Voltage Regulator Multimeter Test

[justincollier]

Is this a accurate test to determine if the VR has failed? My alternator is charging, battery is not. A couple of videos I've seen indicate you should have 1.75 on 20K ohm setting. I've got 360K using my Fluke meter. The VR is an old (probably 30 years old) Borg-Warner, so it may have failed.

[justincollier]

To confirm, with the ignition in the ON position, I should have something close to battery voltage between the blue/white wire on the VR plug and ground?
With the harness disconnected from the bulkhead, I have battery voltage at the bullhead. with the harness connected, I have about a 2.5 volt drop at the VR connector. I have traced that drop all the way back to the bulkhead harness plug. The wire as obviously gotten VERY hot at some point, and will need to be replaced. The sheathing has blistered at the bulkhead harness plug.

I just wanted to verify the soundness of my troubleshooting before wire replacement.

[Bullitt-]

  You defiantly have an issue with the blue wire which could cause an otherwise properly operating voltage regulator to overcharge but that doesn't sound like the problem you are trying to solve. Voltage should be same as battery

The only source I find for checking the VR ohms is this video which is the same 1.75 on 20K ohm as your first post  https://www.youtube.com/watch?v=F04fkFc9SNI&t=3s

[justincollier]

So the issue is that the electrical is undercharging, not overcharging. I can only get 13.5v at start-up on high idle, but as soon as it goes to curb idle (850-900 rpm) it starts dropping, and when you add lights and the factory AM radio, it really drops. I measured voltage at the alternator and have 14.4v. That tells me the alternator is good. I thought I was having a VR issue based on the Ohm reading, possibly too much resistance. I replaced it with an adjustable VR. Same result. That's when I measured the blue/white wire on the connector and only had 10 volts at the VR, and noticed the blistered blue/white wire at the bulkhead harness.

Has anyone removed only that wire from harness, abandoned the damaged wire in place, ran a new wire from the bulkhead harness and spliced in at the VR? Seems like the easiest solution.

[Bullitt-]

 I would think you could jump from any key on power source to test.
The Blue wire is also common to the ignition system so power is down there I would imagine.

Could the voltage drop be between the alternator & the battery? Power travels through the bulkhead connector to the Ammeter first & is often compromised at the connector. My '73 even has an additional connector on the engine harness that failed not found on earlier models. 

Way back in the day I was having low output when under load, lights dimming especially at idle was the noticeable symptom. I went on a program to burnish every connection & ground having to do with the charging system, the issue was resolved...today I would add dielectric grease to prevent future failures

[justincollier]

I wanted to conclude this thread for those that my find it while troubleshooting. Over the past week I continued to clean connections and apply a nice coat of dielectric, without improvement. Believing that the alternator was original, I took a chance on a $22 closeout from Rockauto. That was the fix. I have a consistent 14.6-14.9v depending on accessories running. 

[pschlosser]

I've seen this more than once, as well.  There is more than one rectifier within the alternator, I forget how many.   In time, these can fail one by one.  As they fail, the alternator is able to still make voltage (13vdc+) but less, and less current.   Testing may show it works, but its ability to charge diminishes.