Brake light problem - OEM Part substitutes

[sgtbill]

I have seen in other forums (fora?) that OEM BMW parts and pieces can be replaced by other parts such as Bosch electronic components for instance. In the old days I even took a R75/5 circuit board (under the front engine cover) into the local Bosch shop in Germany and had a new capacitor installed in it. It worked for years after that just fine.

So today I am under the fuel tank on my K100 RS (1986 model I think) and I have the tail light warning module out trying to find a brake light gremlin. It would be easy to just put another module in to rule out the module as the problem. Of course on Sunday the shops are closed but my local NAPA is open and they have lots of Bosch parts. Anyone know of a place to get a list of parts that have been successfully substituted?

BTW, My specific problem today is that I have no brake light when I engage the front brake. The rear (foot) brake gets the light on just fine. I have already replaced the hand lever switch. Obviously the bulb is OK since the rear brake lights it up. I really hate electrical problems.

sgtbill

[dlearl476]

fwiw, Given your symptoms, I would troubleshoot the lever switch and associated wiring first.

[sgtbill]

"fwiw, Given your symptoms, I would troubleshoot the lever switch and associated wiring first."

I have replaced the lever switch and it tested OK for continuity. I will check with Morton's for a module when they are open on Tuesday. I also have continuity all the way to the plug that goes into the module. I'll keep replacing stuff until the brake lights work I guess.

sgtbill

[kbasa]

"fwiw, Given your symptoms, I would troubleshoot the lever switch and associated wiring first."

I have replaced the lever switch and it tested OK for continuity. I will check with Morton's for a module when they are open on Tuesday. I also have continuity all the way to the plug that goes into the module. I'll keep replacing stuff until the brake lights work I guess.

sgtbill

Have you checked for power at the module yet? Do you have power at the switch? If you have all that working fine and the bulb works OK for the rear switch, you probably have a problem with the relay.

What you want to do is test the entire circuit, from top to bottom. That'll tell you where the problem is without having to replace everything. Where ever the power stops is where your problem is.

[deilenberger]

To answer your question - there is no generic replacement for the awfully expensive and sometimes they DO fail (I had 2 go bad on me) brake-light monitor. As others pointed out - the switch is immediately suspect, but since you replaced it - that only leaves the wire between the switch and the monitor/relay as an alternate possibility.

Given the complexity of the BMW wiring harness, and how packed into the frame it is that could be a major PITA to troubleshoot. IF you're skilled at electronics you might be able to pull the relay and it's socket out of the e-box and then jumper the contacts for the front brake switch and see if the brake light comes on. If it does - you've eliminated the relay and it points to the wiring. If it doesn't - you've confirmed it's the relay.

The two I had fail (and original and then a used one I replaced it with) started giving intermittent bulb-out warnings when the bulbs were just fine. BMW's troubleshooting procedure is - eliminate everything but the relay - and if all the other stuff passes - then it's the relay. It was - twice for me.

HTH,

[sgtbill]

Morton's sent me a new tail light checking module. My wife and I installed it this weekend and the brake lights are fine... Our troubleshooting process includes drinking beer or wine and talking about the problem until someone says "... well it can't possibly be XXXX." The we replace XXXX. Repeat until the problem is solved.

Thanks for the advice.

sgtbill

practicing voodoo mechanics since 1902...

[deilenberger]

Glad to hear it's solved. When I still had a K bike I prevented this from failing again by keeping a spare (Eilenberger's Law of Spare Parts: You never need the part you have..) and I never had to replace the one in my K75S.. Keep your eye out on the Fleamarket - one might turn up cheap from someone parting out a K.. and if so - you can invoke the Law of Spares.

[98lee]

(Eilenberger's Law of Spare Parts: You never need the part you have..)

Don,

I truly live by that law.:thumb That's why I have three complete running bikes. No matter what fails, I know I have a good part on the spare bike to get the other two running as soon as possible. :bolt

It also makes troubleshooting of electronic components a breeze. Just because you put a new component in and it works does not necessarily mean that that component was bad. It could have been a bad connection or a bad wire and the act of removing and replacing the part jiggled things around enough to make it work. But if you swap parts with another bike and the problem moves to the other bike, it's a pretty safe bet that the component was at fault.

I didn't really buy the third bike for that purpose, I just got a really good deal on it and planned to use the best components from the two bikes when I was restoring and modifying my wife's bike (she's 5'4" and she's flat footed with both feet on her K75S), but I kept the spare bike complete and running and decided to hang on to it.:dance :dance :dance

[deilenberger]

The strange part is - I accumulated a LOT of K bike parts over 10 years. Some free, some cheap, none expensive. After I sold the K and got the R1150RS I needed some money to Farkleize* the R bike. Ended up selling all the spare K bike parts and I believe I made a profit on them. Basically cost me nothing to have all the spares I ever needed to not use. :)

* Farkleize - the act of installing Farkles** on a bike
** Farkles - necessities for a bike that no one knew would be essential (or make it look better, go faster or just make the rider feel better..)