67-72 Chevy Truck fuel gauge diagnostics 101

[TBONE1964]

Hello guys and gals,

I am going to add some information from a bench test I did today. This is going to show how to wire a fuel gauge loose from the cluster and test.

I will try and make it basic and clear as to what is going on for each photo. This will take some tome so let me get everything loaded before responding please.

I try to keep up with this but very busy with other things. I appreciate anyone who chimes in with good info from their own experience.

Thank you,
Tom

[TBONE1964]

I am using a new AC Delco fuel gauge and a GM 0-90 ohm sending unit.

These photos show how the gauge is wired to the sender and I use a 10 amp batt charger for power.

The ground can be shared between the sender and the gauge.

I use a yellow jumper as the "tan" wire that runs from sender to fuse panel and then to cluster.

I use green lead from my batt charger for the pink power wire from the ignition switch through the fuse panel and to the circuit board in location number 3 on the connector.

It is really this simple. If you can do this, you can test your fuel gauge out of the cluster.

[TBONE1964]

This photo shows the ohm reading (NO POWER / IGN OFF) across the resistor when installed. 42.7 ohms. It will be in the area of 85 ohms when the resistor is loose from the gauge.

[TBONE1964]

This photo shows the voltage in the sender line. Because of resistance, you will never see 12 volts here. The photo shows 6.63 volts at full tank and will show about 1 volt when at 1/4 tank or so. Voltage drops with float movement.

[TBONE1964]

Any questions on above? Please share your issues and lets try and help get you guys happy and not frustrated with fuel gauge issues.

A further note, If you have opportunity to compare your gauge with another used/new one, that helps you understand what the numbers should be good vs bad.

[67 Burb]

Here's what I use for a quick check of a fuel gauge. As a replacement for a sending unit, three 33 ohm resistors in series giving 33, 66 and 99 ohms for 1/3, 2/3 and full, depending where you connect the jumper. hooked up as in the photo, the gauge should read 2/3 of a tank just under 3/4 full.