97 f350 5.8 fuel gauge inop

It looks like the selector valve was moved between front tank and frame but under the hydraulic control for the dump body.
Once I drop this tank, I'll need to replace all of the snapped everything that will come apart so I want to do it once. Customer really can't have this truck down during the day.
It looks like the gauge gets signal from the selector valve which gets it from each sending unit.
What's throwing me off is they think that the tanks stopped working at different times which would suspect the sending units themselves, each.
But I need to be sure and not machine gun parts at it.
Any diagrams or advice. I think I fried my brain

Ohm out the fuel level sensors on each sender. Off the top of my head I think our trucks use 16-250 ohm range.

There shouldn't even be a tank selector valve on this truck! The factory setup uses two fuel pumps, one in each tank - whichever pump is running also has its return line open, when the pump stops that return line also closes. Not sure how exactly that happens, but I can tell you that if you disconnect the hoses from say the front tank the moment the rear tank pump pressurizes the system you'll see fuel gushing out the return line for the front tank as well - but if you somehow plug the front return line the fuel will go back to where it's supposed to go, the rear tank. and again, there is no fuel selector valve on these trucks, the PowerStroke trucks have them but the gassers do not.

In that regard the switch on the dash does two things - it determines which pump gets power to run, and also connects the respective sender to the gauge. Whereas on the IDI trucks most are familiar with here, the switch only controls the selector valve on the frame, and the valve does the connection between senders and gauge. That's why if you replace the selector valve with manual ones you have to do some creative wiring to make the gauge work properly.

So I WAS chasing my tail... doh!

I suppose so Question is, did you manage to catch it?

I can post some wire colors if that will help you some...

Yes please! I'd like to be able to be 100 that it is the sending units or cluster

Alright, on the back side of the switch there will be 6 wires, in two groups of 3:

- red/yellow = key-on power in from the intertia switch
- solid red = power out to front tank fuel pump
- brown/white = power out to rear tank fuel pump

- yellow/white = power in from the fuel gauge
- darkblue/yellow = power out to front tank fuel level sender
- yellow/lightblue = power out to rear tank fuel level sender

Step 1: Basically the selector switch itself is of the on-on dual-pole dual-throw variety, it switches two separate circuits (fuel pumps and senders) at the same time without a center off position for either. So when you have it flipped to the front tank you should have continuity between the red/yellow and solid red wires (fuel pumps power), and the yellow/white and the darkblue/yellow wires (gauge signal). Alternatively with the switch set to the rear tank you'll get continuity between red/yellow and brown/white (pumps), and yellow/white and yellow/lightblue (gauge). Check these with the switch pulled from the dash but still plugged into its harness, and the ignition power off.

Step 2: If the connections check out fine that means switch itself is good, go ahead and unplug the switch from the harness, turn the ignition on (pumps won't run and gauge won't read, that's fine) and check for voltage at the yellow/white wire at the harness connector - this is to verify that the gauge itself has not burned out or something (gauge supplies voltage to senders, senders are grounded on the other end, gauge needle moves based on voltage drop across senders).

Step 3: If you have voltage at the yellow/white wire plug the switch back in, leave ignition on but don't start the truck. Lift the dump way up (prop it for safety, don't want that thing falling on you), put tank selector switch on front tank, locate the front tank pump/sender unit harness connector (should be near the tank itself, follow wires from top of tank to the big fat harness inside the frame rail), then unplug it. Check for voltage on the darkblue/yellow wire on the big harness half of the connector, and also check the black wire for a good ground - this verifies the harness is good. Flip the dash switch to the rear tank, do the same check for the yellow/lightblue (sender power) and again black (sender ground) wires.

Step 4: While the connectors are apart, it's time to measure some senders resistance. Both senders are supposed to read 15-16 ohms when tank is empty and 150-160 ohms when tank is full. Measure said resistance on the sender half of the connectors, across the darkblue/yellow and black wires for the front tank, and across the yellow/lightblue and black wires for the rear tank. Obviously here it helps to know in advance how much fuel there actually is inside each tank, then you'll know what to expect for resistance reading.

Most common failure for Fords seems to be either sunk floats for the senders or simply wore off wiper contacts inside the senders. A sunk float will make the sender show 15-16 ohms resistance all the time, damaged internal contacts will make the gauge read sometimes but not others. Given that, with a dump truck it's usually easiest to just go for sender resistance check first, as the harness and switch and gauge are rarely a problem. But I gave you the complete procedure I use anyways, just in case

Nice! I'll be with the truck tomorrow and go at it, thanks so much