Fuel guage Trouble shoot

If you're indicating the sending unit isn't connected, then I'd inspect the ends of the wiring and the sender. If they don't match, then you may have the wrong sending unit because of some changes made. If the ends are missing, I'd just try putting on new ends and connecting them. Either way, if you have a doubt, use some terminals and a jumper wire to put a 5 amp blade fuse in line so that, if there is a short somewhere, the small fuse will blow long before wiring ir components heat enough to become dangerous or damaging. Standard female .250" spade terminals will fit on a ATC blade fuse nicely and makes a very cheap fuse holder for testing or even permanent installation if you wrap electrical tape between and around the exposed metal. Insulated terminals work ever better not needing tape to protect them when used inside the cab. Improvised protection is better than none at all. Once you verify function and the fuse holds up, you can likely remove the temporary fuse holder and connect the wires to the sender correctly.

You CAN test the sender by connecting a multimeter to the sender terminals and recording your OHM readings at full and at empty at least and see what you get for a range. You could also connect the meter while you fill the tank and watch for it to slowly change over the whole range to look for dead spots or shorts in the sweep of the sender. Also watch to see that it starts moving almost as soon as you start pumping from empty and doesn't stop much before the tank is full.

If you test the sender, you can try to locate the spec for the stock gauge or sender to make sure it's not way off for some reason. Usually if it changes from full to empty the sender won't be bad.

Also, once you have the range, if the gauge is bad, you can use the empty and full readings from the meter to select an aftermarket gauge that is compatible with the sender.

The fuel gauge usually works by passing a 12v signal through the sender. As the float moves with the fuel level, it changes the resistance through the sender which changes the voltage that passes through to ground. The gauge is then matched to show the tank level based on the difference in voltage it "sees" as the fuel level changes. You won't likely see a different method of "sensing" fuel level on a heavy truck. Even VERY few cars have something besides a resistance based fuel sender.

If your sender works, but the gauges does not, you should use the voltage setting on the meter and either ask a mechanic or get a wiring diagram for the truck and test to see that you are getting proper voltage to the circuit at all points. Using the ohm or continuity setting you can test that the grounds are working well by testing from the end of the wire to chassis ground to make sure the ground connection and the wire itself are in good shape.

Hope that helps and is all understandable.

 

great info xTHANATOPSISx thanks.

 

No problem. I'm sure I'll get far more than I can ever contribute.

 

Glad to hear you got it working.

 

Great job.

 

Just an update, I was able to "fix" my sending unit by re-straightening the float wire. all good