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Thread: R90/6 tail light help

  1. #1
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    R90/6 tail light help

    Hi I have a 1975 R90/6 with a tail light not working.The brake light and blinkers all work just not the running tail light. I am not very good with electronics but I looked at the wiring and everything seems to be connected. Any help would be greatly appreciated thanks.
    Trevor

  2. #2
    Benchwrenching PGlaves's Avatar
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    First check the bulb, including make sure the contact on the base end of the bulb isn't corroded/eroded.

    Second, test for 12v at the tail light socket.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

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    I believe I have an after market tail light light set up if that helps diagnose.
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    here is where it connects
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  5. #5
    Administrator 20774's Avatar
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    As Paul said, ensure you have 12v for the taillight. Remove the taillight wire, the top one. Stick a voltmeter probe into the female wire connector and touch the other probe to the frame or good ground. Turn on the ignition. You should have 12v...if not, then you need to figure out why.

    It appears that you have a separate ground path, not just the frame. So you should remove the wire marked ground and this time stick the voltmeter probe into that connector. Switch the voltmeter to measure resistance and touch the other probe to the frame or good ground. You should get zero resistance. If not, then you have a break in the ground return for the light.

    If the brakes work, then ground is probably OK. So, you'll need to figure out if you have 12v for the running lights or not.
    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
    Rally Rat
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    Yup, that is NOT a stock /6 light.
    If the assembly works with everything but the tail light, Paul has the best recommendation.
    Break out the multi-meter and see if you have voltage coming from your lead.
    (My money is on a solderless connector problem)

  7. #7
    Registered User lmo1131's Avatar
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    I am not very good with electronics but I looked at the wiring and everything seems to be connected.
    Which means you may not have a voltmeter/multimeter.

    You can find the same information with one of these... for about $6.


    It will not give you quantitative information, but it will tell you yes, or no. I keep mine in my tool tray.

    To test for presence of voltage; DISCONNECT the tail light wire. Connect the alligator clip to a ground (any point on the frame or the battery (-) terminal) and then touch the tip of the tester to the wire that feeds the tail light terminal. The light inside the tool should come on.

    To test for ground: DISCONNECT the ground wire. Connect the alligator clip to the (+) terminal on the battery, and touch the tip of the tool to the ; the light should come on.
    "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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    Thanks for the info guys. So... I bought a multi meter checked the tail light voltage and got next to nothing. Checked the brake light and got 11.7v and if I checked the resistance properly i got a measurement of 3.3ohms. What do ya think?

  9. #9
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    What resistance do you get when you touch the two meter leads together? If you read something, you need to subtract that from any further resistance readings. If you were measuring one end of the ground wire to something on the frame (ie, another ground) I would have expected quite a bit closer to 0.0 ohms. There will be some resistance just because of the distance. It could be just the way you have it connected and/or some corrosion built up where the ground cable is attached into the harness.
    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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    not getting any reading when I connect the two meter leads and still getting a consistent read of 3.3 ohms when the red lead is connected to the ground wire and the black lead touching the Positive ground on the engine case and about 2.5ohms when it is touching elsewhere on the frame.??

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    I also just realized that my speedometer/tach light does not come one either!!! could this have something to do with the ignition switch?

  12. #12
    Registered User donbmw's Avatar
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    Here is a link to Snowbum site. There is a link for the electrical drawing. Looking at it I think you have a blown fuse.

    http://bmwmotorcycletech.info/

    Don

  13. #13
    Monza Blue 1974 R90/6
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    Check your two fuses located on the connector board. I can't remember if it is the top or bottom one that controls the tail light and instrument lights.

    Barron

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    Thanks to everyone I found the broken fuse (58) it must be the source of the problem. Can anyone recommend a good spot to order an 8 ohm BMW fuse?

  15. #15
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    The fuses are 8A (eight ampere) and the same used in older VW's with the ceramic body and brass fuse element. Most auto parts stores should sell them. Take your blown fuse to an auto parts store and tell them it's for a 1968 VW bug - or ask where the fuses are and find the fuses by the rating (8A) and style.
    Stan

    AH# 13238

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