[G&R's57GMC]

Wire gauge could be a factor also.

If your running a 60 amp alternator 10 gauge would be about as small as you would want to go.

But if your going to run an 85 amp 8 gauge would be the ticket.

Now running electric fans, heater and lights you would want at least 105 amp alternator with 4 gauge wire.

Add fuel injection and A/C 145 amp alternator with at the very least 4 gauge wire.

If you have a large enough alternator and wire on your truck to handle your alternators output and the system is sized for your load then its possible a phase of the alternator is out.

Have the alternator tested off the vehicle.

Looking at your alternator its most likely only good for 60 Amps max.