of a condescending attitude.

Knowing what it does, helps in determining if it is in play here or not.

Thanks and regards,
Rob

Well, never let it be said that we weren't willing to be condescending, but this question hardly seemed deserving.

When my friend Lance told me about the trouble he was having trying to sort out this problem, I went on line and looked for wiring diagrams. Found one in color and have been using it. I just tried to find it again on the web and of course, I cannot. Here is the diagram. If it is fuzzy, it is because it is a snapshot of a PDF file.



I assumed it was a colorized copy of a BMW diagram. That could have been a bad assumption on my part. I now see a disclaimer at the bottom that implies somebody else created this diagram.

Anyway, if you look in box 15, Combined Instrument, you'll see the little box with the bi-polar transistor symbol, that I now know is a regulator for the flashing current to the field on the alternator.

regards,
Rob

That diagram is from here:

http://www.stephenbottcher.net/BMW/wiri ... wiring.pdf

#15 is the instrument pod. #24 in the lower right is the voltage regulator.

Kurt in S.A.

Snakeoil wrote: Anyway, if you look in box 15, Combined Instrument, you'll see the little box with the bi-polar transistor symbol, that I now know is a regulator for the flashing current to the field on the alternator.

regards,
Rob

Well Rob, actually it isn't now I see what you were referring to. I believe that transistor is the symbol for the tachometer. The voltage regulator is item 24 which also shows a transistor symbol so you'll forgive my mistake.

The reason you saw connections to the charging circuit will have been because the charge lamp is in the combined instrument box.

The picture below together with your wiring diagram might help to illustrate how it all connects together. Don't be concerned about it being a different model as this part of the wiring is common across airheads.

Barry's right...the tach works by interpreting pulses from the coils as they saturate/collapse to present what the engine RPM is.

Kurt in S.A.

Anyway back to the original fault.

It's just possible that a fault in the tachometer is killing the engine. Does the tachometer work and if it does how does it behave when the engine dies. To check if an electronic tach is causing a problem you could just disconnect the impulse wire on terminal 10. Might be easier to do that at the other end where it connects to the coil.