The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

**hatzie** · 06-29-2022, 03:28 PM

A & C come off the tank senders to the switched contacts of the SPDT switch inside the valve.

B is the common terminal on the gauge switch inside the valve and the B wire terminates to a molded rubber disconnect on the RH frame rail. The wire runs from there up to the bulkhead plug under the brake booster.

Honestly it's usually not difficult to diagnose and fix the fuel gauge in the dash.

The tank ground wire is one of the more common failure points in the 1973-1986 fuel gauge circuit. It either gets left off by someone replacing the tank or the wire rots off the ring terminal at the frame or the .250" disconnect on the sender locking ring rots off the wire.

If the stock fuel gauge is nailed at 3:00 you likely have a bad tank ground wire.
Easy to test and not awful to correct. Take a set of jumper cables and attach one end to the frame and the other to the pinch weld on the selected fuel tank. If the fuel gauge needle suddenly comes to life with the KOEO (key on engine off) you'll need to fix the tank ground wire between the frame and the sender locking ring. Many moons ago I attached an alligator clip jumper wire from the frame to the pinch weld on the tank on my 78 GMC to get me by for a couple weeks til I had time to fix the problem properly.

The other thing that'll nail the fuel gauge at the 3:00 position is an unplugged sender wire or damage to the wires in the sender circuit. There's only three of them and they're fairly accessible.
If the valve is unplugged you've likely found your problem.
I've commonly seen the sender wire get chowdered up where it runs along the frame rail. It's either unplugged from the harness wire that runs up to the bulkhead plug under the brake booster or something happened to it between the valve terminal B and the molded rubber round pin disconnect on the frame rail or right after the disconnect. None of that is difficult to repair.

No amount of brandy new fuel gauges will read anything without a good tank ground and good sender wires from the tanks all the way to the gauge.

06-29-2022, 03:28 PM	#8
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,916	Re: 85 dual tank gage question A & C come off the tank senders to the switched contacts of the SPDT switch inside the valve. B is the common terminal on the gauge switch inside the valve and the B wire terminates to a molded rubber disconnect on the RH frame rail. The wire runs from there up to the bulkhead plug under the brake booster. Honestly it's usually not difficult to diagnose and fix the fuel gauge in the dash. The tank ground wire is one of the more common failure points in the 1973-1986 fuel gauge circuit. It either gets left off by someone replacing the tank or the wire rots off the ring terminal at the frame or the .250" disconnect on the sender locking ring rots off the wire. If the stock fuel gauge is nailed at 3:00 you likely have a bad tank ground wire. Easy to test and not awful to correct. Take a set of jumper cables and attach one end to the frame and the other to the pinch weld on the selected fuel tank. If the fuel gauge needle suddenly comes to life with the KOEO (key on engine off) you'll need to fix the tank ground wire between the frame and the sender locking ring. Many moons ago I attached an alligator clip jumper wire from the frame to the pinch weld on the tank on my 78 GMC to get me by for a couple weeks til I had time to fix the problem properly. The other thing that'll nail the fuel gauge at the 3:00 position is an unplugged sender wire or damage to the wires in the sender circuit. There's only three of them and they're fairly accessible. If the valve is unplugged you've likely found your problem. I've commonly seen the sender wire get chowdered up where it runs along the frame rail. It's either unplugged from the harness wire that runs up to the bulkhead plug under the brake booster or something happened to it between the valve terminal B and the molded rubber round pin disconnect on the frame rail or right after the disconnect. None of that is difficult to repair. No amount of brandy new fuel gauges will read anything without a good tank ground and good sender wires from the tanks all the way to the gauge. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.