924Board.org

[skrtskrt]

I’ve replaced the alternator and had the alternator tested to make sure it works and it does.

I’ve replaced all the B+ wires that go from the alternator to starter then to battery positive so thats not the problem.

Ground has good connection.

I know it’s somewhere along the excitement wire (D+) that there is the problem (because that is all that is left) and I’ve tried tracking the wiring through diagrams with the Haynes manual I have with no luck.

I’m just trying to figure out where the wire goes because everything I find online is that it goes through the battery light and if that light doesn’t come on then the excitement wire os the problem but my battery light comes on so I’m at a loss.

Any and all help/advice is welcome
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1979 Porsche 924 - The first but probably not the last (Relegated to parts :/ )
1981 Porsche 931 - TURBOOOOOOO

[coup85]

It's also important to check the battery light on the dashboard, as a defective bulb can cause the battery not to charge. I'd start right there.

You can find the cable on page 258 of the Haynes manual. It doesn't appear on Porsche's parts manual as the "excitement wire" is just a single cable, usually blue, that goes from the alternator to the dashboard.




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924 - 79'

[Raize]

D+ goes to the 3-pin white connector above the brake booster, you can pull that apart and check for 12V there with the with the ignition on.

One wire is starter solenoid, one is the coil booster during starting, and one is the D+ wire. So I think you should see 0V on the solenoid wire, 6-9V or maybe less on the coil booster wire, and 12V on the D+ wire.

[skrtskrt]

Thanks for the replies, I’ve already checked the wires up the 3 way plug so those are all good but after that I loose where the wire goes through the harness but seeing that it goes straight to the bulb helps. I’ll give the battery bulb a look.

Would there still be a problem at the bulb if it comes on when ignition is turned on and goes off once car is started?
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1979 Porsche 924 - The first but probably not the last (Relegated to parts :/ )
1981 Porsche 931 - TURBOOOOOOO

[Raize]

[coup85]

You might try to bypass the connection directly from the alternator to see if the problem is on the wiring.
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924 - 79'

[skrtskrt]

Yes I’ve measured the voltage at the battery and it reads around 12.2v, and bypassing it altogether was going to be my next thing I tried.
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1979 Porsche 924 - The first but probably not the last (Relegated to parts :/ )
1981 Porsche 931 - TURBOOOOOOO

[skrtskrt]

[Fifty50Plus]

My race car wasn't charging even though the red light would go out once I started the engine. Turns out that a ground from the alternator housing to the engine block was loose and causing a problem. See if you have a connection there (beside the alternator housing to the engine block near the oil filter).
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1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[skrtskrt]

[Raize]

[TrayC]

To whoever suggested checking the battery light—THANK YOU!

I had the entire dash out a couple of weeks ago to replace the heater core, and afterward the alternator stopped charging. Since I had also removed the alternator to replace the mounting bushings, I was convinced that was the problem.

I swapped alternators, swapped them back, added an extra ground, cleaned every ground I could find, and was completely fed up with the car. Then I saw the post about the battery light. Sure enough, the light wasn’t coming on at all. I pulled the cluster, removed and reinstalled the bulb, and voilà—it started charging again.

What a ridiculous system. I never would have guessed a loose bulb could keep the alternator from charging!

[924RACR]