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Thread: Electric fault diagnosed; physically where do I look?

  1. #1
    Junior Member
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    Electric fault diagnosed; physically where do I look?

    Hi all-so I have been trying for a while now to trace an electrical fault-the main beam headlights don't light(neither does the gauge panel indicator light). I've traced the location on the schematic, but physically, I'm not sure where to start in the actual car.

    The highbeams bring on all 4 headlamps, so I know it's not the bulb. After doing a bunch of tracing with a voltmeter, here's what I've got:

    depressing the console light switch twice brings +12V to both sides of the fuse(as well as the relay, and the blue/red wire at the turn signal wiring, on the steering column). It does NOT bring +12V at either the actual headlamp wiring measured at the bulb, OR at the gauge panel indicator wiring. So I know that is the location in the schematic at which the fault must lie. The problem is that physically, I'm not sure where to start. Tracing fuse #14 downstream, it looks like the wiring runs through the center console, as best I can tell.

    Does anyone have a suggestion as to where physically I can start tracing? thanks for the help!

  2. #2
    Delorean Guru
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    Make sure the low beam filaments in BOTH headlights are not burned out. The two headlights with 3 metal tabs are the low and high headlghts. The bulbs with only 2 tabs are high beam only.
    David Teitelbaum

  3. #3
    Senior Member Christian Dietrich's Avatar
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    Check your wiring harness above the frame in between the frame and the body. Jack up the front and remove the front wheels. Also the head light ground goes to the frame there also. Look in that area.

    Sent from my SM-N920P using Tapatalk
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  4. #4
    Senior Member
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    Could be a bad fusebox - bad crimp on the fuse connector.
    You checked fuse 14, right ?

  5. #5
    Junior Member
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    yup, checked fuse 14 on a DVM. it's good both sides.

    so i pulled the headlamp covers, and the actual lamps. with a DVM across the ground and "main" harness pin of the car, the voltage goes to 12V when i turn on the mains headlight switch. here's where i'm getting confused....

    i tested both male pins of the 3 pin headlamp, using alligator clips from the lamp to the female harness of the right lamp. both pins cause illumination. i would have to assume that this means that both the main and highbeam bulb inside of that lamp are good. confused....maybe the whole circuit of the panel light and both front main lights requires a "closed circuit" for ANY of the bulbs to function? will investigate further, now that i've got the front end disassembled....

  6. #6
    TGTF (Too Greek to Function) AugustneverEnds's Avatar
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    What headlamp switch do you have, original or DMC replacement? Might want to look there and see if everything is firmly connected.

    Did you check the relay for the low/dip beams? Sometimes the terminals are pushed too deep into the socket and not making contact with the blades on the relay
    Nick A.
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  7. #7
    Senior Member
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    What ?

    I don't get your last sentence

    you do have the schematic, right?

    you know that a bulb only has a low resistance and that they are in parallel ?

    Black is GND
    Blue-Red is low beam
    Blue-White is high beam

  8. #8
    Junior Member
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    bulbs in parallel answers my question about the bulb circuit! sorry, i didn't phrase that very well.

    checked again today, and I misdiagnosed the voltages. the high beam socket pins go to 12V, and illuminate, but the low beams only go to 3.6V, and do not illuminate. i distinctly remember the reading on the downstream side of fuse 14 being ~12V, though. Now I need to figure out what junction lies between the fuse and the bulb that could cause the voltage to drop down to 3.6V. A "brute force" jumpering of 12V to the low beam pin of the actual bulb causes illumination, so the bulbs themselves seem to be fine. hmmm....

    Quote Originally Posted by Elvis View Post
    What ?

    I don't get your last sentence

    you do have the schematic, right?

    you know that a bulb only has a low resistance and that they are in parallel ?

    Black is GND
    Blue-Red is low beam
    Blue-White is high beam

  9. #9
    Junior Member
    Join Date:  Mar 2016

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    i don't know which switch. stock, i believe. i did check the relays for both high and low beams-voltages look right, but from my read of the schematics, neither one matters, as both the switch and the relay lie upstream of the fuse, and i'm getting 12V on the downstream side of fuse 14, so if my reading of the schems is correct, there CANT be an issue in either the relay or the physical console switch...

    Quote Originally Posted by AugustneverEnds View Post
    What headlamp switch do you have, original or DMC replacement? Might want to look there and see if everything is firmly connected.

    Did you check the relay for the low/dip beams? Sometimes the terminals are pushed too deep into the socket and not making contact with the blades on the relay

  10. #10
    Senior Member
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    I already suggested to check the fuse connections, many fuseboxes go bad.

    and

    Don't trust your readings, please double check them.

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