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Thread: Fuse failure, turnsignal, brakelight, horn circuit, 76 R90/6.

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    Fuse failure, turnsignal, brakelight, horn circuit, 76 R90/6.

    It (the problem) started after the bike went dark after going over a parking lot speedbump at 4 mph. A local Airhead specialist was able to correct starting and running issue, but the fuse on the turnsignal circuit blows, some times right away, other times 200 miles into a ride. 59000 on the odo and I have replaced the relay, but it's an intermittent problem. Is there anyone that has found a similar gremlin ? I'm about to order a new harness, but I have a feeling that it is something we are overlooking.
    Bill
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    Last edited by tanker4me; 06-27-2013 at 06:57 PM.

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    Administrator 20774's Avatar
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    Sounds like you have a power line that has chafed and touches ground evey once in a while...fuse blows. What you can do is rig a light across the fuse connection and watch for it to light up when you move the harness around. Narrow down the problem that way. I think that should work.
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    '78 R100/7 & '69 R69S & '52 R25/2
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    Registered User lmo1131's Avatar
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    +1 on Kurt's take on the situation. The main harness for the /6 is not an inexpensive item, so I'd definitely spend some time investigating. Tearing into that rat's nest is a little daunting, but well worth the peace of mind knowing what's going on it there. A wiring diagram makes it much easier.

    You mentioned that,

    A local Airhead specialist was able to correct starting and running issue,
    I'm assuming that the entire electrical system failed simultaneously; and now "only" the turn signal circuit is presenting problems. Yes? Did the mechanic tell you what he corrected?

    I'd pull the head light (disconnect the battery first) and start a thorough search for any loose, or pinched wires. Since the bike runs reliably, the circuit from the battery to the starter relay/ignition switch is solid.

    Virtually everything else (coils, head lights) derives power from the Ignition Switch via the Green to the "fuse/connector block" (Terminal 15). From the opposite side of the fuse (at Term 15) it's distributed via Green/Black to the Turn Signal Relay, Horn, Voltmeter, and the Front and Rear Brake Light Switches.

    Look for any loose connections at the ends of these circuits, broken, frayed or pinched conductors. Any of these wires touching a "ground" will blow the fuse.

    Additionally, check the turn signal circuit conductors (Blue/Red - Blue/Black) and inside the turn signal housings; if one of them is grounding while energized it will blow the fuse.

    Here's a wiring diagram (go to the bottom of the page and zoom wayyyyyyyyyyyyyyyy out) > http://bmwmotorcycletech.info/schematic1975,1976.htm

    Here's a /7 color diagram (essentially the same) and easier to follow > http://www.omnilex.com/public/bmw78/78r100wire.jpg
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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    Macrunch MCrenshaw's Avatar
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    The turn signal wiring runs from the headlight shell then under the headlight ears, then under and out the turn signal stalks to the bulbs. It is not difficult for those wires to become pinched or cut and short out, thus blowing the fuse. Yes, it's frustrating to trouble shoot and it may require a disassembly to finally find it and fix. Start with a volt meter and continuity test to isolate where the short is located.

  5. #5
    Registered User lmo1131's Avatar
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    The turn signal wiring runs from the headlight shell then under the headlight ears, then under and out the turn signal stalks to the bulbs. It is not difficult for those wires to become pinched or cut and short out, thus blowing the fuse.
    +1 good point

    The front turn signal wire path is a b*tch; there couldn't be more opportunities for a pinch. Note: The hole(s) in the head light support bracket(s) should have a rubber grommet in it. It's a tiny grommet and doesn't seem to last long ... when it's gone it's a perfect spot to cut through the insulation.

    The grommet isn't listed separately (it comes with the turn signal harnesses). I think I got replacements from Vech ( 31412054282 grommet, fork ear, 4mm ), but you should call and ask.
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    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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    Update

    Thanks, The disassembly/reassembly was the method used, and there were not any obvious places where a short had occurred. So far the fuse has held with about 200 miles since reassembly. A new horn substitute from Max BMW was fitted as well as a new relay from Irv Seaver BMW. The wire harness is no longer available from BMW.
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    Last edited by tanker4me; 06-27-2013 at 06:59 PM.

  7. #7
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    The problem with this kind of short is it can be ANYWHERE.

    One place that is often missed is because it is hidden. The rear harness goes through the seat sub frame. The wires inside the frame often chafe because they are inside this area. You may get lucky and find the problem in a part close to where the wires enter the sub frame but if you have to remove the whole thing it's difficult to replace. Use a wire or a string to pull wires through. Don't make repair FAT. It won't go back in the frame.

    Make the temporary repair by running wires outside the sub frame. Then later you may get around to putting them back inside.

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