1985 k100rs  Front brake lights OK rear brake won't . Checked continuity of switch ..OK. can't find information or whereabouts of the "bulb monitoring unit" that's in the electrical schematic. by passed the switch with a paperclip at the first connection, no light.

Any help appreciated.
thanks
Alan

Lift the rear of the fuel tank and you will see a flat black piece of plastic which will lift away and reveal the relay box which is full of relays and wires etc.

Hey Alan,

I had a similar problem with my 94 K75RT.  It turned out that the rear brake light micro switch (under the rear brake pedal) connector (beneath right side panel) had a worn connector. 

Cut off the black connector housing (which contains two connectors) and spliced in two aftermarket connectors.  Problem solved. 

You have to be careful, as there is not a lot of excess wire to play with. 

Lessons learnt:

• aftermarket connectors are cheap
  
  
• Even a second hand micro switch loom is not cheap
  
  
• There is not a lot of wire to play with coming out of the main loom beneath the fuel tank
  
  
• if you remove the right side footpeg hanger plate watch the spring on the master cylinder, as it took me ages to work out exactly where it was attached to.  Only need to do this if you remove the micro switch loom for the repair
  
  
• lots of stuffing around trying to find the problem
  
  
• no problem with the front brake indication
  
  
• did not even consider the bulb monitoring relay which I guess costs a bit
  
  

I hope this helps. 

Cheers, Dave.

alan we have seen some cracking of the soldered connections around the "bmu "  unit terminals

given that the two circuits ( front and back) both go through the bmu unit ....and that you have shorted the terminals leading to the loom at the connector

it might be an idea to continue on to that unit and have a close inspection

some have fixed their issues by reflowing the solder around those terminals .....the circuit board is easily dug out of the plastic cover for a good "look see "

use the "search" feature   to find some of the posts already made in here ....you will find pics of the unit ...and even circuits

good luck !

alanretired wrote:1985 k100rs   by passed the switch with a paperclip at the first connection, no light.

Switch works different, no brake is switch closed, brake is switch open (no contact)
Unplug the connector near the coolanttank on the right side, now the brakelight should work.
If not, keep searching for a short circuit till the lightcontrol unit.

Wheeldigger wrote:

Switch works different, no brake is switch closed, brake is switch open (no contact)
Unplug the connector near the coolanttank on the right side, now the brakelight should work.

I'll think you have to check your brake light switch one more time.
Open switch = brake light off.
Closed switch = brake light on.

Easy to diagnose if it's a BMU issue:

Remove BMU.

Short the front brake switch wire(Gray/Red) and the rear brake switch wire(Gray/Green) to the rear brake light(Gray/Yellow.)

I took of the tank got to the bmu. Looked like a green box connected to a ton of wires total group about 3/4-1" diameter! put it back. how would I go about checking if that is the problem with my back brake pedal not lighting the bulb? Rechecked the switch and the side connector. pedal down= continuity, up = no continuity. connector= 11+- volts.
Still no pedal stop light, front ok. There is no emergency flash unit under the seat but the red triangle light has been on solid since I got the bike.  The 4 switches on the console are all duds.

Suggestions please. Don't know if I really need the light to come on with the back brake because I always use both front and back, and it lights with the front one.

Also there are a bunch of unused connections under the tank, anyone know where to find out what they are all for?

Thanks,
Alan

The BMU is on the left side of the relay box - it's black. The green one is the flasher relay.

BMU:

Hi All,
Before diving into the technical stuff a few simple checks, is there power on the bulb holder at the rear, one of the common reasons for bulbs not working is the bulb holder. Is the bulb intact or faulty, are the electrical contacts to the base and the earth connection to the side of the bulb perfect, have you treated them with DeoxIT?

Then take a look at the big multi-pin connector in the rear housing that sometimes gets bad connections, after that sstart tracing back till you get to the point where the power reappears.
Regards,
K-BIKE

The blub lights up just fine with the front brake handle so it's fine, it's only the back brake pedal that it won't light up from. 

The bmu must have been relocated by a previous owner because the green thing is on the left side. I'll look at that black thing a soon as I get the tank off again.


Where is this exactly on the bike please " take a look at the big multi-pin connector in the rear housing"   
Thanks,
Alan

K-BIKE wrote:Hi All,
Before diving into the technical stuff a few simple checks, is there power on the bulb holder at the rear, one of the common reasons for bulbs not working is the bulb holder. Is the bulb intact or faulty, are the electrical contacts to the base and the earth connection to the side of the bulb perfect, have you treated them with DeoxIT?

Regards,
K-BIKE

The front brake is working properly so the bulb/holder etc. should be ok, but you're right to say it's common fault. It was with mine, even the wrong wattage bulb could cause the BMU to register a fault.

alanretired wrote:
Where is this exactly on the bike please " take a look at the big multi-pin connector in the rear housing"   
Thanks,
Alan

Remove the tail lamp unit and attached to the inside of the recess is a multi-connector (it's unplugged in this photo):

If you used a paper clip to bridge the contacts at the connector on the right side under the r/h battery cover then that connector needs checking or cleaning and the wires from there back to the harness needs tracking, you can bend the wires as you go to ensure you have no obvious breaks in the wires, sometimes once the insulation is nicked it can cause the wire inside to corrode and go green and not allow power past that point.

Hello Alan  Just use the search tab at top of Forum and type in 'electrical connections', select topics and you'll find comprehensive answers, diagrams and problems others have had.  THE best download is CF explanation of EFI.
Regards kf

I had exactly the same problem with my K100RT and I never replaced anything.  All I did was give the micro-switch on the brake pedal a good blast of CRC Marine and then rapidly work the switch.  It happened routinely, particularly after a lot of wet weather riding and this solution worked every time.

Yeah i second that Westeng though i've only had to do it once.

Thank you all for the help! I found the BMU , was going to bypass it but could not figure out how to take the contacts out of the connector, tried but could not do it. Gave up, put it all back together, thought I'd just use the front brake always at the same time. Then I tried the back pedal and now it works! Did not do anything specific to fix it, guess I just scared it and it decided to work? Happens a lot to me.
Alan

All's well that ends well.

FYI: You don't need to remove the terminals for the connector to jump it.  You can just strip the ends of wires and stick them in the connector terminals.  (Not recommended for long term use.  Just for testing.)