The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Guage pegs when I turn on key (57 3100)

Guys,
When I first bought my truck the fuel guage worked. Then shortly after that the guage would peg full when I turn on the key. When I turn off the key, the guage drops to empty. If anyone can explain electrically it would help. Is empty 0 ohms and full higher ohms. How can I check unit. I have sorta thought the float may be stuck? What you think? I have looked online to see if I could find electrical schematic for the circuit but haven't found anything.
thanks

When the sender wire is grounded the gauge will read full. Either the float is stuck at the top or you have a short to ground. Disconnect the wire from the sender and see what happens. Gauge should stay where it is at no power. Then put an ohm meter on the sending unit terminal. It should read 30 ohms at empty, zero at full.

When the sender wire is open, I think it will read full. I think you may have it backwards Cletis.

That's what I was thinking. The default gauge reading for an open circuit is "full". Sounds like a broken wire to me.

Thanks for the feedback Greg H and Tiny. I hope to get to fool with it this weekend.
later

To check the sender:

Measure ohms between the main terminal on the sender unit and its metal casing. The ohm meter should read near zero when the tank is empty (float is down) and gradually increase to 30 ohms when the tank is full (float is up). The sender unit is bad if you don't get uniform readings between zero to 30 ohms.

To check the fuel gauge:

Remove the wire from the sender. With the key on, touch the wire to a known ground. The fuel gauge should drop to empty. Remove the wire from ground and the fuel gauge should read full. If the gauge fails this test, you either have a bad gauge or a bad wire from the gauge to the sender. Check the wire and its connections first before you spend big bucks on a new gauge.

Norcal49er, thanks for the info. That will be an easy check and is definitely the way to rule out guage or sender. I would really like to see a schematic drawing of this unit and guage but have looked everywhere on the internet and just can't find one. My bet is the float is stuck. Based off of your info, if I had a wire shorted then I would read empty all the time. But as soon as I turn the switch the need bounces off of full. If the float is stuck, is it as easy and just removing assembly and freeing it up. I am sure I will need another gasket to reinstall.
thanks

thanks

NorCal49er is right, pull the main wire off the center of the sending unit. When you ground it, the gauge should go to zero. In my case that test worked which meant the gauge was good. When I pulled the sending unit, I had to laugh. It was completely corroded and covered with old varnish.

I ended up pulling the tank and getting it boiled out, then getting a new sending unit from Jim Carter. The new unit ($32) was kind of a piece of junk made in Argentina. Really cheapo construction compared to the original unit, but it works. You can hook up the new sending unit to the gauge and test it before you install it. Comes with a new rubber gasket.

David Bush

Thanks DavidBush for the comments.
ole skool trucks