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Thread: 91 k75s "Bulb monitoring unit" location?

  1. #1
    Join Date
    Mar 2010
    Akron, Ohio

    91 k75s "Bulb monitoring unit" location?

    I'm getting no power to the positive terminal on my upper tail light bulb. No power (for just that bulb) at the junction box in the tail light compartment. The other bulb is fine. The schematic seems to show that grey/yellow wire leading directly to the "bulb monitoring unit", so I will see if there is a problem there. Where exactly will I find that unit? By the way, the red indicator dash light is on, I gather telling me there is a problem with a bulb.

  2. #2
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    Here's something similar to this on the K100RT...check out post #10:

    Track that P/N to the K75.

    Maybe some background here:
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Join Date
    Mar 2010
    Akron, Ohio
    Right after I posted this question I went and checked the lights again. Does the top tail light only function as the stop light, not a running light? It only has one filament so maybe that is it. If so, it is actually working as a brake light. I thought the center red light on the dash if for a broken bulb. If so, I'm not sure why it won't go out, unless there is a bulb somewhere I don't know about. Both high and low headlights are working.

  4. #4
    Registered User GTRider's Avatar
    Join Date
    Apr 2006
    Nibley, UT USA
    Replace the bulb in the taillight. Filament failure on brick-k bikes would often cause the fault lamp to come on.

    1983 R80ST 1984 R80 G/S-PD 2004 K1200GT w/Hannigan S/C 2010 K1300GT 2018 R1200GS
    BMWMOA#52184, AMA#271542, IBA#138

  5. #5
    Join Date
    Mar 2010
    Akron, Ohio
    Thanks to everyone for the prompt help. I went to the threads that were suggested and read up on it. I didn't realize that using the brakes would turn out the light. When I did that, it went out. Everything seems to be working. Thanks again for the help. Much appreciated.

  6. #6
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.

    The bulb monitor checks the condition of your tail light and brake light circuits (very helpful since you can't see them while on the bike).

    You can tell if your headlight is not working at night because you don't see the light pattern on the road.

    You can tell if one of your turn indicators is out because the side that has a bad bulb will flash twice as fast.

    The bulb monitor checks 4 things; the tail light bulb, the brake light bulb, the front brake light switch, and the rear brake light switch.

    I belive it is checking for continuity to ground through the bulbs in each circuit. This also will detect a bad connection due to a corroded bulb socket. Every time you turn on the bike, it looks for continuity through the taillight circuit (through the bulb filiment), then through the two brake light circuits. If any of the 3 circuits fail, the warning light stays lit. Normal procedure is to start the bike and either before you take off or within a short distance, tap one brake and then the other and insure that the warning light goes out.

    If It doesn't, stop, get off and shut of engine, turn key and kill switch to run.

    #1 check if the tail light is on.

    #2 see if the front brake lights the brake light.

    #3 see if the rear brake lights the light

    If one of the brakes lights the light, the bulb is good, but the brake circuit that doesn't light it has a bad switch.

    If neither brake lights the light, the brake light bulb or socket corrosion is the problem.

    In all these years, I've never checked to see if the monitor will detect a failure after the system has been cleared (while you are riding). I guess something for tomorrow if I can find time.

    The bulb monitor module is located inside the relay box under the back end of the tank in the black plastic box that has the fuse panel on the side of it. It is the largest black relay in the left side of the box, forward of the fuse block. The slightly smaller black relay in the right rear corner is the starter relay.

    Last edited by 98lee; 11-25-2020 at 04:58 AM.

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

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