From what you tell me, I best get my bottle of Patience out and start looking for a short. It was working for a while(several days)and then just started pegging out as soon as the ignition was switched on. Even with it unhooked at the sender, it stays pegged out. If I don't find the "obvious" short in short order, I'll replace the wire from the sender to the gauge and see what happens. Got to keep in mind it's an old truck. It's the original gauge and sending unit.
