The Bulb Monitor Unit (BMU) and LEDs

[frankenduck]

The Bulb Monitor Unit and what it does:  The BMU is a relay of sorts.  It does three things:

1) Tests the brake switches. When you start the bike up, it illuminates the red "triangle light" in the center of the instrument cluster.  Once you've hit both brakes, it should go out.  If it doesn't then one of your brake switches is probably bad.

2) Tests the tail light.

3) Tests the brake light.

It's a safety device so you're not running around with a bad brake switch or lights that are out.

Note that the "triangle light" may flicker or go solid while you're riding.  Sometimes this is caused by a tail or brake light bulb getting old even though it lights up as it should.  Try cleaning the contacts in the socket or replacing the bulb.

Also note that if you've got Run-n-Lites in your rear turn signals that makes the BMU happy even if your tail light does go out. So if you've got Run-n-Lites you might want to visually inspect your tail light bulb occasionally.

LED brake and tail light bulbs: Since an LED is a Light Emitting DIODE, it only allows current to flow one way and has a very small load.  Some LED bulbs are polarized and some are not.  If you get a polarized one then the bulb monitor will not be happy and keep the triangle light lit.

You can cure this by getting rid of the BMU.  The BMU is the black box on the left side of the relay box right in front of the fuses.  Once you've removed it, short the following wires in the BMU socket:

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

Short the tail light power(gray/white) to the tail light wire(Gray/Black)

Note that since you removed the BMU, it's safety functions of testing the switches and bulbs is gone so it's a good idea to add checking those to your pre-ride checklist.  (You've got one, right? :lol: )


LED brake lights and the Audiovox CCS-100 Cruise control:  Similar to the BMU, the CCS-100 sends a small reverse current through your brake light bulb to know that your brake lighting is working properly.  So if you install an LED brake light it might also disable your cruise control.  This can be cured by adding a normally closed relay that grounds the CCS-100 purple wire when the brakes are not in use.

If you use a standard automotive relay for this (a.k.a. Bosch relay) then you want to get a five pin relay that has a normally closed terminal which will be labeled 87a.

Wire the brake light relay as follows:

85: Ground
86: Tapped into yellow wire from the rear brake switch
30: Ground
87a: CCS-100 Purple wire

[frankenduck]

If you're amenable to destroying a perfectly good $130 BMW part then you can take the BMU apart, cut the circuit board out and make it LED compatible.

[bikerboy]

Will taking out the bulb monitor have any effect on the ABS working?

[Inge K.]

Will taking out the bulb monitor have any effect on the ABS working?

You can do whatever you wan't with the BMU, it won't affect the ABS function.

Inge K.

[bikerboy]

Will taking out the bulb monitor have any effect on the ABS working?

You can do whatever you wan't with the BMU, it won't affect the ABS function.

Inge K.

Thanks for that. The reason for the question was that many car ABS systems will not switch on if there is a problem with a brake bulb.

I will go ahead and modify mine so that I can have a full LED array now.

[frankenduck]

2021 Update

I recently came up with a much simpler way to run LED brake and tail light bulbs: Just cut off or break off the BMU pin labeled K. This is the BMU pin that sends a ground to the instrument cluster to illuminate the BMU lamp. No pin, no cluster light!

Note that you will no longer have the front and rear break switch testing illuminate the light either.

On ABS equipped bikes the BMU indicator will still flash until you ride off and the ABS initializes.

[rbm]

Clever.  A reversible option would be to remove the corresponding contact from the plug that would engage with pin "K".  That is a simple procedure and can be reversed simply by plugging the contact back into the connector shell.

[frankenduck]

Clever.  A reversible option would be to remove the corresponding contact from the plug that would engage with pin "K".  That is a simple procedure and can be reversed simply by plugging the contact back into the connector shell.

Good point. That would work as well.

(But one would need to understand how the tang on the terminal holds it into the connector and how to "pick" it out with a needle.)

[Laitch]

Good point. That would work as well.
(But one would need to understand how the tang on the terminal holds it into the connector and how to "pick" it out with a needle.)

For those who haven't learned the secret handshake that unlocks this knowledge, here's a video for those who speak American.


Here's one for those who find Ozian accents soothing.

 

[daveson]

Righto shouldn't be confessing to this, but that hasn't stopped me in the past. I've found a quicker and reversible way. I call it the lazy bastards way, I like it. Wouldn't put money on how many times it's reversible.

[frankenduck]

Righto shouldn't be confessing to this, but that hasn't stopped me in the past. I've found a quicker and reversible way. I call it the lazy bastards way, I like it. Wouldn't put money on how many times it's reversible.

That's only reversible about ten times - until the metal at the bend fatigues and breaks.

[frankenduck]

For those who haven't learned the secret handshake that unlocks this knowledge, here's a video for those who speak American.

Dang. Those are much more complicated than the BMW connectors. I have a spare BMU connector to demonstrate with.  All you need to do is stick a pin or sewing needle into the top of the terminal hole for the white/blue wire's terminal in order to pull the terminal out. (Two white/blue wires on ABS bikes.)