Hello Everyone. Work on the K100RS is progressing well. Just fitted a replacement rear brake switch but light still not working. The new switch is working fine and I've got power to the switch (green/black wire). Front brake light switch is working fine. I next job will be to take the tank off and check out the wiring beneath and the bulb monitoring unit. Any advice on resolving this problem. Best regards. Dave

Remove the BMU relay and check that you got power to terminal E2, (gray/green wire) when the switch/brake is activated.

If you do and cleaning connectors doesn`t help, it seems like the BMU relay is ready for the bin.

Inge K.

Thanks Inge K for your speedy reply. I will certainly follow your advise. After studying the wiring diagram could I just miss out the BMU as a tempoary bodge to make the rear brake light work for the MOT test? Dave

Check post 5 in this thread.

Inge K.

Many thanks for your help Inge K. I followed your advice. There was power at the E2 terminal but none at the 54A terminal so the BMU was indeed faulty. I went ahead by removing the BMU relay and shorting out the terminal as suggest in the thread you sent to me. Both brake lights now working fine. Thanks again. Dave

I'm also having issues with my brake light. My bike has been modified from original. I do not have the stock instrument cluster, or rear tail light. I have a single bulb, dual filament running/brake light. The other night both stopped working. I got home and saw that one of the filiments was burnt out, yet the remaining one was also not working. I bought a replacement bulb, but now the running light seems to work, but neither the front brake, or rear brake activates the brake light.
I have seem to be getting minimal current to the rear wire like there is a short somewhere in the line. I tried to trace the wire, and thought I found a culprit in a burnt connection plug, but replacing the plug did not seem to help. I visually checked the BMU for any signs of cracked soldering, or faulty connections, but didn't see anything that stuck out at me (as suggested by http://bmwmotorcycletech.info/K-hints.htm). I cleaned the connections with DeOxit, cleaned the rear brake switch, and the first plug from the front brake. Nothing changed.
I have not yet checked Inge's suggestion from an earlier post here to see if there is power to the BMU, but in the meantime, any other ideas why by brake light won't work?

Thanks in advance!

If both switches are out of action there will be a common cause and the common thing is the power to each switch. When there is no power the fuse is the place to start looking. don't just look at the fuse it can be faulty and not blown so measure with a meter, it should be 0 ohms if you dont get a reading replace all the fuses with new ones. They are all the same age and if one is crook then all are suspect.

Thanks RicK G! I'll check the power to the switch and the fuse tonight...Keep my fingers crossed as I'm running out of things to check.

RicK G wrote:If both switches are out of action there will be a common cause and the common thing is the power to each switch. When there is no power the fuse is the place to start looking. don't just look at the fuse it can be faulty and not blown so measure with a meter, it should be 0 ohms if you dont get a reading replace all the fuses with new ones. They are all the same age and if one is crook then all are suspect.

I checked the fuses, and they all read 00.something Ohms.
What is the easiest way to check for power to the E2 terminal? If I have one test lead on the E2, where should I put the other lead?

Negative terminal of the battery or engine case or gearbox case or frame.

Makes sense...I was trying to connect to the brown wire in the BMU plug, think that was how the circuit was being completed.
Thanks, I'll give it another go tonight. My wife is wondering when she'll get the garage back...

RicK G wrote:Negative terminal of the battery or engine case or gearbox case or frame.

t1slc wrote:...I was trying to connect to the brown wire in the BMU plug,

That should also function, since the brown wire is ground.

If that's the case, and I don't get a reading when the front or rear is depressed, what is the next step? I assume the lines from the front and rear brake switches meet somewhere and since neither activates the light, it seems logical that would be where to look. Any idea where that join happens? (I don't have my electrical diagram in front of my right now to check.)

Inge K. wrote:

t1slc wrote:...I was trying to connect to the brown wire in the BMU plug,

That should also function, since the brown wire is ground.

Well here's something interesting. When I check the volts at the brake light plug, I get a reading of about .2, but when a brake is pressed, the voltage drops slightly. What's going on!
Also, when I disconnect the BMU and check the grey/black wire (the tail light), I don't get a reading, but when I check it at the plug, it gives me 12V. So when I check the Grey/yellow brake light wire, I also don't get a reading (but get the low V reading at the plug.)
I am so confused, worried I'm doing something wrong, and generally pretty frustrated...But hey, I did manage to mount some new turn signals and got those working...

t1slc wrote: Also, when I disconnect the BMU and check the grey/black wire (the tail light), I don't get a reading, but when I check it at the plug, it gives me 12V. So when I check the Grey/yellow brake light wire, I also don't get a reading (but get the low V reading at the plug.)

It's normal that you don't have power at the grey/black and the grey/yellow wire when the BMU disconnected, this is the outputs from the BMU........

But you should have power here when BMU is connected (light on, brake[s] activated).

I should clarify, I was testing at the plug that goes into the Bmu not at the light plug.

