Fuel Gauge Way Past Full

[1981 C20]

I'm a newbie to this forum with average to decent mechanical skills, but my understanding of electrical issues is subpar - so I could use some assistance. My '81 C20 with dual fuel tanks is having fuel gauge issues. When I bought the truck a month ago, only the right fuel tank was usable and the gauge measured it's contents reliably. After doing some research on this forum, I decided to replace the fuel tank selector valve in an attempt to use both tanks. Now, after paying a mechanic to have the valve replaced and the left tank full and the right a couple gallons shy of empty, the gauge reads significantly past full all the time, no matter which tank is selected. The mechanic said he could source the issue and fix it, but it may take him several hours. I'm afraid that his bill, when it's all sorted out, will tap me dry if I have him work on it any further!

Could the switch in the cab be the source of my problem? Or something else?

Any help would be appreciated. Thanks.

[Dead Parrot]

IIRC, a reading way past full, guessing yours is at about 3 o'clock, is a missing or bad ground from the tank in question to the frame.

Since one of yours was working properly before the repair attempt, thinking that somehow you lost ground on both tanks. On mine, there is a wire that runs from the tank sender to the frame. Not easy to get to and both ends are exposed to both rain and road splash. After 30+ years of corrosion, probably worth checking out.

Another possibility is the mechanic either bought the wrong switch or connected it wrong.

[1981 C20]

Thanks! Yes, you are correct - the gauge is constantly at the 3 o'clock position. I'll start with the ground connection. What is the best way to access the wire? Would be easier to unbolt and remove the bed to get at the wire than to remove both tanks since both are now full? I suppose the other option is to keep an eye on the odometer until both tanks are near empty and then drop the tanks ...

[Jeepwm69]

My 83 did that and the pink wire from the tank was actually cut in half. I spliced it back together and the gauge worked fine after that.

No idea how it got cut, but it was cleanly cut.

[Dead Parrot]

If you need to replace the ground wires at the tank, removing the bed is probably easier if you have several self portable bed lifting units you can bribe with sufficient quantities of recreational beverages. Plan on replacing the 8 bed bolts and start treating with penetrating oil several days before the planned removal. (That 30+ years of corrosion again.) Also plan on replacing all of the rubber gas lines while you have easy access. Don't forget to unhook the wiring harness(es) at the back of the bed. I can speak from experience that dropping and installing an empty tank by yourself is a PITA.

Before doing the bed removal, I would double check the wiring at the switch since that is the last thing changed and one side was working before the mechanic touched it. Easier to do if you pull the ash tray first.

[slotard]

I'll have to try adding a ground to my tank, right now my gas gauge is at almost 3 o clock and I've never seen it move.

[LBJ]

Similar problem on my 77 Bonanza. Gauge goes way to the right when full and by the time it gets to 3/4 tank indicated, it's down almost 10 gallons. I'm used to it after 39 years, but it's a nuisance.

[dieseldawg142]

Originally Posted by Jeepwm69

My 83 did that and the pink wire from the tank was actually cut in half. I spliced it back together and the gauge worked fine after that.

No idea how it got cut, but it was cleanly cut.

this^^
if it hangs at 3, the sender wire is most likely damaged. start at the switching unit & trace the pink. if you want to test it quick, cut the pink at the switcher, hook up a new wire, then tap back into the pink under the hood at the plug. it is faster than searching the entire wire, which is 30+ yrs old anyways....
mine was cut by a clamp.

[hatzie]

Originally Posted by 1981 C20

I'm a newbie to this forum with average to decent mechanical skills, but my understanding of electrical issues is subpar - so I could use some assistance. My '81 C20 with dual fuel tanks is having fuel gauge issues. When I bought the truck a month ago, only the right fuel tank was usable and the gauge measured it's contents reliably. After doing some research on this forum, I decided to replace the fuel tank selector valve in an attempt to use both tanks. Now, after paying a mechanic to have the valve replaced and the left tank full and the right a couple gallons shy of empty, the gauge reads significantly past full all the time, no matter which tank is selected. The mechanic said he could source the issue and fix it, but it may take him several hours. I'm afraid that his bill, when it's all sorted out, will tap me dry if I have him work on it any further!

Could the switch in the cab be the source of my problem? Or something else?

Any help would be appreciated. Thanks.

If it worked before he changed the valve then he fouled it up. He didn't finish the job. I wouldn't bring another vehicle to him and I wouldn't reward his poor performance by paying him to fix this.

Since it's not working on either tank I doubt it's the grounds. You can test this by measuring the resistance from the tank to the frame. If it's close to 0 ohms the ground is not the problem.

First place I'd look is the wire from terminal B of the valve to the bulkhead plug under the brake booster. In 1981 this will be a Pink wire with no stripe. There's a molded rubber disconnect in the wire not far from the Weatherpack connector on the valve. Verify that it is not disconnected... If your parts changer was too ham handed he could've disconnected it.

Another possibility is the sender switch in the valve is defective. Ohm it out with a meter.