Fuel Gauge pegged - where to start looking

I have a 94 F-250 HD 4x4 Vin K truck with a NA motor. Systematically going thru it and making repairs. Next is the fuel gauge. I have replaced the the dual tank switch on the dash as it was broken. The gauge sits fully pegged on full regardless of which tank it is switched to and I know the tanks are near empty. So I am thinking what are the odds both sending units are bad. Makes me think it could be a common connection before the tanks. Anyone have any thoughts about where to start looking? I looked underneath at the tanks and I don't see where you can get to the sender without dropping the tanks. Wow, it looks like I need to remove skid plates to remove the tanks. Any simpler ways to troubleshoot this issue? If gauge is pegged what would be most likely the cause?

I am thinking it may be easier to pull the truck bed to work on all this. While it is off do a little rust prevention too. Does that make sense? I do have a fork truck at my disposal.

Obviously, I rather not take things apart if I don't have to.

Stickies, grounded wire,sender

Fwiw my kid used to lift 1 side of the bed up @ the stealership to access senders, pegged f sounds like a short to groung, as an open circuit reads m t

The guage wire goes to the switching valve on the frame. The switching valve has a little motor in it, that is all the switch in the dash does, is tell the motor which direction to move and select the proper pump. The electrical switching of the sending units is actually performed inside the switch valve itself.

I would get to the fuel switching valve, the wiring should all be there.

The yellow/white wire is the wire going up front to the fuel gauge. If you ground this wire and then unground it with the key in run, the plug unplugged on the fuel valve, the fuel gauge should swing back and forth in the dash if that wire and the gauge are good.

Leaving the fuel selector valve, the wire going to the front sending unit is the darkblue/yellow wire. The yellow/lightblue wire is the one going to the rear sending unit. You can try the same test. First turn the switch front to rear and back with the key on, engine off, you should hear the valve on the frame making a clunk sound each time you move the dash switch.

If that is working, if you select the front tank, then you should be able to ground and unground the darkblue/yellow wire and make the guage swing back and forth. I would unplug it from the tank first though. You usually can get to the front tank without dropping it. If you can get your head up inbetween the driveshaft and the tank, you usually then can see and feel around the top of the front tank to unplug the wire. The rear tank can be difficult.

Here's a link to a diagram. https://easyautodiagnostics.com/ford/4.9L-5.0L-5.8L/fuel-pump-wiring-diagram-1

IIRC, if the gauge reads empty, it's the sender. If full, then bad ground or grounds. Read it here a while ago.

Do you know if you have an anti slosh module in the instrument cluster. I think they can cause the fuel guage to peg when bad. My fuel guage was pegged for a while from me not pushing a connector fully together at the tank from the sender after I worked on it. Could try to locate that connector and wiggle it, push it together.

franklin2 said:

Here's a link to a diagram. https://easyautodiagnostics.com/ford/4.9L-5.0L-5.8L/fuel-pump-wiring-diagram-1

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You posted a Diagram for the gaser engines.
It show`s the fuel pumps in the tanks.
Could confuse some one.

Goat

Like has been said, it sounds like you have a short to ground somewhere in your sender wire. I would check each of the sender wire connections for continuity to ground, starting with then fuel selector switch plug. You can narrow it down until you know which stretch of wire has the short. I just went through all this a couple months ago, except mine was pegged on empty. Turned out the flat flexible circuit board on the back of my instrument cluster is cracking, and I had to run a separate wire to the fuel gauge to get it to work.

Old Goat said:

You posted a Diagram for the gaser engines.
It show`s the fuel pumps in the tanks.
Could confuse some one.

Goat

Click to expand...

You are right, thanks for catching that. The other wiring should be the same besides the fuel pump wiring which you can disregard.

Here is a diesel version. While this diagram is a 1986 version, the wiring colors still look the same and it looks like it's hooked up the same as the later trucks.

I’ll actually be messing with this tonight. Generally I agree with the comments from experience that grounded out is “F” and signal issue “E”. That’s all makes and models not just ford trucks. From @franklin2 diagram, I have the FSV unplugged and I’m reading “F”. I’m going to jump y/w to y/lb as a test and if I get a reading it will be interesting that the missing sending wire results in “F”. The other option, and this has happened to me before, is when you swap sending units and they don’t have a rubber spacer and you end up grounding the sending unit to the tank. It’s something you don’t think about in the moment till it happens to you a few times.

The sending unit should have it's own ground wire in the wiring plug. While it should ground itself through that wire, I don't see what harm it would cause for it to ground itself through the tank, though that ground would be very iffy at times with the padding on the tank straps.

You can see in the diagram above, #57 black is the grounding wire in the plug harness. If you study the sending unit, you will find this ground goes directly to the sending unit body.

franklin2 said:

The sending unit should have it's own ground wire in the wiring plug. While it should ground itself through that wire, I don't what harm it would cause for it to ground itself through the tank, though that ground would be very iffy at times with the padding on the tank straps.

You can see in the diagram above, #57 black is the grounding wire in the plug harness. If you study the sending unit, you will find this ground goes directly to the sending unit body.

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I don’t disagree with you on this application. I’ll update what I figure out.

quickster said:

IIRC, if the gauge reads empty, it's the sender. If full, then bad ground or grounds. Read it here a while ago.

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This is confirmed incorrect now. I bet whoever said that was referencing other vehicles and assuming it applied to this circuit. I have experienced that behavior with Jeep.

The wiring diagram Franklin and I were going back and forth on really says it all.

Open sending circuit:


Closed circuit with jumper:

When my connector to the tank was not pushed all the way together, aka open circuit, my fuel gauge was off the chart full all the time.