First I apologize if this seems a dumb question, but being over in the UK I don't have another truck handy to compare it to.

Does anyone know why my gas gauge should be stuck on "full" all the time? I thought it was broken, but now I've seen another on e-bay which sits the same way, and the seller (who I've dealt with before) says that they worked fine when he was running the truck about 4 years ago. Obviously I don't want to bid on it if this one's broken too.

Only thing I can think of is that the sender is faulty, but is there any surefire check on this one? Should add that it does twitch a little when the ignition is switched on, so there must be voltage going to it.

Hello, and welcome. I think your grounding wire to the gas tank is severed. Test my theory by running a "jumper" wire (any bare piece of wire, to jump from a shiney part of the tank, to a shiney part of the cab).

Well even the wire running to the sender could be at fault as well.They go to full when the wires are unhooked.I would check hte ground first as it's easiest,and then trace the sender wire back to the gauge,and replace it if necessary.

They go to full when the wires are unhooked..

Thanks Brainchild; that was the bit that was puzzling me, most gauges I've seen go to empty when there's no power to them.

I had the same problem on my '62. There should be a black wire running from the sending unit over to the drivers side door, and down under the metal floor plate (where it says "GMC"). Then it comes up right next to the high/low beam switch straight up to the fuse block.

It should plug into teh one that says "gas tank" or something like it, if not, make sure it is plugged into one that only has power with the key in the "on" position. Definately make sure there IS a good connection at the terminal on the sending unit on the gas tank, as well as on the fuse block.

If you have an ohm meter, you can test the gas tank sending unit to see if there is a reading, if not, it probably is the sending unit. Mine turned out to be a bad sending unit, with the wire unplugged from the fuse block.
David