For the electric lovers. Problem on my k100rs 1984.

Ive lost the tail light on my k now it doesn't work when the ignition is on. And there is no power to fuse 2. However when the bike is in park im getting voltage to fuse 2 and the tail light works perfectly.

The front brake light works regardless whenever i press the switch.

Now ive tried cleaning the switches on the right hand side. Ive tried a set that definetly work. Still nothing.

The bmu has been changed for a known working one. It doesn't matter if the side stand is up or down. The right hand cluster had been swapped still nothing.

Tail light sensor stays on as the rear brake has stopped working as well randomly.

Any idea I work nights so need to sort it for tomorrow.

Check if you have power at the grey/blue wire at the connector for the
ignition switch (under the tank).

Yes: the problem is in the ignition switch.
No: the problem is in the light on/off switch, or the related wiring.

Hello Inge there is no power to the grey blue at the ignition at all when on or off. In park the grey wire has the power as expected at ignition.
If I put a wire across from the green straight to it then it works fine.

I've just noticed that the clocks aren't lighting up either however if i do the above the light up as well.

Edit.

Looks like you were right inge. Rear brake light is still a mystery however. The tail light is now working the right hand big block the grey blue wire im getting 12 volts. If i pull on it im still getting 12 volts however the lights go off.

So it will last me for now but looks like im into more repair work before long.

TheJoker wrote:Edit.

Looks like you were right inge. Rear brake light is still a mystery however. The tail light is now working  the right hand big block the grey blue wire im getting 12 volts. If i pull on it im still getting 12 volts however the lights go off.

So it will last me for now but looks like im into more repair work before long.

Inge K is always right....

If your tail light is now working, the brake light works when operating the front brake lever but not when operating the rear brake pedal (bulb therefore is good) and the red triangle on the dash is permanently lit (BMU doing its job correctly), then I reckon your problem lies with the rear brake pedal microswitch. It is not unusual for corrosion to build up on the switch contacts - especially after riding on lovely wet, salty UK winter roads. When you operate the rear brake, the corrosion/crud that has probably build up on the contacts prevents an electrical circuit being made and results in no brake light and, as far as the BMU is concerned, you haven't operated the rear brake to clear the logic (both front and rear brakes need to be operated to clear the BMU light).

The rear brake micro switch is accessible by removing the right hand foot peg plate (3 bolts) - with care, you don't need to disconnect the brake hydraulics gubbins and can access the switch at the front of the foot peg plate - a bit of fine wet&dry paper between the microswitch contacts and/or a squirt of deOxit should do the trick. In the worst case, if the switch is cactus then it is an easy enough job to replace it (about £40 from motorworks - which is why is worth trying to clean it up first)

Or if you're working on the rear brake anyway, just fit a hydraulic switch in place of the rear banjo bolt. I hate those 'orrible BMW switches.

The most common problem with the early rear brake light switch is that the
plunger sticks and don't move all the way to the outer position.
Try to get some WD40 into the switch along the plunger, then move the
plunger in and out several times...add more WD40....move the plunger etc etc
Sooner or later the plunger moves all the way (4 mm).

Do this as a regular maintance to avoid this problem.....
If this don't solve the problem...well, nothing lasts forever...time for a new one.

The same switch is used for brakes, clutch and choke, if you don't need a
choke on warning light........

Hopefully these tips will help me isolate my tail light problem.  The red triangle light is also not lighting.

Al