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
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.
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.
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
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.
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
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
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
Another way is to get rid of the
BMU altogether and jump the wires to its connector with
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
An alternative more compact relay
could also be used as described at
© 2021 Drake Smith - Please do
not use or reproduce this elsewhere. Feel free to link
to it though.