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MK4 TDI alternator circuit problems-

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  • MK4 TDI alternator circuit problems-

    The alternator went out in my TDI over thanksgiving weekend, but when it happened I did not get a battery warning light. I removed the alternator and took it apart. Everything looks fine electrically. Nothing is burned and the brushes still make contact with the slip rings, but the rear bearing is shot so it's getting replaced anyways before it fails completely. When I turn the key to the on position, all the warning lights but the battery light light up. I looked at the wiring diagram for the circuit and that light is wired in series with the circuit that powers the armature. I tried providing a ground for that wire with a test light, but neither terminal had power with the key on or when the car was running. The circuit in question has a blue wire, but both wires going to that connection are black, so I'm unsure which wire comes from the instrument cluster and which one comes from engine control module.



    Before I tear into the dash to get at the cluster and look at that light, is there any other thing I should look at? I really need to get this back on the road and it looks like simply throwing the new alternator in it won't help since the armature isn't going to get any power.
    -Noah
    '95 Wolfsburg Cabrio, '90 Wolfsburg Jetta.
    Apprentice Shimologist.
    "Everybody should drive a Mini; everyone should own a Mini at some point or you are incomplete as a human being." -James May

  • #2
    Noah, the warning light should turn on if you ground that L wire. The wires in that harness have a sheath on them so you would need to carefully peel the sheath back to see the wire color. Should be the wire closest to the front of the car. If you don't have any voltage at that terminal there may be a break in the circuit somewhere. That wire goes through the 4 pin connector that sits next to and parallel to the starter. You could try checking it there.
    Joe
    91 Jetta 16v
    78 Transporter
    www.germanautoltd.com
    I have a fender roller. Contact me if you'd like to rent it.

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    • #3
      I grounded the L wire and got nothing. I'll check that connection next time I'm looking at it and hopefully that's where the issue is. Fixing that will be a lot easier than getting into the instrument cluster.
      -Noah
      '95 Wolfsburg Cabrio, '90 Wolfsburg Jetta.
      Apprentice Shimologist.
      "Everybody should drive a Mini; everyone should own a Mini at some point or you are incomplete as a human being." -James May

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