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Thread: Gauge lighting - 1988 R100RT

  1. #1
    gbrickell
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    Gauge lighting - 1988 R100RT

    Did a little searching and didn't come up with anything, so figured I'd ask.

    The cluster lights (speedo/tach) on my '88 R100RT are inop. The backlighting, that is. The turn signals, neutral light, etc, all work properly.

    My only other (brief) airhead experience is a 1975 R90/6. If I recall back in my dim memory, there was a flexible printed circuit board of some kind inside the cluster that tended to go bad. Was wondering, before I start tearing into things I know little about, if the RT of my vintage uses the same "guts" inside the gauge cluster as the old /6 did. Either way, advice and or link to any DIY or tech tips anyone knows of regarding how to troubleshoot and or repair the gauge backlighting would be appreciated.

    Sorry if this is probably a noob question but I didn't turn up much on Google either. Thanks for any guidance.

  2. #2
    Registered User lmo1131's Avatar
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    Anything else not working; parking lights, license plate light, clock, voltmeter?

    If so, the 8A fuse (right side of diagram) is a likely culprit. Gray/Black wire feeds the back lighting (as well as the other components mentioned).

    If not, it may be just the bulb(s)

    Here's the diagram. Kinda hard to read, but if you concentrate on it it's workable. Kinda.

    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  3. #3
    gbrickell
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    Thank you for the reply and the diagram.

    Your post made me want to double check that everything else was working. Glad I did. The gauges and indicator lights all function properly aside from back lighting. The back light to the clock on the fairing IS working but not the lighting to the voltmeter next to it, also fairing mounted. The fact that one of the fairing gauges has lighting but nothing else might imply something.

    Also, and I probably would not have noticed this outdoors, my brake light seems to be on all the time when the bike is running. When I turn the key to the first position, the taillight comes on, and when I turn it to the second position to get ready to start the bike, the brake light comes on full intensity. When the bike is running, the brake light stays on all the time. Glad I tested this indoors to see that brake light against the garage door.

    Not sure if the brake light issue is related to the gauge backlighting or not.

  4. #4
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    Instrument light bulbs can burn out - having both speedometer and tachometer bulbs bad at the same time may be such a coincidence. The brake light on all the time can be: the rear brake switch is stuck/corroded or the hand brake master cylinder switch is stuck/corroded or (and it much less likely) a wire has grounded from the handbrake or rear brake switch. It's good to check the lights regularly. Good luck!
    Stan

    AH# 13238

  5. #5
    Registered User 88bmwjeff's Avatar
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    Yes, there is a printed circuit board in the instrument cluster, but it is different than the one for your 75. bulbs could be out for the remaining ones. Check the wiring going to the back of the light for the voltmeter. Both of mine broke off. The wire gets brittle with age and with the vibration, they can break. Be careful when going into the instrument cluster, since the case is NLA. If you do go into the cluster. Don't forget to take note of the order of the three wires for the tach (if you remove them). The tach can be damaged if the wires are installed incorrectly. Also, the main wiring harness is NLA, so treat it with care as well.

    With regards to the brake light, check the switch for the rear brake and make sure the brake pedal is hitting the switch. It's located on the inside of the frame in case you didn't know.

    I hope this helps.

    BTW, I see you have a Bermuda Blue bike as well. Oddly enough every 88 RT I've noticed seems to be in Bermuda Blue. I wonder if there are any other colors for the 88 RT.
    Jeff in W.C.
    1988 R100 RT (the other woman)
    "I got my motorcycle jacket but I'm walking all the time." Joe Strummer

  6. #6
    Registered User lmo1131's Avatar
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    The fact that one of the fairing gauges has lighting but nothing else might imply something
    The clock and the voltmeter are powered by the same wire (gray/black). If the clock is working, but the voltmeter is not, then I would suspect either the bulb, or a broken wire (both is possible, but not probable).

    Not sure if the brake light issue is related to the gauge backlighting or not.
    What Stan and Jeff said..

    Before you go sleuthing around inside the head light nacelle, it wouldn't hurt to disconnect the battery until you've got the headlight off and have a chance to check the general condition of the wiring. Once you know that there aren't any frayed conductors or errant loose ends flying around you can re-connect and start testing continuity with either a multimeter or simple continuity light.

    edit - check that the Brown ground wire is continuous. It daisy-chains across the various components (to say that that diagram is difficult to read is an understatement...)
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  7. #7
    gbrickell
    Guest
    Thanks for all the input. I figured the brake light thing was probably a simple switch somewhere and I'll troubleshoot that issue first now that I found it. Appreciate the advice. If I find something, I'll post it.

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