Chevrolet - Suburban :: 1986 - Alternator Intermittent Overvoltage

86 Diesel Suburban

Lately the voltage gauge sometimes starts to wander upwards toward the 18v red zone. When it does it wanders up and down a lot, but mostly well above the center 13v. Sometimes I left of the throttle a little and it goes back and stays at normal, and sometimes it doesn't and bounces up again as soon as I press the throttle. Doesn't happen on every drive, just sometimes.

I thought it unlikely this was a bad voltage regulator, since it's a pretty new alternator, 2 years tops. So I thought I'd see what other possible causes could be. (Why do they make the voltage regulator internal to the alternator now anyways, since they seem to go bad more often than the core? Can I just add an external regulator like the older trucks had?)

I've heard over-voltage can ironically sometimes be caused by a short, since the regulator dumps more voltage into the field coils because the main coils are showing they aren't delivering enough current (because it's all being soaked up by the short), resulting in an overall over voltage as the alternator tries to compensate for the voltage drop of the short.

How do I tell if this is the cause of the overvoltage or if it's something else instead? If it is a short, where could it be? I'm assuming it must be somewhere weird, because almost any normal place I could think of for a short would simply blow a fuse. Or is there such a thing as a partial short-- high enough resistance to not blow a fuse, but low enough to drive the alternator nuts?

Could it be related to overheating? I initially thought that was a possibility, because it began around the time the weather turned warm here, but on any given day it seems pretty random and not tied strongly to excessive speed or going up hills.