How to test the gas gauge

[Tuff Gong]

Is there a way to test the gas gauge while it is still installed? I put in a NOS gauge and it continually reads full (not past full). I thinking it might just be bad. The older gauge seemed to work.

[pjmoreland]

Disconnect the sender wire from the tank, and the gauge should point past full, 3:00.

Touch the sender wire to a good ground, and the gauge should point to empty.

Ignition needs to be in the run position during these tests.

Did you transfer the ceramic resistor from the old gauge to the NOS gauge?

[Tuff Gong]

My sending unit is really hard to get to because it is a blazer tank. Not sure on the resistor. I assumed it came completely ready to go. I will check in the AM

[pjmoreland]

Alternatively, you can disconnect the sender wire where it plugs into the fuse panel.

[pjmoreland]

There should be a resistor mounted to the fuel gauge posts and clamped against the back of the gauge cluster. It will be hidden behind the flexible circuit board. I have heard that some reproduction gauges have the resistor built in internally, so if it was a reproduction gauge that you replaced, then maybe there was no resistor.

Older gauge clusters, like the '67 shown in the photo below, had resistors that were made of resistive wire wrapped around a thin board. Later on, GM switched to resistors encased in white ceramic. Your NOS gauge will need the resistor.

There are some photos of the resistors in this thread.

https://www.67-72chevytrucks.com/vbo...d.php?t=515846

[Grumpy old man]

When I was building my new dash cluster I odered a new fuel gauge and wondered about this also . I used an orange one from an extra cluster , I haven't checked it yet . I looked for new replacements from the regular parts suppliers and didn't find any . I did find this chart from Wilcox Corvette that lists the resistors And I'm thinking the orange one is correct given the fuel sender is 90 ohm ? maybe one of the guys rebuilding clusters will show up and give us the correct info on if they are needed with a new gauge or not ?

[1967-C10]

sending unit should be 0-90 ohms. use ohm meter to verify.

[Tuff Gong]

The resistor looks good and I can't test the sending unit. I'm just going to buy a new gas gauge and see if that's the problem

[pjmoreland]

You can access the tan sender wire under the dash where it plugs into the fuse panel. Unplug it and see if the gauge moves to 3:00 with the ignition in the run position.

[Tuff Gong]

It just stays at full. Nothing moves

[Tuff Gong]

I'm guessing the NOS gauge is 40 years old and just locked up

[Tuff Gong]

OK so I've got a few more questions. Please look at the picture and tell me which direction is correct for the resistor. I may have had it on backwards and I think it caused a short in the circuit board. Any other opinions on why it shorted out

[pjmoreland]

The resistor is oriented with the color side away from the gauge cluster. Then a set of washers and nuts holds it down. Next is the circuit board, and another set of washers and nuts goes on.

If you had the resistor facing the other way, then the resistor contacts may have shorted the posts to the gauge cluster housing (ground). Power comes in on one of the posts, so it would have been shorted straight to ground, which would explain the burnt trace on the circuit board.

[Tuff Gong]

It was facing up like the picture shows. I just purchased a new printed circuit and reproduction gas gauge. Fingers crossed. The repo gas gas does not have a resistor so I assume it is internal as mentioned

[pjmoreland]

Does your NOS gauge have these black rubber insulators on the posts?

[Tuff Gong]

I will check