Intermittent and low voltage on alternator exciter wire

1989 CADILLAC FLEETWOOD BROUGHAM
106,000 MILES • 5.0L • V8 • 2WD • AUTOMATIC
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TROY D
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When the temperature outside is in the low 30's I get 0 volts on the exciter wire with it plugged into the alternator with the key on and with the engine running, this of course causes the alternator to not charge. After the temperature warms up to 50 degrees F, I get 2.1 volts on the exciter wire plugged in and battery voltage on the exciter wire with engine running. The no charge light on the dash comes on with key in on position, and goes off when engine is started with exciter wire having 2.1 v.
I don't know if the dash light goes off or not when there is 0 voltage on exciter wire. will check that next time it doesn't charge. Am I supposed to have battery voltage at the exciter wire, 2.1 volts? seems odd. Thanks.
Jan 20, 2021 at 4:19 AM
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KASEKENNY
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Just to be clear, you have 12 volts on the B+ wire shown in the diagram below? Then you have 2.1 volts on the wire going to the L terminal? If that is the case then you most likely have high resistance in the circuit or the voltage regulator has failed internally.

I would disconnect the fuse and the connector on the generator. Then measure resistance on from the fuse terminal on the generator side of the fuse contacts and the connector at the generator. You should not have much resistance. I would expect it to be around 1 -2 ohms.

If this is okay, then I would replace the generator. Let me know what questions you have. Thanks

https://www.2carpros.com/articles/how-to-check-a-car-alternator
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Jan 21, 2021 at 3:36 PM