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Thread: Instrument Failure

  1. #1
    shieldhouse
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    Angry Instrument Failure

    The last time I took my 1978 R100RS out, it started right up just fine -- after several weeks stuffed in the garage while there was a constant threat of rain. The problem is that the clock, voltmeter and tachometer all decided to stop working simultaneously.

    This machine is 34 years old and it's possible, but not likely, that all three instruments died simultaneously. The voltmeter has been jumpy for the year or so that I've owned the machine and the damp environment the past few weeks may have done some kind of nastiness.

    Still, I have to think something easier (and cheaper to fix) happened. I popped open the headlight and saw a small group of fuses in there and all appeared to be fine. I have a Haynes and Clymer's manual. The Clymer's, I believe, says the fuses are under the tool tray. I looked there and didn't see anything that resembled a fuse box.

    Can anyone feed me some ideas on how to tackle this?

  2. #2
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    1. Dig out the wiring diagram and see if they are all on the same fuse. If so, replace that fuse regardless of whether it looks good or not.

    2. Look a the wiring diagram for any other commonalities (e.g., ground, power feed) and check those.

    3. The other possibility is that the problem is 'mechanical.' By that I mean that the wires for the three instruments happen to be in close proximity and somehow have physically damaged.

    The weather may be a red herring. It's unlikely humidity caused a problem like this.
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  3. #3
    Administrator 20774's Avatar
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    The voltmeter and clock seem to get power through the fuses plus there are other connnections. The tachometer is electrical and gets its signal from a single wire that comes off one of the coils. It would seem that if the tach doesn't physically operate, that failure is independent of the simple power that is sent to the other two devices.

    The /7s have the big plug that goes into the back of the speedo/tach pod. The power for all the bulbs as well as the signal for the tach comes in through that plug. Corrosion on the pins of the plug can create issues with the display. But if the voltmeter and clock are located somewhere else, you have two problems.
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  4. #4
    Sir Darby Darryl Cainey's Avatar
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    Very unlikely they would fail at the same time.

    What I would do is pull out your volt meter, & run test wires directly to your battery, making sure that - & + are connected corectly!

    If the volt meter works you have to look elsewhere!

    Good luck!
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  5. #5
    Registered User bmweuro's Avatar
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    Quote Originally Posted by 20774 View Post
    The voltmeter and clock seem to get power through the fuses plus there are other connnections. The tachometer is electrical and gets its signal from a single wire that comes off one of the coils. It would seem that if the tach doesn't physically operate, that failure is independent of the simple power that is sent to the other two devices.

    The /7s have the big plug that goes into the back of the speedo/tach pod. The power for all the bulbs as well as the signal for the tach comes in through that plug. Corrosion on the pins of the plug can create issues with the display. But if the voltmeter and clock are located somewhere else, you have two problems.
    That is incorrect. The clock is a constant hot wire that is red and unfused and the volt meter is fused inside the headlight bucket and the wire is green w/ a black tripe. Before you do anything remove the gas tank and clean the ground wires that are at the coil mount(brown wires)
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  6. #6
    Administrator 20774's Avatar
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    Quote Originally Posted by bmweuro View Post
    That is incorrect. The clock is a constant hot wire that is red and unfused and the volt meter is fused inside the headlight bucket and the wire is green w/ a black tripe.
    Well, there I go trying to interpret wiring diagrams! I looked at one of them that's on-line...seemed like they both routed through fuses.
    Kurt -- Forum Administrator ---> 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!

  7. #7
    shieldhouse
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    Cool Big Plug

    20774 was correct. The big plug behind the instrument cluster was disconnected. After I mashed the thing back in, the tach again began working, as did the neutral light, which had crapped out earlier. I assume that as the plug worked it's way out, various instruments became disconnected.

    Plugging it back in did nothing to correct the problems with the clock and the tach. When I get more time, I'll remove the tank and examine the grounds, as recommended. I suspect the voltmeter was on its way out anyway. While riding, it would bounce around like it was on crack. Is it common experience for these electric clocks to keep ticking -- probably without service -- for 34 years?

  8. #8
    John D'oh
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    Quote Originally Posted by bruno78 View Post
    20774 was correct. The big plug behind the instrument cluster was disconnected. After I mashed the thing back in, the tach again began working, as did the neutral light, which had crapped out earlier. I assume that as the plug worked it's way out, various instruments became disconnected.
    There is a small Phillips oval head sheet metal screw that holds the connector to the back of the instrument housing. Check to see it is still there and if its still in place, tighten it up if you can. Thats what holds the connector tight.

    Quote Originally Posted by bruno78 View Post
    Is it common experience for these electric clocks to keep ticking -- probably without service -- for 34 years?
    If its a White face clock then no. Green face clocks seem to do better. Its due to the type of movement used. The auxiliary instruments have a common ground which is probably the cause of their inactivity.

  9. #9
    Fixes Gauges tvrla's Avatar
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    I agree with all that.

    Namely - two separate issues. And you found one of them - the loose plug. And the other is most likely the ground. When several things stop working or there are multiple weird electrical problems, suspect a bad ground!

    Or it could be three problems - the volt meter gave up the ghost and the clock lost power. By the way, the VM can be rebuilt.

  10. #10
    Atomic City Boxer 154048's Avatar
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    And you may have to put a small flat washer on that phillips head srew that holds the electrical connector 'plug' onto the instrument pod, as the pod has a tendency to work its way over the screw...as my RT recently proved...
    Steve in Santa Fe
    1980 R100RT
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