Fuel gauge stuck on full

[dieselfreak5.9]

I have seen lots of problems with fuel gauges going to empty but mine is stuck on full and will not budge. I have banged on the side of the tank to see if the float is stuck, no luck. I have disconnected the electrical connector at the sending unit, no change on the gauge. I see on other posts that the sending unit does not go directly to the gauge but rather to a controller.



Anyone out there troubleshoot this before? Any help would be appreciated.

 

[kompressor]

Just think, you'll never have to fill up

 

[dieselfreak5.9]

In this economy not having to ever fill up again would great! But seriously, anyone out there have this problem or have the factory service manual for some troubleshooting guides? My Chiltons does go into these problems.

 

[Regcabguy]

Coincidentally,mine was still on full after 125 miles today. It started to budge right at home. Hope I'm not in the same boat.

 

[sag2]

From 2005 Service Manual.

OPERATION

For Fuel Gauge Operation: A constant input voltage source of about 12 volts (battery voltage) is supplied to the resistor track on the fuel gauge sending unit. This is fed directly from the Engine Control Module (ECM). NOTE: For diagnostic purposes, this 12V power source can only be verified with the circuit opened (fuel tank module electrical connector unplugged). With the connectors plugged, output voltages will vary from about . 6 volts at FULL, to about 7. 0 volts at EMPTY. The resistor track is used to vary the voltage (resistance) depending on fuel tank float level. As fuel level increases, the float and arm move up, which decreases voltage. As fuel level decreases, the float and arm move down, which increases voltage. The varied voltage signal is returned back to the ECM through the sensor return circuit.



Both of the electrical circuits between the fuel gauge sending unit and the ECM are hard-wired (not multi-plexed). After the voltage signal is sent from the resistor track, and back to the ECM, the ECM will interpret the resistance (voltage) data and send a message across the multi-plex bus circuits to the instrument panel cluster. Here it is translated into the appropriate fuel gauge level reading. Refer to Instrument Panel for additional information.

 

[Prairie Dog]

So deriving from what Sag wrote above, if you were to unplug the sending unit and short out the wires, it should read empty. If not then either you have an open in the wiring or the ECM is crapola. If shorting them does cause the guage to show empty, you have a bad sending unit.

 

[dieselfreak5.9]

Thanks sag and prairie dog. I understand how it works now and will try shorting. You guys know if finding the two wires at the ECM would be easier than the sending unit. Thanks again...

 

[dieselfreak5.9]

Finally got out there to verify a few things and something weird is happening. With the sending unit plugged in the gauge is stuck on full. When the sending unit is unplugged the gauge goes to empty, this is not shorted. When I short the pins the gauge still reads empty. In one sense this tells me that there is a short from sending unit to ECU but it does not make sense that when it is plugged in the short is gone. At this point it still seems like the sending unit/pump needs replacement. OReillys near me wants $420. 00 for the unit. Any advice would help.

 

[Prairie Dog]

Can you read 12V at the connector when unplugged?