Fueling systen snafu

[Clb]

The obs in my sig line ....
I did the switching valve a while back, now I just noticed the gauge does not read when I checked the EDIT FRONT tank, but its full, truck will run.
Both tanks suck to the bottom/e on gauge.

I know that this is just a "go troubleshoot the effing thing" situation...
Just tapping the combined knowledge here .

So where to look?
I want it to be easy like the switch,or gauge, or loose wire ,and last the sender.
Where have you guys found the issue most often?

 

[franklin2]

99% of the time it's the sending unit. What you can do is switch to the rear tank, and then pull the wires off the rear sending unit. Hopefully you can get to them.

The rear sending unit wire should be a yellow with a darkblue stripe. There should also be a black ground wire. If you can get to these wires, with the key in run(engine off) and someone watching the fuel gauge(make sure you are switched to the rear tank) ground the yellow/darkblue wire. The fuel gauge should swing full scale empty or full(forget which). And then take the grounding jumper off and let the plug hang in the air. The fuel gauge should then swing full scale opposite.

If it passes this test, your wiring, switching valve and fuel gauge are good, your sending unit in the tank is bad.

 

[Clb]

Thanx
Yep grounding the lead it should swing to full.
Multimeter time...

 

[madpogue]

Grounding the lead will make it read EMPTY. Opening the circuit will make it read past FULL. http://userpages.chorus.net/elephant/FuelTankSelector.pdf . If it's reading empty on both tanks, it's most likely the gauge head itself. If it were anything in-tank, it would have to be two sunken floats, or two senders shorting to ground, both at the same time.

Does the FSV actually switch fuel feed/return properly?

Disconnect the FSV electrical connector. If the gauge does not peg past full when you do that, then the gauge head is bad, or a bad power wire to it, or a bad fuse.