OK, did a search and came up with a lot of good things to check, but,,,,
The 91, separate regulator. Started out of the blue after I stopped at a traffic light, the vibrating volt gage trick. Reved up the engine a little, no good. Drove on home and watched it. It was intermittent, sometime it did sometimes it didn't. At home I noticed the lights just barely flickering, but would stop if you reved it up.
Troubleshooting:
Hooked up an analog gage and started poking around a number of different places. The gage vibrated most anywhere I hooked it up, 15v round about. I started checking the wiring. Grounds were still good. I pulled them all and added a few when I replaced the battery. Pulled apart the bulk head connector. No indication of any problem there. Sense I had a spare, I threw a different regulator on there and, OK this is different. The volt meter (on the truck) read high and was still vibrating. Pocked around at the same places with the analog and got a rock solid 15v every were. The only thing I noticed that was weird was on the ground side of the field off the alternator, the side that goes through the regulator. It was making my analog vibrate.
Questions;
Why only on the field ground now? Diode?
Why is my volt meter reading higher with a different regulator, yet the voltage is normal everywhere else with the test meter?

 

My '91.5 W250 has been doing the same thing for about 4 yrs. now.....don't know why, but nothing has happened or went wrong. Needle flips all over the place with the headlights on at idle......running, everything is as it should be......mystery!

 

Mine does too. I think the gauge itself is bad. When I check voltage everywhere else it is fine.

 

My gage is steady once the heaters settle down, despite all the electrical gremlins in my system. I'd check for whatever capacitors are in the alternator. You must be getting noise in your sound system right?

 

The external voltage regulators regulate by controlling the ground side of the field circuit that is most probably why you see your voltmeter wiggle and that terminal. I was told years ago by a mechanic friend that those voltage regulators had a bad habit of not grounding correctly. Apparently they like to vibrate loose. You might try polish in the ground for it, also checked the wire between it and the alternator. Just my two cents, Mike.

 

I think your truck has had it. I'll give you an address in Anchorage to drop it off, and I'll take care of it when I come up to visit in the summertime.

 

Ain't even dead yet and the vultures are circling.

Santaclaus, That is always the first one I check due to experience. All the other grounds follow.

ppiggppenn, No noise in the sound system, and with what I listen to you'd hear it.

I'm leaning toward an alternator problem on the field side. Bad brushes and/or slip rings.

 

It controls the ground side but it senses on the hot side.

https://www.dieseltruckresource.com/...highlight=volt