troubleshooting nonworking fuel gauge
 

A guy I work with has a '79 chevy 4x4 1/2 ton with dual tanks, the gas gauge is pegged past full when the truck is running, shut truck off and it drops down to full until you start it again and it goes past full. I belive he said it does it on both tanks. Any ideas where to start looking? I would have done a search cause I'm sure this has been asked before, but am in a hurry. Thanks in advance.

i have heard that a ground could cause this

definitely a ground issue by the tank. have you had the tank out recently?

I don't think he has had it out, whats a good way to check this? Could I clean a spot off on the frame and tank down to bare metal and run a wire between the two spots and see if the gauge works?

on mine mine goes to half way when i fill up ad somethimes its goes past half way and hopes around and when i get about a 1/4 tank it hopes arounds alot

So is the conclusion to make sure the ground is hooked up and both ends cleaned and it should work?

it should

I tried running a wire from a bare metal spot under the tank to a bare metal spot on the frame and it didn't work. Maybe its gotta be grounded to the top of the sending unit and not just the tank. Or there is some other problem

It does need to be grounded at the sending unit on each tank. There is a spade to hook the ground wire to on the sending unit. Run a wire from the spade to the frame. The wire from the sending unit to the gauge should be pink if you have to go looking for a failed connection or short. It is very possible that there is a problem with the dual tank switch as it also switches the gauge from one tank to the other. I know that Haynes manuals has their shortcomings but I have found the wiring diagrams in the back of the book to be invaluable when fixing the screwed up wiring on my '82 C30.

Odd, my fuel gauge does the same thing.

However, one thing I have been thinking is that it may have something to do with a fault in the dual tank switch. Mine worked flawlessly by hooking it up to a bare wire I had running down my frame rail, until I pulled out the dual tank wiring, at which point I suddenly have the pegged all the time syndrome.

Maybe try changing the switch out and see if that does anything for you? I'm just gonna run a new wire from my gauge to the back in my truck.