Mechanics

F250 FUEL GAUGE, ONE TANK EMPTY, ONE TANK FULL

1997 Ford F-250

Electrical problem
1997 Ford F250 V8 Two Wheel Drive Automatic 259000 miles

My front tank stays at empty. If I unplug the sending unit the gauge then goes past full. If I leave it plugged in and cut the ground wire and splice in another and run it to ground the gauge goes past full. Bad sending unit for the front tank? My rear tank was also staying empty and not moving. I unplugged the sending unit and it went past full on the gauge. I spliced in a wire and went to ground and now the gauge reads past full when I have the rear tank on? Both wires I used were the smaller of the 4, the yellow and white I think. Do I have 2 bad sending units?
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Nsenyk117
September 18, 2010.




Hi nsenyk117,

Thank you for the donation.

The Yellow/White wire is correct for the sending units but in our database it is stated as Yellow/White up to the fuel tank selector switch and beyond that, the rear fuel tank should be Yellow/Blue whereas the front fuel tank is Dark Blue/Yellow.

Where did you unplug the sending unit connectors and grounded it?

One thing that seems out of place is the gauge showing past full when the connector is unplugged. Under normal curcumstances the gauge should drop to empty when the sending unit is unplugged.

KHLow2008
Sep 18, 2010.
At frst I unplugged the sending unit at its electrical plug in connector. I then tried to ground the female end of the plugin to the truck. The key was in the run position. As soon as I unplug the connector the fuel gauge goes all the way full. To avoid this I left it plugged in to keep the tank reading empty and cut the yellow/white wire on the sending unit side of the electrical connction. Then splicing it to another wire and running it to the negative side of the battery the gauge moves all the way past full again. Also, with the key in the run position, I put my test light into the female ends on the wiring harness side of the connectors to test for power, it never lit up on all 4 wires. The truck will run and use the fuel from both tanks. If I unplug the connector and try to start the truck it wont so it must work for the pump. Why do both tanks read all the way past full when the electrical plug in to the sending units is unplugged? Would it have anything to do with the dome light coming onvery time the brakes are pushed?
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Tiny
Nsenyk117
Sep 19, 2010.
Here is what I have for the wiring at the front tank.
Sending unit side Wiring harness side
Black Black
Black/PinkRed
Gray/RedOrange
Yellow/WhiteYellow/Blue


http://www.2carpros.com/forum/automotive_pictures/338784_f150fuelsending_1.jpg


Tiny
Nsenyk117
Sep 19, 2010.
That kept me wondering. If the connector is unplugged, the fuel gauge should not move from bottom of gauge.

Plug in the connectors. Turn ignition ON and then OFF again. Unplug the connector and then turn ignition switch ON. Does the gauge reading go past Full?

Problem seems to be from gauge rather than sensing unit.

With connector unplugged, test the resistance between the Yellow/White of sending unit connector and body ground. Is resistance between 5 to 100 ohms?

If possible, get 2 different resistor of 10 and 50 ohms. Connect either one in series between the Yellow white wires and turn on ignition switch. Note gauge reading. Change resistor and retest.

KHLow2008
Sep 19, 2010.
With the connector plugged in its empty. With it unplugged its pegged past full. When I press the brakes the dome light comes on and when I open the drivers door the dome light does not come on. Do you think this could all be related? It appears the guy before me had speaker stuff hooked up. I found a cut aftermarket wire run into the fuse panel. Do these systems share the same wiring?

Tiny
Nsenyk117
Sep 19, 2010.
For the fuel gauge to register Empty, the sending unit must be providing a high resistance, most probably in the region of 100 to 105 ohms to ground.

In this instance, it could be a bad/poor ground that is increasing the resistance.

This could be related to the interior lights coming on when the brake pedal is depressed. Check the ground circuit at rear taillights. Clean and retighten them to see if the problem is resolved.

The driver door not operating the dome light could be due to a faulty door switch.

KHLow2008
Sep 19, 2010.
I found the ground under the drivers side tailight. It had 2 white wires to the frame. I cleaned it up and nothing. I replaced the door jamb switch and nothing. The passenger side does work however, always did. The fuel gauges are the same. Im going to bring my meter home tommorow and check the ohms. The dome light still comes on when breaking. It appears to me that the passenger side door jamb switch wiring runs along the dash into the fuse box and then to the drivers side switch. Could a bad gauge do all of this?

Tiny
Nsenyk117
Sep 19, 2010.
A faulty meter is not likely to cause the symptoms described and I believe it could be a bad ground for the meter assembly.

Check the ground circuits at : 1. Behind bottom of left kick panel.
2. Left front of engine compartment, on upper radiator support.

Is your vehicle equipped with keyless entry?
Does the warning system for the door chime module works when the headlights are left on with ignition switch turned off?
Is your vehicle model heavy or light duty?

KHLow2008
Sep 20, 2010.
Both grounds are good. No keyless entry. No, the door chime doesnt work at any time. Heavy Duty model. After dinner I am going to check ohms. Would a bad tank selector do this?

Tiny
Nsenyk117
Sep 20, 2010.
Ok, I got under the truck with my meter. With the key off I tested the Black and Blue/Yellow wire on the wiring harness side of the midship tank. I got 155 ohms. Then I went from the yellow/white wire to the gas tank as my ground on the midship tank 1494 ohm. I could not produce any results with the rear tank. I tried both the ways as the midship tank and switched the fuel to rear tank, nothing.

Tiny
Nsenyk117
Sep 20, 2010.

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