[Mick65]

Originally Posted by tincan1966

Assuming the truck is original( still has external regulator) it's possible the regulator may be getting a bit weak.

With the truck idling, turn on the headlights and the heater fan. The gauge should drop to 12 but stay steady. If it does, then run it like that for a few minutes, then shut the lights and heater off- see if the gauge stays steady. If it still fluctuates, I would guess the regulator is getting worn and not compensating for the load.

This happens a lot in vehicles that are not driven a lot, especially in situations where more alternator output is needed. simply put, these trucks, when original, have very little draw on the charging system, so at idle, there isn't much need for a lot of alternator output. With modern updates, like HEI, stereos, etc, which draw higher amperage demands, the alternator is almost always charging, even at low RPM.

Sorry, I should have been more descriptive in my original post. I do have an internal regulator, HEI, and an electric fan.