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Thread: 1977 R100RS Electric Problems

  1. #1
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    1977 R100RS Electric Problems

    I push the starter button, nothing no sound no start. I push the starter button again, I smell electric wire burning, and a puff of smoke from the right handle bar switch assembly (turn signal, kill switch, starter switch/button). So I remove the right handle bar switch and the green wire looks burned as does the one next to it. Also, the 3 lower wires look factory, the others look like someone has soldered them, comments?


    So I look inside the headlight bucket and find; 2 burned/partially bare green wires and a totally burned/melted headlight bulb. The 2 green wires, both connect to the circuit board, top right connector tab, labeled "15 U". The one green wire goes into the wire bundle that goes to the right handle bar switch. The other green wire looks to go into the wire bundle that goes up to the keyed ignition switch (not sure though). The headlight bulb is a 100/80W (ahhh who put that in?). And the bulb holder is toast. So the 100/80W bulb is the culprit it seems. Comments pls, plan of attack to resolve?

    Questions:
    Replace or repair handle bar switch, I have fair soldering skills.
    If I repair the switch, what should I do with the melted green wire from the switch that is in the headlight bucket?
    Replace 3 prong bulb holder? Are they BMW specific or generic car/bike holder?
    Maximum bulb wattage?

    thank you for all comments and suggestions and directions. I will check back but this is a big project. Also, many family issues pull on me so my time working on the bike is back burner. Know that I do appreciate and thank you for your help.

    chuck
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  2. #2
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    Bulb OEM is a 60/55w H4 BMW Part 63121354619.
    Dave Hutton '78 R100S (mfg 7/77)

  3. #3
    John D'oh
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    The bulb melt-down is significant but there is possibly a catalyst. There is a factory service bulletin on first year R100RS that indicates the need to install a fuse in the dash clock circuit - these bikes had mechanical clocks which were powered at all times. That was where the problem lay since under specific conditions, (points closed when engine has stopped rotating) key off - un-switched voltage could be distributed to the system. However unlikely it may seem that this could be a problem now, owners back in 1977 reported melted wires, wiring harness's and in a few cases burned-up bikes. Melted bulb though - that's a new one for me anyway. An event like that you might remember if riding at the time however your observations suggest you were unaware of the melted bulb - or it happened as you pressed the starter and you simply didn't notice as the headlight is shrouded after all.

    I did find a 77 RS that had no fuse as original. It had been purchased new, ridden a few times an sat forgotten until 2007.

    I'd look for another switch since there are probably melted wires in the bundle that now connect everything to everything. Also, check the relays by looking for correct continuity.

    If you haven't already, I'd disconnect the battery completely. From here you'll need a self powered test lamp, volt meter or both.

    Another explanation: Bulb ran so hot it melted fusing the high low and ground and melting the wires adjacent to it. Then as resistance went up connections heated up on the start switch which opened primary wiring circuits and caused a cascading electrical failure. Don't know why it stopped.
    Last edited by Na Cl K9; 10-04-2018 at 04:15 PM. Reason: forgot to add the simple explanation...
    John D'oh

  4. #4
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    Thank you for the replies, and to continue with NaCl K9 comments. I was outside in the sun and did not notice if the headlight was on. I disconnected the battery right away when the smoke occurred.

    Interesting comment about the bulb melting and shorting and here is why. In the photos you see that the 3-prong bulb holder got hot and melted some. However, the brown, yellow, and white wires from the bulb holder do show signs of heat or melting. At the bottom of the bulb there should be a plastic/or similar material that seals the bulb and connects the internal bulb wires to the external 3 prongs so the bulb can plug into the 3-prong bulb holder.

    The photo of the bottom of the bulb does not show the plastic. It got so hot and was so brittle that it fell apart when I tried to pull the bulb out. Sounds and looks like a short occurred right at the base of the bulb, doesn't it? Does anyone know what the "15U" on the circuit board connects to or powers?

    I was going to contact Rick Jones of Motorrad Electrik and get his opinion. Anyone else with guru like BMW electric skills?

    thanks, chuck

  5. #5
    John D'oh
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    15U - left side of top fuse/connectors (solid green wire) go to right switch (emergency stop) and to the ignition/headlight switch. I noted that the top 8amp fuse looks fine - is that true? The other side of the fuse connects to the coil which connects to the starter relay which is connected to the battery among other places. There is a factory wiring diagram for all 1977 /7 motorcycles that has a reliable circuit diagram for your RS. I have a PDF. I need to figure out how to get it to you...
    John D'oh

  6. #6
    John D'oh
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    1977 R100RS factory wiring diagram pdf - Lets see if this works.
    Attached Files Attached Files
    John D'oh

  7. #7
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    Thank you much, great diagram.

    Let me sort through this but it appears at first look that there is a correlation for the two shorts/failures.

    chuck

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