Well it has been a long while since I acquired this 91 RS (life happens). I have basically been through this entire bike. Tip to tail, top to bottom every system has been gone through. Clutch to cleaning every contact everything. Well almost 5 years later I have to get this project back online and complete. I am basically back to putting the fuel tank and a couple body panels on. Woo hoo!!  After sourcing a replacement right side foot plate and installing it yesterday I can’t get the brake light to come on from foot pedal or hand lever. First of all I have not started the bike as the tank is not on yet. First question is does the bike need to be running ( I would guess no) but we all know how the BMW engineers think. Lol. It is my only thought at this point. Everything I check seems to be fine. Unfortunately I have a rented garage and I have been warned way to many times about working on my bikes and tenants above my garage smelling gas ‍. So I am hoping for the last minute to crack open the gas can and fill her up outside of course. Any thoughts from the brain trust . Thanks in advance,

All other lights, signals, horn and dash work fine.

Bike does not have to be running BUT the kill switch needs to be in the on position for the brake light to work.

If that isn't the issue:

Check Fuse 1.

If you're not careful putting the right side peg plate on then you might pinch the wires for the rear brake switch between the peg plate and transmission which can cause a short. BTDT.

Kill switch is on, bike cranks. I never use them on any of my BMW’s. Learned that a long time ago.  Lol. 

Was SUPER carful routing that wire it is behind all of the plastic tabs. Also tested the rear switch it toned out fine. 

Fuse 1 is good. 

Also I am using the clymer book of lies. Wire coloring does not match up with the bike. ‍�

Thanks

Check the bulb. Turn signals use the same bulb so you can swap one of those in.

Make sure that the BMU is the relay box is connected.

All checked and double checked

Here's the BMW K1 diagrams which are more or less the same as for a K100RS4V:

http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/kwiring/K1.K100RS4V/K1.SLP.Wiring.Diagram.V2.0.pdf

Take the BMU out and try jumping 12V to the terminal for the gray/yellow (brake light) wire.

Well I just quadruple checked the fuse.  

Seems I should have looked DEAD ON IN the fuse box.

#1 fuse was missing. 

OMFG!! I have been chasing this all day 

Embarrassing to say the least. I had this bike down to nuts and bolts. 

Thank you sir and I have a feeling Santa is going to be good to you. 

Don't be embarrassed.  Someone I know rather well tore a bunch of his bike apart because it wouldn't start...

...only to find the kill switch was in the "Off" position.

Well that makes me feel a wee bit better. 

In my own defense, Sadly I haven't touched this bike in almost 5 years, and it was raised up waist hight on my table.

I am a pretty good wrench, no-one touches my bikes but me. 

But this was a new level of amateur hour for me. 

I hope to have the tank rebuilt and mounted tomorrow, and ready for a final tune and dial in for a shake down ride when this bitter cold hopefully next week. 

Thanks again for helping the newbie out. 

I am glad that this and many other forums have not fallen to FB and died a slow death. 

Ride Safe
Ride Long
Ride Far

Merry Christmas & happy New year to all. 

I will update in a few days, maybe I can figure how to post pix of the steed. 

Thanks again,
Chris

Anch'io ho una K 100 Rs del '91 e poco tempo fa ho avuto lo stesso problema, una cosa pazzesca, per poi scoprire che era il blocchetto di accensione posto sotto la chiave di accensione a creare il problema. Sostituito il pezzo e tutto ha ripreso a funzionare.
Ps, il pezzo non è più disponibile quindi bisogna cercarne uno usato.
Un saluto.

recomu wrote:Anch'io ho una K 100 Rs del '91 e poco tempo fa ho avuto lo stesso problema, una cosa pazzesca, per poi scoprire che era il blocchetto di accensione posto sotto la chiave di accensione a creare il problema. Sostituito il pezzo e tutto ha ripreso a funzionare.
Ps, il pezzo non è più disponibile quindi bisogna cercarne uno usato.
Un saluto.

Google translate:

I too have a '91 K 100 Rs and recently I had the same problem, a crazy thing, only to discover that it was the ignition block located under the ignition key that created the problem. Replaced the part and everything started working again.

Ps, the piece is no longer available so you have to look for a used one.

Part #: 61322305292   (K100RS4V, K1 & K1100)

Testing continuity for the ignition switch is at the bottom of this page:
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/4vignsw.forklock/4vignsw.forklock.htm

The BMU is a problematic part on these bikes. I bypassed mine.