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Thread: R90/6 Electrical Problem

  1. #1
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    R90/6 Electrical Problem

    I'm wondering if someone can advise me on a problem I'm having with my '76 R90/6.

    This has happened several times; I get the bike started as usual and warm it up, or have done some riding and then restart it, and then turn on the headlight switch which causes the instrument panel lghts to go off and the bike dies. Several times after I've left the bike sit for awhile after this has happened; I turned the ignition key on, the instrument lights go on and I can start the bike. Once when I turned on the ignition key, after having left the bike sit for awhile after it had died, the instrument lights went on, but blinked off when I hit the started button and the starter wouldn't work.

    Any ideas??

  2. #2
    Administrator 20774's Avatar
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    The first thing I think of is the starter relay under the tank. This relay controls virtually all of the power routing for the bike. Pull the relay and clean up the contacts...see if that helps.
    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!

  3. #3
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    Quote Originally Posted by 20774 View Post
    The first thing I think of is the starter relay under the tank. This relay controls virtually all of the power routing for the bike. Pull the relay and clean up the contacts...see if that helps.
    OK, thanks. I'll give that a try. Do you know if those relays need to be replaced periodically?

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    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    Only when they fail. I replaced mine a few years ago, and have a spare, just in case somebody needs one.
    F.O.G.Rider, Rounder #6,
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    BMWRA Wisconsin Region Rep, camping chair

  5. #5
    Administrator 20774's Avatar
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    Not sure about periodically, but certainly they don't last forever. More often, it's just the pins that need cleaning...usually just by pulling it out and pushing it in a few times does the trick. But internally, the contacts can corrode and not actually latch effectively. There is a red wire that comes into the relay which goes right back out to the rest of the circuit. There are other wires that are part of the relay section...these are associated with the providing power to the starter. But this other smaller red wire is the culprit. I've seen on Snowbum's website where he suggests connecting a wire to the incoming and outgoing red wire, thus bypassing the pins on the relay. What this does is effectively eliminate the problem with corrosion building up on the pins which can affect this red wire "pass-through".
    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!

  6. #6
    R100GS, '89 Guenther's Avatar
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    turn on the headlight switch which causes the instrument panel lghts to go off and the bike dies
    That doesn't sound like the starter relay. My guess would be a ground connection problem. Maybe the battery ground connector or the grounding post where the headlight and the ignition (coil) are connected (sorry I don't have a wiring diagram).

    /Guenther

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    Registered User dmftoy1's Avatar
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    Get yourself a trouble light and next time bike is dead, take off the gas tank and see if both red wires on the starter relay will light it. If the relay is bad (as described above) then only one side will have power. When my 74 R90/6 failed it happened when I flipped the high beams on . . . Just pushed it over the edge. A volt meter showed voltage on both sides, but a test light only lit up on one

    Good luck,
    Dave

  8. #8
    Monza Blue 1974 R90/6
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    I think that the symptoms you describe (no lights or power at all) is either loss of connection at the main ground (easy to check), or loss of power from the red wire that starts at the + battery post and winds is way through the starter relay diode board, and ignition switch. This one is harder to troubleshoot, but if you follow that red wire, you can see there are "sections" that lend themselves to methodical troubleshooting. For example, from the battery to the starter relay. From the relay to the diode board. From the diode board to the ignition switch. From the switch to the connector board in the headlight bucket.

    As suggested above, use a test lamp (load) to trace the voltage. A digital meter will pick up 12+ volts and make you think things are ok, but in reality, the corroded connector or few strands still connected in the broken wire won't carry a load, even though they pass enough voltage for the meter to register.

    The other method is to make a long (6') jumper wire connected to the + battery post. Move the other end to the connection points following the red wire in the harness. Once you get dash lights back, you have found the bad section and can narrow it down to a loose connection or broken wire.

    I've had problems with the red wire connections at the starter relay and with the red wire where it connects to the diode board. Finally, less common, but I've read a couple of reports of the red wire being broken in the harness in the area near the steering head where the harness flexes when the handlebars are turned (or the harness was pinched by the lower triple clamp). In all instances, symptoms were as you describe - either no lights at all, or lights initially when the key is turned on, but as soon as a load is applied (starter button, headlight, blinker) everything goes dark.

    Keep us posted. Electrical problems are possibly the most frustrating and a lot of folks will weigh in with their experiences if you keep describing the symptoms and results of your troubleshooting.

    Barron

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    Thanks Baron, that's very helpful! I'll work on it this weekend.

  10. #10
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    Quote Originally Posted by GUENTHER View Post
    That doesn't sound like the starter relay. My guess would be a ground connection problem. Maybe the battery ground connector or
    or.. the connection at the starter.

    Whenever I've experienced those symptoms it was a loose wire at one of those two locations.

  11. #11
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    I followed the red lead from the positive battery terminal to the left Dyna Coil terminal (my trouble light lit there). I have no red lead from there. I checked for juice at the starter relay at both red wire terminals (nothing). I then checked for juice at the red wire terminal on the diode board (I have juice there). I have no juice at the ignition switch (as expected). Is my diode board shot? Any ideas??

    Thanks.

  12. #12
    Registered User dmftoy1's Avatar
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    I'm pretty sure there's a red wire running from the diode board to the inboard center terminal on the starter relay that you say shows no juice. Snobum has really good wiring diagrams on his website and I think diode board troubleshooting. (If not check the airheads archives on micapeak.com)

    Any chance you removed front cover w/o disconnecting battery?

    What are you using for a ground for your test lamp when checking the starter relay? (. I tend to use engine fins)

  13. #13
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    "I'm pretty sure there's a red wire running from the diode board to the inboard center terminal on the starter relay that you say shows no juice"

    No - that would be true for a 73/74 /6 bike but my wire diagram for 75/76 /6 bike shows:

    One large diameter red wire from the battery to terminal 85 on the starter relay
    then one smaller diameter red wire from terminal 30 on the starter relay to terminal 30 in the headlight bucket
    then from terminal 30 in the headlight bucket to the ignition/lighting switch

    One large diameter red wire directly from the pos terminal on the starter motor itself directly to the diode board
    like the /7 models

    so your test at the diode board shows that the very large black wire to the starter motor and the red wire from the starter motor to the diode board are ok

    you say you get no juice at either red wire at the starter relay - this means there is a problem with the red wire run from the battery to terminal 85 on the starter relay - look at this better - try a jumper here and see what happens

    "red lead from the positive battery terminal to the left Dyna Coil terminal" this is not BMW schematic (no red wires on the coils)

  14. #14
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    Quote Originally Posted by dmftoy1 View Post
    I'm pretty sure there's a red wire running from the diode board to the inboard center terminal on the starter relay that you say shows no juice. Snobum has really good wiring diagrams on his website and I think diode board troubleshooting. (If not check the airheads archives on micapeak.com)

    Any chance you removed front cover w/o disconnecting battery?

    What are you using for a ground for your test lamp when checking the starter relay? (. I tend to use engine fins)
    Yes, I removed the front cover (the alternator/diode/points (although I don't have points now with my Dyna Coils) cover) without disconnecting battery. Is that bad?

  15. #15
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    Quote Originally Posted by reachfar View Post
    Yes, I removed the front cover (the alternator/diode/points (although I don't have points now with my Dyna Coils) cover) without disconnecting battery. Is that bad?
    It can be. There are sections of the diode board that are "hot" and if you touch the cover to that section while another part of the cover is touching the engine case, then something can be fried. I used to take the cover off for years...I was clueless. Then it was pointed out how easy it might be to short something, I began to remove the battery ground before working under the timing cover.
    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!

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