Last summer my wife and I scooped up an 86 LeBaron off an older lady for free. She had put I think 3 alternators in it in a few years and finally got fed up with it and replaced it with a mid 2010's TDI Passat. I noticed it had a mopar voltage regulator piggy back installed on the alternator when I scooped it up but it wasn't charging. I just drove it onto my tow dolly, and then drove it home. Ordered a new Chrysler style alternator last fall, and then finally got around to replacing it about 2 weeks ago. I removed the external voltage regulator, got the reman alternator in, fired it up, and confirmed it was charging around 13.7 volts just fine. Took it for a 20 minute trip, and halfway through it popped a power loss light. I did a small launch with it (and discovered it had a torn vacuum line because it hit boost cut) and the voltage surged to the top of the gauge on the dash. Pulled it back in the barn and cut it off.

I then disconnected the wires going to the ECU, and tried hooking up the external voltage regulator by itself, and it wouldn't charge at all. Fed it a 12V key on source to the field + wire, and it would instantly shoot up to 17V. I tried feeding it a source directly from the battery through a relay and get the same results. Bought a new external voltage regulator and had the same results. I did some research and thought it might be a bad ground so I added a large ground wire directly from the alternator to the voltage regulator case with no change in the results. Not sure what to do right now, Either thinking of putting on the old alternator to see if I can replicate this issue with it, or if it charges and I can return my new alternator to rockauto/ACDelco.

Here's the only photo I have for a reference of the alternator I'm dealing with, it is the alternator I removed.

https://i.imgur.com/GRY263I_d.webp?m...idelity=medium