Author Topic: Bulb Monitoring Unit Pinout  (Read 13692 times)

Offline pallum

  • ^ Proficient Motobricker
  • Posts: 192
Bulb Monitoring Unit Pinout
« on: August 28, 2011, 03:44:45 AM »
While troubleshooting some electrical problems I incidentally learned a whole mess about the Bulb Monitoring Unit (BMU). After chasing multimeter readings around the whole wiring harness I concluded that my BMU was dead/dying, so I decided to bypass it in hopes of catching my electrical gremlin. I found a bypassing the BMU guide elsewhere on the internet, but nothing about reconnection if you forget where everything goes so I'll just put it all down here in once place.

To bypass the Bulb Monitoring Unit:

1.) Remove spade connectors Grey/Yellow, Grey/Red and Grey/Green from the BMU main connector and connect all three together. The smaller female spades of Grey/Red and Grey/Green will slip onto the loops of the larger Grey/Yellow female spade.
2.) Remove spade connectors Grey/Black and Grey/White from the BMU main connector and connect both together. Just offset one and they will be able to slip together.
3.) Cover both of these separate connections with electrical tape (or something else insulating, heat shrink tubing if you are making this bypass a more permanent mod) so they don't short on each other and leave the rest of the unused connectors in the BMU main connector.
4.) Remove BMU to keep it out of the way inside the relay box and you are done.

To reconnect the Bulb Monitoring Unit:
After I discovered that the BMU wasn't my particular problem, I decided to reinstall mine (personal preference). Problem was... I didn't remember where all the wires went inside the BMU main connector. So I figured it out:

Each line in the BMU main connector diagram represents one slot in the BMU main connector, long lines for the larger slots, short lines for the smaller ones. Labels to the left of each slot are labels on the BMU itself and labels to the right are wire connections and their purposes for clarity. Diagram is if viewed from above after removing the BMU itself.
  • Federal Way, WA
1994 K75RT 45,000 miles (Apr 2020)

Offline frankenduck

  • Adrninistrator
  • ^ Quintessential Motobricker
  • Posts: 3653
Re: Bulb Monitoring Unit Pinout
« Reply #1 on: August 28, 2011, 07:00:34 AM »
Correct pinout for BMU:

PinWire ColorTo
58AGray/blackTail light
54AGray/yellowBrake light
E1Gray/redFront brake switch
KWhite/blueTell-tale light in instrument cluster
15Green/blackFuse 1 and accessory connector
58EGray/whiteFuse 2 and front parking light
31BrownGround and flasher relay
E2Gray/GreenRear brake switch

(Source: BMW wiring diagram.)

How to eliminate the BMU (for example to run LED brake and tail lights): http://www.motobrick.com/index.php?topic=331.0
Once I had a Collie pup
Dug a hole and covered him up
Now I sit there by the hour
Waiting for a Collie-flower

Offline pallum

  • ^ Proficient Motobricker
  • Posts: 192
Re: Bulb Monitoring Unit Pinout
« Reply #2 on: August 29, 2011, 12:42:42 AM »
Yea, messed up the BMU pin labels-- flipped the images upside down in my mind one too many times, fixed.
  • Federal Way, WA
1994 K75RT 45,000 miles (Apr 2020)

Offline frankenduck

  • Adrninistrator
  • ^ Quintessential Motobricker
  • Posts: 3653
Re: Bulb Monitoring Unit Pinout
« Reply #3 on: August 29, 2011, 12:21:23 PM »
Nice diagram.  Thanks for posting it.
Once I had a Collie pup
Dug a hole and covered him up
Now I sit there by the hour
Waiting for a Collie-flower

Offline SinNH

  • Motobrick Curious
  • Posts: 36
Re: Bulb Monitoring Unit Pinout
« Reply #4 on: June 06, 2019, 03:57:35 PM »
1993 K1100LT
Good information, thanks. Did the jumpers in the socket method this morning, surprised when the blinking triangle still was active. Under closer inspection, I have two w/b conductors to terminal K. The clymer manual shows just one w/b wire from bulb to BMU,  so realizing the monitor dash light was being fed from somewhere else, snip one wire and now all is well. Just though I would mention this, as I didn't think that I saw any print on this issue before. 
  • NH
  • 93 K1100 LTA greenish

Offline Scott_

  • Forum Moderator
  • ^ Quintessential Motobricker
  • Posts: 2105
Re: Bulb Monitoring Unit Pinout
« Reply #5 on: June 06, 2019, 06:27:21 PM »
Just fyi for you.... the +12v goes to the lamp in the inst cluster 1st(fed from the FI relay), then leaves on the W/B wire to the BMU to be grounded.
Sounds like that 2nd wire is grounded somewhere that it's not supposed to be.
I've looked thru a bunch of schematics and haven't found one that calls for 2 wires on that 'K' terminal of the BMU.
  • My Garage
1997 K1100LT 0302488 (R.I.P.)
1995 K1100LT 0302044
1997 R1100RT ZC62149
"One who does not ask questions is ashamed to learn" Danish proverb

Offline daveson

  • ^ SuperNatural Motobricker
  • Posts: 715
Re: Bulb Monitoring Unit Pinout
« Reply #6 on: June 07, 2019, 06:09:35 AM »
[quote ...The clymer manual shows just one w/b wire from bulb to BMU, ...
[/quote] SinNH

But the ABS system is shown on a separate page (718) and shows a white/blue wire from the bulb monitor to the ABS control unit, so I guess the Clymer manual is still correct.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline Scott_

  • Forum Moderator
  • ^ Quintessential Motobricker
  • Posts: 2105
Re: Bulb Monitoring Unit Pinout
« Reply #7 on: June 07, 2019, 07:41:54 AM »
[quote ...The clymer manual shows just one w/b wire from bulb to BMU, ...
 SinNH

But the ABS system is shown on a separate page (718) and shows a white/blue wire from the bulb monitor to the ABS control unit, so I guess the Clymer manual is still correct.

