I have a 84 gmc jimmy with a 6.2 diesel and i am having trouble with my glow plug coming on and stay on after driving for about 5 to 10 miniutes also the voltage meter in the cab is bouncing around like i have a short in the system

The glow plug system on the 84 and older trucks is notorious for having issues. There's a controller in the coolant blockoff plate at the rear of the driver's side head. That tends to go bad from what i've heard.

Try un-plugging the connection on the GP relay on the driver's side fender. see if that helps.

I'd just set up a manual relay setup for the glow plugs if i were you.

i have went as far as replacing the glow plug controller and think it might be the alternator but need more advice

The pulsing voltage is caused by a bad regulator. Does it bounce about twice per second at a constant interval? Regulator failure can cause weird GP issues in the 82-84 trucks.

Forgot to add, voltage pulsations caused by the GPs cycling are normal. The voltage drops, headlights dim, blower slows down etc when the GP light comes on.
The GPs should cycle for about 10-30 seconds *after* the initial glow cycle when the engine is started, typically one second on, two seconds off- all varies with temperature.

The switch iamdave0887 is referring to is the GP inhibit switch. It's used to disable the GPs completely when the engine coolant is above 120*F or so. As the engine gets more and more worn out, compression drops and the GPs are required even when it's warm- when they are disabled by that switch, you wind up with a no-start. Bypassing the switch usually remedies this.

Finally, the old 82-84 style GP system *is* notoriously unreliable, I had three genuine GM controllers fail within two years. Everything else was tested and found to be in spec, so I just said the hell with it and wired in a manual button. What's more, unless you modify the system it dosen't work so well with the slower heating AC60G plugs.

rgm