[Doug Schmidt]

An update (in case anyone was following this):

Finally figured it out: loose alternator field wire – DUH. The field wire was poorly crimped in a butt splice near the alternator, and a little dab of oil had worked its way into the splice – as the engine heated up, the butt splice was loosening enough (or the resistance was rising enough) to basically kill the field connection and force the alternator offline (hence why the alternator usually worked at first then died after the engine warmed up). Luckily I don’t think replacement of the battery or voltage regulator was wasted – they were both bad and trending worse anyway, but they simply weren’t the problem.

Replaced all of the wires between the alternator and the voltage regulator and everything works great now. I think this was a pretty rare circumstance, but hopefully this helps anyone who runs into a similar issue.

[Doug Schmidt]

Sorry, forgot to mention – it’s a ’79 AA-5B. Update on the issue:

We replaced the voltage regulator with a Zeftronics R15v00, replaced the battery, and bypassed the separate overvoltage relay (the Zeftronics has its own internal overvoltage protection). We’re still seeing the ammeter go negative occasionally, although with much lower frequency (and it usually seems to self-recover if the load is reduced). Still no popped circuit breakers.

Now we’re thinking that the alternator brushes (or something else in the alternator) are bad…

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