This has intrigued me..... looking even deeper in the BMW wiring diagrams, the extra W/B wire from the BMU to the ABS controller is only on ABS-1 bikes.....
So if you are bypassing BMU on an ABS-2 bike, or a bike without ABS, you don't have to worry about doing anything with terminal K.
  • My Garage
1997 K1100LT 0302488 (R.I.P.)
1995 K1100LT 0302044
1997 R1100RT ZC62149
"One who does not ask questions is ashamed to learn" Danish proverb

Offline Fstntq

  • Motobrick Curious
  • Posts: 15
Re: Bulb Monitoring Unit Pinout
« Reply #8 on: June 17, 2019, 08:54:09 AM »
This old thread helped me a ton to track down my lack of brake light on my new to me 90 K1.  Thanks to all who posted to it.  Initially no joy from either hand brake or foot brake so odds of both being blown pretty low on what was a well kept bike.  So Far my issue points to the BMU as all works fine with a bypass.  Instead of removing the connectors from the block I just jumpered them according to the pin out listed here as shown in the pics.  I had to bend the tangs a bit as they were a bit small to hold but they were all I had on a Sunday.
Best source for, dare I ask, an inexpensive BMU?
thx
Mike


  • Portland Maine
  • K1

Offline daveson

  • ^ SuperNatural Motobricker
  • Posts: 715
Re: Bulb Monitoring Unit Pinout
« Reply #9 on: June 18, 2019, 07:52:31 PM »
It looks like you have opened it,  look inside again,  it might just need heat again to the relevant pins solder. If your really lucky it only needs cleaning of the circuit board with deoxit.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline Fstntq

  • Motobrick Curious
  • Posts: 15
Re: Bulb Monitoring Unit Pinout
« Reply #10 on: June 19, 2019, 08:32:16 AM »
Thanks.  Seems silly not to give it a shot.  It was tougher than I though to remove the board form the cover as there was a fair bit of crud in the internal "slide rail"  All in all looks pretty good save some corrosion around this central post that may have migrated between joints.  I'll reflow the pin in/out connections and see what happens.


  • Portland Maine
  • K1

Offline rbm

  • ^ Quintessential Motobricker
  • Posts: 2055
Re: Bulb Monitoring Unit Pinout
« Reply #11 on: June 20, 2019, 12:13:39 AM »
While the board is out of the enclosure, test the diodes with a multimeter to see that they are working correctly.  Most multimeters these days have a diode test feature.
  • Regards, Robert
Toronto, Ontario

1987 K75 - Build Blog @http://k75retro.blogspot.ca/

Offline Fstntq

  • Motobrick Curious
  • Posts: 15
Re: Bulb Monitoring Unit Pinout
« Reply #12 on: June 20, 2019, 10:48:41 AM »
Well my multi meter is as old as the bike so no diode test feature.  That said not sure I'd know the diodes from the straight resistors either.  At this point pretty easy to just drop back in the bike to test.
  • Portland Maine
  • K1

Offline rbm

  • ^ Quintessential Motobricker
  • Posts: 2055
Re: Bulb Monitoring Unit Pinout
« Reply #13 on: June 20, 2019, 06:19:39 PM »
Simple test.  Diodes on that board are either black with a white stripe or glass with a black stripe.  The stipe is the diode's cathode (-ve pole).  The other end is the anode (+ve end).  Use the resistance setting on your meter.  Test for low resistance with the Red lead on the anode and Black lead on the cathode. Reverse the test leads and the reading should be high resistance.  If you don't get this behaviour, then your board may have bad diodes.  These diodes steer current in the circuit and could account for your BMU behavior.
  • Regards, Robert
Toronto, Ontario

1987 K75 - Build Blog @http://k75retro.blogspot.ca/

Offline Fstntq

  • Motobrick Curious
  • Posts: 15
Re: Bulb Monitoring Unit Pinout
« Reply #14 on: June 20, 2019, 08:19:57 PM »
Thank you for the new life skill!
All the diodes checked out and each was VERY close in measured throughput.  Back in it goes, though maybe after the inspection, given my luck.
  • Portland Maine
  • K1

Offline szabgab

  • ^ Proficient Motobricker
  • Posts: 346
Re: Bulb Monitoring Unit Pinout
« Reply #15 on: June 26, 2019, 04:49:00 PM »
Hi, given, that ebay is full of old K bike relays, you can source one for a low amount, probably the lowest of all the BMW parts. After multiple attempts to get my burnt BMU working again, I have bought one for 5GBP+postage, since then my bulb monitoring works again, as it should...
  • Budapest, Hungary
  • K75S 1985 model

Offline Fstntq

  • Motobrick Curious
  • Posts: 15
Re: Bulb Monitoring Unit Pinout
« Reply #16 on: June 26, 2019, 05:05:55 PM »
The trick on used is knowing before hand if they work or not!  Can get NOS new for about $65 but I got lucky.  Thanks to rbm, I checked all the diodes which tested good and reflowed all the main contact solder joints and the BMU is currently functioning as it should.  Top result!
  • Portland Maine
  • K1
Agree Agree x 1 View List

Tags: