Intermittent Charging Issue

On returning from a club event last weekend the alternator quit charging . I drove the last 5 miles with the battery running the ignition and fuel injection and the voltage dropping. Today I went to diagnose the issue attached a volt meter to the field connection and started the engine. About 5V on the field coil and 13.8 from the alternator. Put the headlights on the field voltage went up to about 8V and the output remained near 13.8V. Conclusion, it’s an intermittent problem but I have no idea if it’s the voltage regulator or the alternator. Any suggestions. It is a factory set up with relatively new alternator and regulator (about 2 years old). I think it’s a solid state regulator but don’t recall for sure.

1970 351C-4V 65 amp Duralast rebuilt alternator, more car detail in the signature below

I’m not an expert but had a similar issue on my '70 when replacing the alternator recently. It sounds like the VR is exciting the alternator to charge as it sits right now so if that wasn’t happening on the drive home, it might be an intermittent fault in the wiring from the VR to the alternator.

With the car running and measuring the voltage at the pos terminal of the battery, try messing around with the wire harness from the alternator back up to the VR to see if the alternator stops charging at any time while you’re jiggling things.

Shouldn’t the output be around 14.2? You said one word that tells me it is probably your alternator…Duralast.
I worked at Autozone for a few months when I was around 18 and we had so many returns on parts especially alternators and starters that I really don’t know how they stayed in business. Granted that was roughly 30 years ago and they should have improved in quality, but I still don’t trust their parts.

Do your due diligence though and swap out the VR for a known good one and see what you get voltage wise. If you don’t have a spare, NAPA sells them pretty cheap.

Just my 2 cents. Keep us posted on your findings.

P.s. the original amp alternator is not going to keep up with all the additional electronics you have installed and it will overtax the alternator so that it dies an early death most likely. Consider upgrading to a 3G style at least 95 amp output. (Dash amp meter will have to be converted to volt meter and a 6 gauge wire will need to be added in addition to your original alt output wire) see the alt upgrade post on this forum:
Specifically the link on pg 2.

Fixed - Focus on the voltage regulator, alternator, alternator wiring harness, connections, grounds, etc. didn’t find the problem. The alternator was not tested, AutoZone’s tester was not working, they just traded it out. Still didn’t charge. Got out the sharp ended volt meter probes, stabbed into the wiring for measurements, and found ~14V at the alternator side of the connector to the main engine to fire wall harness and ~12V on the battery/car side. Further investigation found a break inside the connector on the engine harness side. New heavy-duty connector soldered in. Working well again.