Fuel Gauge stuck on full....

[Nodnarb76]

Hello everyone, I have an 87 R20 Suburban with a 40 gallon tank.
When I got the truck the fuel gauge wasn't working that well so I replaced it last summer with a unit from spectra, was working ok (reading was always not right and it fluctuated, but was better than when I got the truck).
Now, it is stuck on full regardless of how much fuel is in it.
Should I suspect the crappy spectra unit or is the signal wire from the sending unit bad?
Any ideas?

[wilkin250r]

It's easy enough to test.

Stuck at full is an indication of an "open circuit". It means the electrical path is broken somewhere. It could be broken inside the sending unit (very likely), but it could also be broken somewhere outside the tank. It could be as simple as a bad connection.

The gauge is just a resistance meter, and it's expecting to see zero ohms at empty tank, and 90 ohms at full tank. A broken wire is showing it a million ohms, so it's going to show up as full full full. The test is to pull the sending wire off the tank and short it to a good ground path (zero ohms). The gauge should read as empty. If it still reads full, then your problem is somewhere outside the tank, in your wiring and gauge. I would first suggest cleaning all connections.

If you want to get fancy with testing the gauge, check out this thread on how to check gauge accuracy by testing with resistors (warning: it requires a trip to Radio Shack and $3 worth of test equipment)

http://67-72chevytrucks.com/vboard/s...d.php?t=609792

[hatzie]

Usually this is the ground between the sender and the frame not being re-installed when the sender or tank was serviced. Since the TBI fuel pump shares this ground, and she runs, I doubt that's the problem.

The RV series trucks have fuel pumps in the tank that share a ground that runs through the top of the sender along with the fuel pump power and 0-90Ω gauge signal wires... I assume you still have TBI and it's not converted to carburetor. Carb senders are slightly different and rely on a ground wire between the sender ring area and the frame.

Check the sender connection at the 2 position WeatherPak. Unplug the connection there and ground the Tan sender wire. If the gauge goes to E with the ignition on then you may have a bum sender.

Look over the sender wires, especially at the tank, to make sure they didn't get pinched when you re-installed the tank.

[Nodnarb76]

Great info guys, thank you, will crawl under soon and start checking.
Thanks again!

[motornut]

I found cut/nicked green corroded fuel pump power wire over the wheel well drivers side
on my 87