2021 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.

The BMU (Bulb Monitor Unit)

Warning: If, for whatever reason, you ever need to hit the kill switch while riding, note that it also kills the power to the brake light so if you are braking with the kill switch engaged then the traffic behind you will not know that you are braking. (In my opinion, this is not a very thoughful design by BMW engineers.)

The Bulb Monitor Unit and how it works: 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 until both the front brake lever and rear brake pedal have been applied.

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. Once you've hit both brakes after turning the bike on, it should go out. If it doesn't go out then one of the brake switches or one of the rear light bulbs may have failed.

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 then that makes the BMU "happy" even if your tail light does go out. So if you've got Run-n-Lites installed then you might want to visually inspect your tail light bulb occasionally to make sure that it is illuminated.

 

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

To address this you can make it so that the BMU does not light the cluster warning lamp but the LED brake and tail lights will still work. (Note that this also disables the BMU warning lamp if you have a faulty front or rear brake switch.)

 

2021 Update: The approach below this box works but in hindsight I was overthinking things. Here are a couple of simpler ways to disable the BMU warning lamp when using LED brake and/or tail lamp bulbs.

Non-ABS bike: Remove the lamp from the cluster behind the BMU "triangle light" red lens. The brake and tail lights will continue to function normally with LED bulbs installed but the warning light in the instrument cluster cannot illuminate since there is no bulb. (This could also be accomplished by cutting the white/blue wire at the BMU connector in the relay box. The white/blue wire from the BMU sends a ground connection to the instrument cluster which illuminates the BMU warning lamp.)

ABS bike: On ABS equipped K75s and K100s the BMU warning lamp in the instrument cluster doubles as a second ABS warning lamp and flashes if the ABS is in fault mode. If you wish to keep this functionality but keep the BMU from illuminating the warning lamp when LED brake and/or tail lights are used then you can cut off the pin marked "K" on the bottom of the BMU. This will keep the BMU from sending a ground connection to illuminate the BMU warning lamp in the instrument cluster but the instrument cluster BMU warning lamp will still function as an ABS warning lamp.

If you don't want to damage the BMU or  for whatever reason want to implement a mod that is reversible then another option is to remove the terminal for the white/blue from the BMU connector. All you need to do is stick a pin or strong sewing needle into the top of the terminal for the white/blue wire. (Or two white blue wires on an ABS equipped K bike.)

Simply stick the pin/needle into the connector as shown in the photo below and the terminal should pull out of the back of the connector.

MY OLD APPROACH

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:

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

2) Short the tail light power wire (Gray/White) to the tail light wire (Gray/Black)

Note that since you removed the BMU, its 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.

There are a variety of ways to do this.  One is to take your BMU apart, cut out the circuit board and then solder wires to the insides of the terminals.

Another way is to get rid of the BMU altogether and jump the wires to its connector with Posi-Tap connectors:

LED brake light with 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: Tap into yellow wire from rear brake switch

  • 30: Ground

  • 87a: CCS-100 Purple wire

An alternative more compact relay could also be used as described at this link.

2021 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.