Brake Lights Stuck On - K100RS

[WHITE91K100RS]

Hey all, hoping someone here can point me in the right direction...


1991 K100RS 4v - Just completed spline lube and failed u-joint replacement (pictures here

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Quick history - After completing spline lube, I decided I would also bleed the rear brakes to get some fresh juice before getting back on the road. During the process, I blew a 25yr old brake hose, so I replaced rear brake hoses with Spengler stainless steel. Also rebuilt the rear master cylinder with brand new cylinder/spring/orings from BMW since it was leaky and tired looking. After all this was complete - I noticed that my rear brake light was stuck in the on, braking, position.

In an attempt to trouble shoot I unplugged the rear brake wire and brake light remained lit/engaged.
Hit the front brake with rear brake wire unplugged and brake light unchanged.


Then I unplugged the front brake wire and brake light remained lit/engaged.
Hit the rear brake with front brake wire unplugged and brake light goes OFF.
Release rear brake with front unplugged and brake light goes back ON.


I read a few posts here about brake light sticking and tried a few of the common solutions (cleaned grounds, reinstalled RMC) and still nothing. Any ideas where the ass backwards brake lighting issue might be coming from? Thanks in advance!

[WHITE91K100RS]

I thought the same thing - pulled the right side foot peg plate off, removed brake switch and cleaned it a bit. No pinched wires found

[WHITE91K100RS]

Light at the tail end of the bike that is responsible for letting other people know I'm stopping

PS, what does removal and reinstallation of swingarm have anything to do with it?

[Laitch]

In an attempt to trouble shoot I unplugged the rear brake wire and brake light remained lit/engaged.

You might have a switch stuck on or wires crossed. You'll need to check wiring from the tail and brake light sockets right up to both brake switches. Use your multimeter. Clean your switches with electronic cleaner. It could be coincidence but you probably crossed up something during disassembly and assembly.

[The Mighty Gryphon]

What happens when you unplug the bulb monitoring unit which is in the relay box? 

BTW, your u-joint failure was probably caused by someone installing the driveshaft with the u-joints out of phase.  Did you make sure the phasing was correct when you put everything back together?  It's important to get it right.

[kioolt]

Don't k100RS's have only one u-joint?

[WHITE91K100RS]

Unplugged bulb monitor relay and both brake lights go out

[WHITE91K100RS]

I also swapped bulb monitor relay out of a friend's kbike and same issue... so nothing wrong with bulb monitor relay


And hey - thanks for mentioning shaft phasing! No, I dont believe I phased properly so im going to have to dig back into the final drive so I can pull the drive shaft out and ensure phased properly. Thank you!!

[Laitch]

Don't k100RS's have only one u-joint?

The '90–'92 models have paralever suspensions so they have that drive shaft arrangement.

[WHITE91K100RS]

Problem solved - I dug into the switch in the front brake lever and found a rolled up piece of electrical tape previous owner wedged inbetween switch and contact because wasnt making full 'click' when squeezed. Fixed it proper and works find now.

[Laitch]

Fixed it proper and works find now.

:clap: