75gmck25
Full Access Member
- Joined
- Oct 13, 2016
- Posts
- 2,305
- Reaction score
- 2,282
- Location
- Northern Virginia
- First Name
- Bruce
- Truck Year
- 1975
- Truck Model
- K25 Camper Special TH350 NP203
- Engine Size
- 5.7
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
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