HI Guys,
New on here but have had bikes for 40 yrs and this one has me a bit stumped.
1991 K100 RS 16 v completely stock

When activating either brake stop lamp comes on without engine running,little dull for the first 3-4 activations and then brightens up to what you would expect.
Engine running, activate either brake, cuts out instantly.

I have
1.Cleaned up all earths and bulb contacts.
2.put a test lamp across contacts ,all good even with engine running ,(possible resistance discrepancy?)
3.switches appear ok,but its doing the same thing on both,so have ruled that out for now.
4.fuses all seem to be ok,and everything seems to reset after the engine cuts off and will restart as normal.

I am left with thinking it is the BMU gone,but I am a bit old school with electrics so any suggestions or enlightenment would be greatly appreciated.

Thanks in advance.(This problem is almost Italian (ducati) in nature,no offence to all my Italian friends!)
Eric

Egilchrist wrote:When activating either brake stop lamp comes on without engine running,little dull for the first 3-4 activations and then brightens up to what you would expect.
Engine running, activate either brake, cuts out instantly.

I am left with thinking it is the BMU gone, . . .

It could be possible to test that hypothesis by following guidelines found in this thread.

had a mate with similar issues

turned out to be  the top shock absorber mounting bolt - nut assembly crushing the cabling going to the rear lamps

so have a good examine of that area ...before you do anything else


good luck

Been thinking about this since I read the post earlier today and it has me intrigued. I recon it will be something like Charlie has suggested.
One of the places that gets chaffed is around the steering head and one other is the pink resistance wire for the heated grips gets overheated and melts the surrounding wires usually in the same area.

OK been following this one and going to ask a few questions.....

Both brakes kill the engine.....but I can't see how chafing at rear shock and shorting can kill it.

But I can see that if on activating one brake power also feeds into the other brake switch wiring......and the problem is up at the headstock shorting into some other wires.....such as ignition wiring. Tricky because I have been in the wiring on a few Ks and the brake switch wiring is generally well separate.

One little thought I have is that if its an ABS model either a movement of a brake line or a wiring short from the ABS wiring could cause the problem?

The brake light brightening after 3-4 applications is strange.....earthing issue?

If the tail light is removed,  check if the dimness problem disappears. Maybe check if you have 12 Volts at the positive of the brake light with the dim problem, and also for 12 Volts at the negative.

Egilchrist wrote:I have

2.put a test lamp across contacts ,all good even with engine running ,(possible resistance discrepancy?)

You mean that when you put a test lamp instead of the stop lamp the problem disappears? Actually, where did you connect your test lamp?

Have you tried with different powers of test lamp? Or did you try with a new stop bulb?