Ok I have a 02 Gmc with the p0380 code, also low voltage, sometimes the voltage comes up like it should. Sometimes it doesn't, it might go a few days of to and from work driving with low voltage then it comes up. I know that the p0380 code is glowplug related but what would cause the voltage to be low. Thanks.

The truck runs fine, like it always has. Starts fine to. Could it be the alternator?

I had the same Code on my '01 and ended up being the Glow Plug controller and the alternator. Not sure if that was your issue but the symptoms were the same. Just a thought.

You probably have a phase out in your alternator there are three phases.

Should I replace the controller first?

Should I replace the controller first?

What ever the cheapest and easiest do first.

So I don't think its the alternator. We have had some warm weather here this week. Voltage is up. Its like there is somthing telling the alternator not to turn on.

As long as the glow plugs are on the alternator want start charging. Alternator starts charging once glow plugs turn off

So I don't think its the alternator. We have had some warm weather here this week. Voltage is up. Its like there is somthing telling the alternator not to turn on.
Could it be loose/dirty wiring to the alternator? Also, if the intake heater or glow plugs are stuck on, that can keep the voltage down indefinitely. I know my voltage won't come up until the intake heater and glow plugs are all off.

I doubt the glow plugs are sticking on, since they would burn out pretty quick. Try checking how long your intake heater stays on. If it is sticking on, it won't hurt anything (that I know of), but it will suck up a ton of power.

From what I can tell the code P0380 covers the glow plugs and intake heater, so it could be either one tripping it. Good luck!

I just got done fixing this problem today its the alt go havce it tested it'lll fail

Replaced the alternator, and everything is fine.