A little bit of my trucks history. Bought it very cheap, it didn’t run and there is not an ounce of rust anywhere! Got it running but I only have one tank (forward) rear tank is on the ground. Here are the issues. With key in the run position, the fuel gauge is pegged past full. The tank select switch has to be in the rear tank position in order for it to run. I did a test on the white/yellow wire to the select switch by grounding it. The fuel level dropped to an 1/8 tank which is correct.

I have to admit, I’m stumped. Any help would be greatly appreciated.

 

Your vehicle has from the info I have , the system 4 fuel supply set up.
There is a high pressure fuel pump in each tank.
You can jump the fuel pump relay (under the hood) to send power to the fuel pump assy that has been selected by the tank selector sw.
When you first turn the key on , the selected pump should run for 2-3 seconds. The PCM provides the ground for the fuel pump relay causing it to close and send power from the fuel pump relay through the inertia sw on down to the selected fuel pump assy. If no RPM signal is received , the PCM opens the fuel pump relay and no fuel is sent to the engine.
When the wire to the fuel gauge from the fuel pump module is grounded it should cause the fuel gauge to go to full.
Not quite sure , tank on the ground, but if there is no ground path gauge may not work correctly. Let us know how you make out with this , thanks.

You might want to get a fuel pressure gauge connect it to the fuel rail and see what you have. Fuel pressure running should be 30-45 psi.

 

The truck runs on the one tank, and I'm trying to get the fuel gauge working cause it's pegged past full. What I did find however, was the tank select switch will only allow the truck to run when the switch is switched to the rear tank. I bypassed the tank select valve so it's all good. I'm just needing to get the gauge working. I'm stumped on having to switch it to the rear tank just to run and when I switch it to the front tank, the engine dies. Would you happen to know the voltage draw is for the fuel gauge or the cluster? When I was testing the tank switch, I found that the yellow/white wire fo rhte gauge barely lit my test light. Thanks for you help by the way!

 

The truck runs on the one tank, and I'm trying to get the fuel gauge working cause it's pegged past full. What I did find however, was the tank select switch will only allow the truck to run when the switch is switched to the rear tank. I bypassed the tank select valve so it's all good. I'm just needing to get the gauge working. I'm stumped on having to switch it to the rear tank just to run and when I switch it to the front tank, the engine dies. Would you happen to know the voltage draw is for the fuel gauge or the cluster? When I was testing the tank switch, I found that the yellow/white wire fo rhte gauge barely lit my test light. Thanks for you help by the way!

 

This is what I would try.
Unplug the plug from the tank you want to pull from.. With the key in, what does the fuel gauge show? It should show empty.
If it shows full the wire from the plug that goes up to the gauge is grounded someplace.. Do you at least get this result?
Keep in mind the tank switching switch also switches the wire signaling the fuel gauge also.
Maybe you should unplug both plugs at both tanks and see what you get.
With the key on.the gauge should read empty with both plugs unplugged, does it?

 

No matter what I do, the guage reads past full. Even with the rear tank sitting on the ground and not hooked up to anything, the guage pegs past full. One thing I did stumble on was checking the fuel tank switch with a test light and one the yellow/white wire (the fuel gauge) and the fuel gauge falls to about an 1/8 of a tank and I verified that is accurate. I also used a volt meter at the switch and was surprised to see the voltage on most terminals was about 1/2 a volt.

 

If you unplug and remove all wires from the tank switching switch, with the key on what does your fuel gauge read?

 

It’s pegged past full.

 

That tells us the wire from the switch to the gauge is grounded someplace ,,,,OR,,,The gauge is damaged.

What I would try, open the dash carefully, so you can see the back of the fuel gauge. WITH THE KEY ON, One wire to the gauge provides the source of power, the other wire brings the signal to the gauge. If you unplug the signal wire, the gauge should drop down to empty. If it does that wire is grounded somewhere. If there is no change , the gauge is faulty.

 

I'll check that out thanks. Did I mention that I grounded with yellow/wht wire (for the fuel guage) with the aid of a test light and the fuel guage went from past full to 1/8 of a tank which is about right. That and the test light barely had a glow to it. Volt meter read .5 volts