Fuel Gauge

If nobody has mangled the original wiring, there should be a wire junction for the sending unit wire just inside the frame rail next to the tank. There will be two wires coming through an insulated bushing in the hole in the frame rail. One wire will be grounded to the frame and the other will have a round bullet-shaped junction you can pop apart.

Take that junction apart, or if necessary cut the wire at that point.
- Turn the truck on and verify that you are getting 12 volts coming on the wire from the gauge. Now, if you ground the wire you should get a full gauge reading. This would verify that the gauge and wiring is connected and working. If you can find some small resistors (I miss Radio Shack) you could insert resistance that varies from 0 to 90 ohms to see what gauge readings you get. I don't have the table handy, but GM has documents that show what resistance should match each gauge reading.
- Then test the sending unit. With the tank close to empty, test the resistance between the wire coming from the tank sending unit and ground. It should vary from 0-90 ohms, depending on how much gas is in the tank. The meter should read 0 ohms empty, 90 ohms full.

My experience is that the sending units take a lot of abuse from being first immersed in gasoline and then exposed to air in the tank, and their resistance values get less accurate over time. If it works fairly well to change resistance based on the fuel in the tank, its probably good enough.

Bruce

Dmack said:

If it rests last full, at 3 o'clock, it usually means you have an open circuit. Generally a broken or disconnected wire, or broken sending unit connection, as some mentioned before.

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So how can you tell if the sending unit connection is broken? I checked out all my wires and most of them look alright other than the ones on the bulkhead that look like they have some oily gunk around them.

How do i test a ground, and more specifically how do I test a particular ground such as the one from the frame to the tank?

The best way to test the sender wire is with an ohm meter. Unplug the wire at the sending unit and put one meter lead on the post and the other to ground. You should get a reading between 0-90 ohms. If not, you have a bad sending . Then put one lead at each end of the wire and you should get basically zero ohms (shows no resistance through the wire run). If there is high resistance of infinite, you have a problem in the wire. If so, there are a couple of sections with connections you can test independently.

For the ground test, put one lead on the sender base metal and the other on a good frame ground. If you have no resistance, you have a good ground. If it is open (infinite resistance), you need to fix a good ground.

You can also try putting the meter on both sides of the wire, and should get no resistance.