69 F100 charging problem; fluctuating gauge

[banderson7156]

I'm having an issue with the charging system on my 390ci engine. When I start it the voltage at the battery shows 14v at idle. The gauge in the dash is pretty much pegged all the way to the right toward the "C" side. Once I rev past idle, the dash charging gauge starts to alternate between the center and all the way right about once per second. It continues to do this until I go back to idle. Then it returns to the way it was before. Anyone have any idea what is going on here? I've checked the wiring diagram on this site and it shows four wires connected to each of the regulator pins but mine only has three connected(904, 152, 35 if the color codes are the same; the #4 white wire does not exist in the wiring harness on this truck). (At least I can't find it.) I'd like to know what's up if anyone knows. Any help would be greatly appreciated.

[Nitekruizer]

If you turn on your headlights do they flicker while this is happening? The trucks with ammeters or amp guages are wired differently than trucks with "idiot lights" (this also applies to Ford/Mercury cars of the era). The white/black stripe wire (wire #4) connected to the "S" terminal of the voltage regulator in the diagram is the stator wire. My truck has an amp guage. It does not have this wire. There's no wire connected to the stator terminal of the alternator. My '70 Mercury has an ALT "idiot light" and it does have the white wire connected to the "S" terminal of the VR. The other end of the white wire is connected to the stator terminal of the alternator on that one. So, in other words, if the truck has the ammeter it won't have the white wire.

[banderson7156]

Thanks for the reply.Yeah, they do flicker when the engine is at anything other than idle. No wire is connected to the alternator's stator wire. Just the ground, field and battery terminals are connected. I'm starting to think it might just be a bum alternator or regulator. Any thoughts on that?

[Nitekruizer]

I would check the alternator. I'm thinking it might have bad diodes.