Only at night, or whenever the headlights are on, and I turn on AC/Heat or just vent, the volt gauge will creep towards the left. In D or R it will go all the way to the N where normally it sits on A. If I turn the fan or AC off it jumps right back to A. Did these trucks just come with low amp alts? would it be worth it to upgrade to a 200a-300a aftermarket one? I just gotten used to it. On freeway or driving the gauge and lights are fine but when stopped I just turn fan to low. Again only with headlights on.
Thanks '92 F-250 in Sig

 

Factory alt should be a 90 amp and on a 92 should be a 2G. Look up 3G alt upgrade in the search on this forum. Very good upgrade to a 130 amp without a fire hazard.

 

I've heard things about that, good things. I need to do that. I've just gotten used to the dimming lights at stops. Thanks!

 

3G alternator upgrade was the best modification I ever did to my old '92 F350.

 

don't get used to that.do the 3g swap and you'll kick yourself for not doing it sooner.great low buck upgrade.

 

X2

 

I would check the resistance on both battery cables and confirm all the ground connections are clean before I changed alternators.

 

Although thorough troubleshooting is advocated here the symptoms the OP posted are very characteristic of the factory 2G performance.

Since the same OP did not state they had issues with battery charging, slow/hard starting, etc. I feel confident they are experiencing the symptoms of a poor alternator design.

 

I'm having the same symptoms as the OP,in my 93 F-250 with the 5.8. except I have slow hard starting, most of the time. Only sometimes will the truck not have a slow hard start.
The starter barely cranks, like the battery is dead. but if I keep the key turned it will eventually fire up.
What is the likely cause of my problem?
I have a new alternator. I bought a new one not long ago, but it was apparently bad, so Oreillys warrentied it and tested the new one before they gave it to me, but I am still having the same problem.

 

I would definitely check all the cables. The negative cable gets overlooked and causes a lot of problems. I had the local NAPA make some custom 1 or 0 gauge cable for the Bronco for just a bit more than the 4 gauge replacements they sell off the rack.

 

Point well taken. Never overlook the obvious!

 

First, have your local auto parts store test your battery. If your battery is shot it won't hold a charge. If it tests good, then check all your cables and connections for deterioration or corrosion. All the cables, both negative and positive. Clean and or replace as necessary. See where that gets you.