Fuel sender unit

BrissieJeeper · **Posted:** Thu Jun 04, 2009 4:46 am

Anyone ever had a problem with their fuel sender unit? I had the 90k service yesterday and I asked that my fuel guage be checked because it is reading inaccurately i.e. shows only 7/8 full when tank is actually full.

Anyway they didn't get a chance to check it (slackers!) but said that it was most likely to be a faulty fuel sender unit....which they would be happy to replace under warranty..... I then informed them that my car is not under warranty. I was then presented with the price tag to recover that small portion of my fuel tank guage....$1200. If their is no other way to fix this then i think i'll just get a texta and mark a new full line on the guage!!!! Has this happened to anyone esle? is their anything else that could be causing it?

2JeepFamily · **Posted:** Thu Jun 04, 2009 4:54 am

not sure but it could just be the float level, if goes down without any problems (fuel gauge) then I dont think
that there is really a prob with the sender, more like it's just not rising to the top. With old quadrajet carbs
we use to get a problem with the floats becoming saturated with fuel then a flood of fuel would come flowing out of the breather as it wouldn't float enough to shut the seat, but this is mearly speculation as i haven't had a problem with
mine, but worth looking into.

G'ME MUD · **Joined:** Tue Jul 17, 2007 7:55 pm **Posts:** 370

mine went haywire for awhile there, but at the other end. The gauge would read empty but it would only take 48-50 ltrs to fill, then after about 10 tank fulls it came back to normal taking around 62-65 ltrs to fill. nothing to spend 1200 on to fix anyway.

Glend · **Posted:** Thu Jun 04, 2009 6:25 am

Here is the relevant page from the Service Manual. I get the impression that you might be able to check the voltage going to the PCM from the Sending Unit to determine if it is in the range of normal full (0.6V). If you can see that (0.6V) state on the sensor return circuit then the tank and associated float and sender should be OK. If the PCM is receiving the correct voltage but the gauge is displaying something else then the signal coming from the PCM maybe suspect. In that case, perhaps powering down the PCM (ala disconnecting the battery or relevant fuse for an appropriate amount of time to allow capacitors to discharge) might cause the PCM to default back to original powerup settings. Or I could be completely bonkers.

Whatever you decide to do, don't mess around with the wiring on the tank sender (as it will be carrying a small current to the resistor track), and we don't want any smal sparks down there do we. Try reading the PCM end of the sender return circuit.

SENSOR-FUEL LEVEL SENDING UNIT
DESCRIPTION
The fuel gauge sending unit (fuel level sensor) is attached to the fuel pump module. The sending unit consists of a
float, an arm, and a variable resistor track (card).
OPERATION
The fuel pump module has 4 different circuits (wires). Two of these circuits are used for the fuel gauge sending unit
for fuel gauge operation, and for certain OBD II emission requirements. The other 2 wires are used for electric fuel
pump operation.
For Fuel Gauge Operation: A constant current source of approximately 32 milliamps is supplied to the resistor
track on the fuel gauge sending unit. This is fed directly from the Powertrain Control Module (PCM). NOTE: For
diagnostic purposes, this 12V power source can only be verified with the circuit opened (fuel pump module
electrical connector unplugged). With the connectors plugged, output voltages will vary from about 0.6 volts
at FULL, to about 8.6 volts at EMPTY (about 8.6 volts at EMPTY for Jeep models, and about 7.0 volts at
EMPTY for Dodge Truck models). 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
PCM through the sensor return circuit.
Both of the electrical circuits between the fuel gauge sending unit and the PCM are hard-wired (not multi-plexed).
After the voltage signal is sent from the resistor track, and back to the PCM, the PCM 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.
For OBD II Emission Monitor Requirements: The PCM will monitor the voltage output sent from the resistor track
on the sending unit to indicate fuel level. The purpose of this feature is to prevent the OBD II system from recording/
setting false misfire and fuel system monitor diagnostic trouble codes. The feature is activated if the fuel level in the
tank is less than approximately 15 percent of its rated capacity. If equipped with a Leak Detection Pump (EVAP
system monitor), this feature will also be activated if the fuel level in the tank is more than approximately 85 percent
of its rated capacity.

BrissieJeeper · **Posted:** Sat Jul 04, 2009 8:07 pm

Sorry for the very late reply all!! Thanks for your insights! I haven't had a chance to take the car back in yet....hopefully it is just somthing simple or fix itself. I'll let you know what jeep say when i finally get around to taking it back.

Fouros · **Posted:** Sun Jul 05, 2009 7:49 pm

1200 does not sound right for a fuel tank sender is that including labour

i had mine go out on me a while ago

turned out that i dislodged it from the clips in the bottom of the tank, took em next to no time 1- 1 1/2 hours at the most

BrissieJeeper · **Posted:** Tue Jul 07, 2009 5:57 am

1200 is just the guestimation they made...i got the impression the guy I was talking to didn't really know what it costs to fix anything that isn't covered by warranty...... hopefully it is something simple like that....but despite how annoying it is i'm just gonna wait til the next service to deal with it...other more expensive, non car related things to deal with at the moment...

Fuel sender unit

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