924Board.org

[coup85]

I deleted my previous thread to simplify things — too much backstory that wasn’t helping . Here’s the actual issue:

I’ve rewired my 1979 924 following the factory diagram. The previous owner had a very non-standard setup: he had removed the wire going to pin 16 of the starter entirely and ran a direct wire from the battery to the alternator. The car ran and charged — but obviously, something was off.

Now, with the wiring restored:

-If pin 16 is connected to the ignition coil (black/yellow wire), the engine starts, but dies immediately when the key returns from START to ON.

-If pin 16 is disconnected, the engine starts and runs normally — which I can’t understand.

I have no idea what’s going on.

Thanks in advance — open to any tips or suggestions!


Wiring diagram: https://i.imgur.com/YqOcH9y.png
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924 - 79'

[Raize]

I had to leave that wire disconnected, it seemed to be shorting to ground.

Could be shorting out on an exhaust heatshield for example.

Never had a cold start problem. Probably not the best to have it shorting out while cranking but doesn't seem to have done any harm over 15 years of being like that.




Considering you are in Madrid I think you can very safely disconnect and ignore this cold-start feature.

[coup85]

I actually do have cold start problems, not now as temperatures are quite high, but it gets cold down here (sub-zero even) during the winter.

In any case, I'd like to know what is going on. The wiring between the starter and the 3-pin connector in my case is fine, as I just re-made it.
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924 - 79'

[Raize]

You can check the resistance to ground at the coil end of the wire with it disconnected, it should be infinity.

You could also check the continuity between the 3-pin connector and the end of the wire that goes onto the coil just to make sure it's actually the same wire since your car seems to have had some "custom wiring" in its past.

You can check the coil voltage with ignition on, with and without that wire. It should be 9V without the wire and 12V with the wire (in reality both will be less due to voltage drop).

I assume your car has the transistorised ignition unit, not points?

[coup85]

Thanks a lot for the detailed reply — really helpful! I’ll go ahead and test all the things you suggested. I’ll report back with the results.

As for the transistor ignition unit… it’s driving me a bit crazy. My car is a ‘79, so according to the diagrams, it should have it, but I can’t find it anywhere on the car, not even in the parts manual.

I’m starting to wonder if some units just didn’t come with it, or if it’s located somewhere completely unexpected.
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924 - 79'

[Raize]

[coup85]

I'll post some pics tomorrow because I'm a little embarrassed to admit I can't tell the difference between the VW and the Porsche one:)

But I'm inclined to say Porsche's.
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924 - 79'

[coup85]

Here it is:




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924 - 79'

[Raize]

I think what that is, is an aftermarket electronically controlled points distributor.

It’s certainly not anything that came standard on a 924.

There will not be an ignition box anywhere, the dwell is controlled inside the distributor by some electronics. Probably someone replaced one of the early 924 points distributors with this thing.

Doesn’t really effect the coil wiring though.

[coup85]

That's really helpful, thanks as always. I didn't have the time to run more tests but this actually explains why I couldn't find the transistor
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924 - 79'