Well I have been having electrical issues with my 37 Plymouth with 96 model 360 since I bought it in May.

Had starting issues early on replaced the battery, starter, battery cables and added grounds then a second new starter. Unfortunately, due to my own negligence I didn't get one of the wires to the starter back into the loom clip and it got loose and wrapped around the end of the steering column causing a short and all the wires under the hood to burn up. Luckily I always have a fire extinguisher in the car so it was confined to the wiring.

Anyway, I rewired the entire car with a new EZ harness and things were looking good once more. Then the electron demons reappeared. Driving down the road on my way home and noticed voltmeter not registering any charge. Made it to the driveway and checked belt good and tight, all wires with good connection etc. Next morning started it with a jump and voltmeter reading 12 volts. So off I go only to look down a few miles later to no reading on the voltmeter.

Pulled the alternator and had it checked everything shows good. So I swapped out the voltage regulator. Things seemed to be good after I jumped it again and made it home.

Started the car yesterday again with a jump and started the stopwatch on my phone. A couple minutes after I started it the needle on the voltmeter slowly started falling from about 12 volts and at three minutes and 14 second after starting it hit zero.

At this point I am clueless as to where to turn next. I am not the brightest bulb in the pack when it comes to electrical and was actually surprised when everything but the brake lights worked after the rewire.

Is there an internal voltage regulator in a Denso alternator or something else internally that could be heating up and shutting it down? Any suggestions would be appreciated I'm at a loss.

Your alternator is dead. Your voltage regulator can regulate voltage when it is being created, but if it is not being created, there is nothing to regulate.

Charge battery, start car, you should be at 14.4-14.7 volts while charging.

12 volts running means no VR or alternator charge current going to battery.

A voltmeter reading of 12 volts with the engine running means it's not charging, with the engine running the voltmeter should read 14+ volts.

What combo of Denso alternator and regulator are you using?

I ain't up on a Denso but they did test it & they said OK. I would get the wiring CONFIRMED correct (including ground paths) & see where you are at. With that (wiring) confirmed then we can (re)check the alt/reg if still no go.

Always consider just because a part is new doesn't mean it is good, smack the alternator when you are testing it!

double check your wiring, if the reference voltage wire isn't hooked up, it won't charge. I'm guessing you have the very common denso on the mid 80's toyota trucks...