Fuel Gauge only works when ignition off

[Pops Truck]

I (or rather my grandmother) have a 1979 C10 with one passenger side tank.  I have seen a number of posts with fuel gauges not working or being pegged past full.  In my case, the fuel gauge reads close to correct when the ignition is off.  Right now that is around 1/4 tank.  As soon as I turn on the ignition, it goes straight to empty. 

I checked and cleaned the ground from the fuel sending unit that is attached to the frame.  Using an ohmmeter on the pink wire coming from the sending unit and that ground wire, I get around 10 ohm when the ignition is off.  As soon as I turn on the ignition on, nothing registers on the ohmmeter.  I get a similar result but around 13-15 ohm when I try the contacts on the back of the plastic board behind the gauge cluster. Then, similar result with ignition on. 

The plastic board looks to be in good shape.  No cracks, warping, etc.  To add to the confusion, when we drive around in the truck, the fuel gauge will sometimes read that there is fuel and sometimes not.  Typically the amount it reads is within a few gallons of correct.  When it does read, it bobs around sometimes as well. 

I am thoroughly confused.  Any help is greatly appreciated.

[rich weyand]

Is the cab grounded to the engine, and the frame to the engine?  There should be a ground strap from the rear of the passenger side engine head to the firewall, and a ground strap from the front of the driver side engine head to the frame rail behind the steering box.

If the engine, the dash, and the frame (to which the fuel sender is grounded) do not agree on what 0 volts is, the fuel gauge will act flaky, particularly when the ignition is on and current is flowing to other accessories.

[Boone83K10]

a bad ground when dealing with the fuel tank would peg it past the FULL mark.

try this..

[rich weyand]

a bad ground when dealing with the fuel tank would peg it past the FULL mark.

I think that depends on whether it is a bad ground at the sender or the gauge, and whether parasite currents drive the ground voltage higher.

[Boone83K10]

my fuel selector got stuck halfway between tanks and gave me full readings on the left and half on the right... replaced it along with the entire right tank/lines/sender which were full of "sludge pudding" and now it works like a charm.

[randyp]

Hello, I am having very similar issues as OP (guage on E with ingnition on, goes to 1/4 when off). Here's my question; currently I am only using one of a two tank system and my fuel tank selector switch is missing/not connected. Could that be causing the problem? Is the wiring using the switch to complete the circuit? Please forgive my electrical ignorance.....

[rich weyand]

The sender wires from the two tanks are also selected by the switch.  If they are just disconnected, there is no connection from the sender wire to the gauge.  If the fuel lines are patched around the selector valve, the sender wire from the same tank also needs to be patched around the valve.

[randyp]

Thanks a bunch! I also discovered the other thread which confirms. Pretty much everything from a dual tank systems needs to be connected/working, otherwise re-wire around it.......I think I'll just work through completing the dual tank system. Thanks again!

[Pops Truck]

Is the cab grounded to the engine, and the frame to the engine?  There should be a ground strap from the rear of the passenger side engine head to the firewall, and a ground strap from the front of the driver side engine head to the frame rail behind the steering box.

If the engine, the dash, and the frame (to which the fuel sender is grounded) do not agree on what 0 volts is, the fuel gauge will act flaky, particularly when the ignition is on and current is flowing to other accessories.

We had a new engine installed last year and there was only a ground wire on the rear of the engine to the firewall, not the front to the frame.  Just installed a ground from the front of the engine to the frame.  That did not solve it. 

Then, tried the suggestion from Boone83K10.  I disconnected the wire from the tank and sure enough, it pegged full.  I know the ground from tank to frame is OK, so I will be dropping the gas tank to check the sending unit. 

Thanks to all for the help thus far.  Will post again after I pull the gas tank and install a new sending unit that I bought.