Have a 66 f100 with an in cab tank. My gas guage reads full all of the time until it's completely out of gas then it will move slightly down but still close to full. I guess my question is how can I tell if it's the fuel guage (aftermarket) or the sending unit.( I'm guessing original) that is creating this problem. Any help is appreciated!

 

Take the wire off of the sending unit, turn the ignition key ON, and see what the gauge does. Then, with the key still ON ground the wire and see what the gauge does. If it doesn't change then it is the gauge or the voltage regulator for the dash. The regulator usually affects all of the gauges though.

You can also remove the sending unit, hook it up to an OHM meter and run it through the range from full to empty and watch the meter readings. The readings should increase or decrease steadily.

 

If I take off the wire going to the gauge the gauge goes to empty. And if I ground that wire it it goes back to full. The only other gauge I have is a temp gauge which seems to work fine.

 

It's probably your sending unit then.

 

How many ohms should it read full and empty?

 

I know I've seen that info on other posts on this topic. I don't know them off the top of my head though.

 

There's a chart at the bottom of the page.
Fuel Tank Sending Unit Tech - FORDification.com

Lotsa stuff on youtube.

 

1961/77 F100/750 in cab fuel tank sending unit same; D7TZ-9275-G replaced C1TZ-9275-K