I have a 1973 F250 that has the in-cab behind the seat tank. I pulled the sending unit and it had a hole in the float. I bought a complete new sending identical unit installed it and it does not work. I just purchased this truck and the gauge has never worked. So I have a new sending unit installed. I have the gauge assembly out as I'm cleaning all behind the dash and wiring and instrument panel. How can I bench test the gauge or can I. If the gauge works and I have a new sending unit where do I go from here?

Would like to bench test the gauge.

 

fmc400 will probably scold me, but what I've done in the past is use a 9v battery and touch the terminals to the two gauge studs for just a second, long enough to see if the needle moves.
The gauge actually operates on ~5.5 volts so that's a little on the high side.
When the instrument cluster is installed, to check the gauge and all the wiring all you have to do is turn the ignition on and ground the wire that goes to the sending unit. If everything is okay the needle will swing all the way to full.

 

YES!

I tried the 9 Volt battery for just a second and the needle moved slowly to full.

So that means the gauge works and the fuel sending unit is new. I touched it right on the gauge terminal.

Could the IVCR or whatever that's called be bad and not letting the info get to the gauge?

So how do I chase this down?
Good Gauge and New sending unit but not working?

 

Hook everything back up, but then disconnect the sending unit connection at the tank. Connect a test light to the connector that went to the sending unit. The light should blink about once per second. Report your results.

 

I deleted everything in this post, forgot you had an in-cab tank instead of aft axle.
What I posted doesn't really apply.