Inge K. wrote:

t1slc wrote: Also, when I disconnect the BMU and check the grey/black wire (the tail light), I don't get a reading, but when I check it at the plug, it gives me 12V. So when I check the Grey/yellow brake light wire, I also don't get a reading (but get the low V reading at the plug.)

It's normal that you don't have power at the grey/black and the grey/yellow wire when the BMU disconnected, this is the outputs from the BMU........

But you should have power here when BMU is connected (light on, brake[s] activated).

Thanks to some great step by step instruction from ibjman, I was able to figure this out. After running through his diognasitc steps, I was able to determine that power was working at all the switches and plugs. Leaving only the BMU to be the issue. Turns out there was some flux on the back side of the BMU that was causing a short. If you look closely at the photo, you can see a few area of brown around the solder points (I already scraped off where the short was happening at the connector before I took the photo.)
When I first looked at the BMU, I didn't realize that the brown stuff was flux (and since the light used to work, I assumed that it was supposed to be there.) And so while all the solder points didn't show signs of cracks, the flux was the issue. I cleaned it off, and just like that the light worked. From the link to the BMU jumping page I mentioned in a previous post, it's possible that the modified light that is on my bike is pulling more current through the circuit and maybe some excess flux has been burning off?
You all were such a great help and this was a beneficial learning experience for me! I owe you all some beers.

I hope all is well now.
Personally, I've never seen flux cause such an issue. I wonder if by chance you could have actually just (temporarily) reconnected a poor solder joint somewhere without actually knowing it. If so, and it recurs, at least you'll know right where to go.

Electrical Gremlins can be spooky on the K's.

I don't remember if we covered cleaning all the various contacts, but if you don't have it you might just as well get some of this:

http://www.allelectronics.com/make-a-store/item/D5S-6/DEOXIT-R-CONTACT-CLEANER/REJUVENATOR/1.html

Use it to clean all & every place where 2 things connect together such as the plug & pins that connect the BMU.
It could be that you again inadvertently connected a defect by simply pulling the plug off & reconnecting it.

Another place that is famous for problems is corrosion on the multi ground stack bolted to the left side of the frame backbone about midway back from the front of the fuel tank.
Definitely need to unbolt all those tan wires and give everything a good scrub with de-oxit & a wire toothbrush.
Regards, Ibj...

A wire tooth brush now just the thought makes my eyes water or is it that I need new glasses.

better go back again in there
flux does not conduct at all
possibly you have moved the connections to provide temporary conduction


having a look at that pic I can clearly see several cracked and poorly soldered joints

in particular one of the output terminal to board connections

il edit the pic to show you where

i know this is an old post, I also have a problem with the tail/brakelight not working but the front brake works it activates the brake light, the switch behind the foot peg hanger died, I have a replacement, hooked it up and still no brake light, the wires going to the switch have less than 11 volts anyone have any ideas, cheers Gary

Corrosion on the bulb?

Hi Dai, no the bulb works off the front brake, seems like there is not enough power going to the switch

Garyk100 wrote:. . . the bulb works off the front brake, seems like there is not enough power going to the switch

Tracing the wires checking their integrity, looking for kinks and bare wire faults is the way to go. Often there is a kink in the wire down by the pedal assembly that interferes with electrical function. Take a close look there.

Separate the rear brake switch connector from the harness located under the right side battery cover and test that the switch closes when you press the rear brake pedal (use a multimeter set on Ohms setting and look for 0 Ohms when the switch closes).

While the connector is separated, set the multimeter to DC Volts and measure the voltage on the Green/Black wire on the connector at the harness side.  You should read +12V.  If not, you have a wiring problem or a blown Fuse #1.

If that switch diagnosis demonstrates a working switch, then check the E2 connection on the Grey/Green wire going to the BMU.  If you don't get +12V at E2 at the BMU when the brake pedal is pressed, the rear brake light will not function. Reconnect the rear brake switch connector.  Set the multimeter on DC Volts and measure between the Grey/Green wire and ground at the BMU connector.  Normally, the reading will be 0V.  When you press the pedal, the voltage should jump to +12V.  If not, there is a wiring problem between the brake switch and the BMU.  If it does, you have a faulty BMU.  One possible explanation is that solder joints inside the BMU can go bad; it's possible to repair any cold solder joint by reheating it with a soldering iron.

Easy way to narrow it down to a bad switch or bad BMU:  Short the two terminals on the wiring harness side of the rear brake switch connector.  If that lights up the rear brake then the switch is the problem.  If that doesn't then there's an issue either with the wiring harness or BMU.

