924Board.org

[Tank77]

I was hoping someone smarter than I could help me try and diagnose the nightmare of an issue I'm having. I have an 81 924. It's been in my family for years but I just recently acquired it. A couple months ago the alternator just quit charging. Tried replacing the regulator with no luck so I bought a new upgraded 120amp, installed it and still no luck. Noticed the charging light wasn't coming on when I turned the key on. I noticed some cracking in the trigger wire right before it goes into the heat shield so I repaired what I could and still no luck. I wired the trigger direct to battery and the charging light came on but when I turned the key on the light turned off instead of staying on. With the trigger wire still hotwired I fired it up and it seemed like the alt was somewhat working but only at 11.8. I disconnected the hotwire and it held voltage for about a minute then started tanking. I tried hooking it back up and no luck. Charging light came back on just like before but still nothing. I'm so damn confused as to what the issue could be. New battery, new alternator, a couple new wires but still no luck. I'm about to say #### it and rig up a GM single wire alt unless someone has some ideas.

[Eliza]

The alternator idiot light is connected to the battery on one side and, via the exciter wire, to the alternator on the other side.

Ignition on, engine not started: The idiot light comes on via the battery as the alternator at that point does nothing, so the exciter wire acts as a groud path for the voltage from the battery > light lights up.

IF the exciter wire is intact, once the engine starts and the alternator starts charging, the light "sees" voltage at both connections > light goes out.

The alternator needs to see the voltage via the idiot light to establish a magnetic field and start charging. If it doesn't, it will eventually self-induce at around 2000 rpm.

Now that you've established the lamp itself is good, check the blue wire from the instrument wiring to the three-pole plug in the engine bay. I suppose there's your problem.
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1985 924 NA RoW
1981 931 S2 RoW

[Tank77]

I think I checked that plug already but I will check it again this weekend when its not 45 degrees and raining.

[Goldwolf]

Do you have a ground strap between the alternator and block? There needs to be one. Or there was one originally. Car will not complete the circuit without it.
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1981 Weissach Edition #179