Thank you all for the replies, if I cross the wires together there still is not a brake light, less than 10v at the connector above near the coolant bottle, so it's time for a good look thru the wiring, and bmu, if I find the fault maybe it will also fix the gear indicator that thinks neutral is 3rd gear and of course can only be started with the clutch in.
I will reply with the results soon, I suppose while I have the tank and bits off, I can clean the bugs and the other items the radiator has collected over the last 19 yrs, hide the bright yellow battery, repaint the bellypan as the only brake hose I have not replaced split, 
And I have my old exhaust that rattled,it had been welded many times and must of Brocken something inside. I have a duel muffler that I am going to see if I can make my own exhaust.
Have owned my K for nearly 20 yrs, the older I get the faster it seems, I do all my own maintenance and enjoy tinkering as much as riding, thanks to the Krew on here you help me to keep my ride goin, 
Cheers Gary (now to google what a bmu is)

A gear indicator that shows "3" when the transmission is in neutral means a defective Transmission Gear Position Indicator (TGPI) switch. Solution is to remove the TGPI switch, service it and replace. That will require removing the rear swingarm.

BMU mean Bulb Monitoring Unit, an electronic module in the relay box that monitors the state of incandescent bulbs for the rear brake and running lights. When a bulb fails, the BMU lights a dash indicator. Consequently, the brake switches don't control the brake light directly; they only provide inputs to the BMU. Also, the BMU does not play well with LEDs unless it is modified or eliminated.

ok thanks for the replies, done some other things which I will post in the relevant sections (thermostat stuck open) now electrics, not my strong point, like many others if the light at the top of the traffic lights is on its red, on the bottom it's green, was going for my powder monkey certificate years ago but thought it might be dangerous red and green flags and all. So to recap, rear brake switch had worn the little button off so a new switch from RS industries, small mod to make it fit but for $6 delivered worth it.
New switch fitted no brake light, low power to the switch, less than 11v.
Cleaned earth from Battery  to engine, it has 2 earth wires running to it, is the any other ground/earth wires, have read there is 1 under the tank, mine is 91 16v is mine different?
Removed and opened the bmu relay, can't see anything wrong but don't know what I am looking for and if I did know can I see it.
So hook the bmu back up, now I have no tail light at all no brake lights and if I turn the indicators on they don't work but the clock disappears, 
I am thinking the bmu, help, Gary

A quick test would be to remove the BMU and jump the connections to complete the circuits. Tail lights should work normally but you won't have the monitoring feature.

Thanks Holister, tryed jumping the terminals, no change, but took the tail light out to make it easier to see, replaced the tail bulb it was still working but has a chrome look to it, I made a mistake before the brake light does still work off the front brake switch, headlight works no tail light and low power to the back brake switch so no brakelight, put the bmu back in all is the same but the warning light on the dash comes on as it should then it used to go off when I tap both brakes, if I turn indicators on they don't work and the clock fades out, hmmmmm

Inge K. wrote:Remove the BMU relay and check that you got power to terminal E2, (gray/green wire) when the switch/brake is activated.

If you do and cleaning connectors doesn`t help, it seems like the BMU relay is ready for the bin.

Inge K.

So I get it right with the multimeter one pointer in the E2 on the plug other pointer to ground point on the frame

I was just reading the workshop manual to make sure I connected the wires to the fuel pump correctly, I must have as the bike starts and runs, but found this in the tank removal section, I don't play golf [url=https://servimg.com/view/18791023/131][url=https://servimg.com/view/18791023/131][url=https://servimg.com/view/18791023/131][/url][/url][/url]

Try hanging around the local golf club, look miserable and sob occasionally. When they ask you what's wrong tell them the missus is sick of being a golf widow and has stolen all your tees. They will feel sorry for you and give you tees and will invite you to the bar for drinks to cheer you up.  If you sob  occasionally  and look really grateful it is unlikely they will ask you to return the shout, if they do tell them she stole your wallet as well.  Win Win. 
Regards Martin.

Garyk100 wrote:I was just reading the workshop manual to make sure I connected the wires to the fuel pump correctly, I must have as the bike starts and runs, but found this in the tank removal section, I don't play golf 

Don't play golf.......looks like you have a secret yearning.....four (sic)! 

Actually, for those slightly longer periods of tank removal, I insert a couple of old ear plugs into the fuel supply and return lines. Great for keeping the dirt out and bright yellow, so I don't forget to remove them later!

ear plugs I suppose it would give my ears a clean, helmet might smell of fuel tho, anyway, this morning I decided I would put the "custom exhaust" on put the brake light switch on and start putting the bike back together, hanging out to see what it sounds like, started it up a few coughs and a splutter till it got a belly of fuel, sounds nice not loud at all, thought I will try the indicators they work:1B: front brake light works and WTF now the rear brake light switch works, still thinks it's in 3rd when it's in neutral, maybe it take a good ride to repair itself:lol!: 
Cheers Gary

Garyk100 wrote:ear plugs I suppose it would give my ears a clean, helmet might smell of fuel tho, anyway, this morning I decided I would put the "custom exhaust" on put the brake light switch on and start putting the bike back together, hanging out to see what it sounds like, started it up a few coughs and a splutter till it got a belly of fuel, sounds nice not loud at all, thought I will try the indicators they work:1B: front brake light works and WTF now the rear brake light switch works, still thinks it's in 3rd when it's in neutral, maybe it take a good ride to repair itself:lol!: 
Cheers Gary

like the white wheels looks neat