Recently my gas gauge has been spiking at start up and then sticking for a bit before dropping from lets say half a tank to what is really in the truck. I didn't know if it was the sending unit or something else. My ex is telling me that due to him running it to EMPTY and letting it sit like that for awhile and he ran it once with no fuel in it that it could be the problem. But even with gas in it now, 1/4 of a tank its still spiking and dropping. I am at a loss on what to do, and the two mechanic's in my house are clueless... Its an 03 Dodge RAM 1500 Quad SLT 5.7L HEMI MAG... help please.

 

I am no expert, and someone will correct me if I am wrong. But, sounds like the fuel sending unit is getting flaky to me. It should be nothing more than a variable resistor. If you had an ohm meter or a digital multimeter and can locate the wires from the sending unit, you can measure across them and measure the resistance (ohms). I would measure with vehicle showing the correct fuel level, then get it to show the incorrect level and see if the is a difference in resistance. If reads the same, could be something else. If different, then something wrong with sending units or gummy/tarnished contacts in it. You could possibly try to run a can of sea foam in it with a full tank of gas and see if that helps by the time you run out most of the gas. From what I understand, to change the pump or sending unit requires dropping the fuel tank, or removing the bed of the truck.

 

Found this in the service manual. Go to Weed's sinature and download you a copy of the service manual. Yes this is from the 03 Service Manual.

² Actuator Test - Each time the cluster is put
through the actuator test, the fuel gauge needle will
be swept to several calibration points on the gauge
scale in a prescribed sequence in order to confirm the
functionality of the gauge and the cluster control circuitry.
On vehicles with a gasoline engine, the PCM continually
monitors the fuel tank sending unit to determine
the level of the fuel in the fuel tank. On
vehicles with a diesel engine, the ECM continually
monitors the fuel tank sending unit to determine the
level of the fuel in the fuel tank. The PCM or ECM
then sends the proper fuel level messages to the
instrument cluster. For further diagnosis of the fuel
gauge or the instrument cluster circuitry that controls
the gauge, (Refer to 8 - ELECTRICAL/INSTRUMENT
CLUSTER - DIAGNOSIS AND TESTING).
For proper diagnosis of the fuel tank sending unit,
the PCM, the ECM, the PCI data bus, or the electronic
message inputs to the instrument cluster that
control the fuel gauge, a DRBIIIt scan tool is
required. Refer to the appropriate diagnostic information

 

Again, from service Manual. The fuel sending unit and fuel pump are all one module.

FUEL LEVEL SENDING UNIT /
SENSOR
DESCRIPTION
The fuel gauge sending unit (fuel level sensor) is
attached to the side of 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 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.

 

From this data, what I gather is your main players in this are the fuel pump module (sending unit and fuel pump), PCM, Gauge Cluster, Gauge, and wiring in between. Based on your symptoms I would first see if you can inspect or even see this connector where it plugs into the fuel pump module. Sometimes a bad or dirty connection could cause this. Or perhaps blindly unplug and replug this connection a few times and see if that helps. If not I would suspect the sending unit. Afterall it is just a variable resistor that increase/decreases the resisitance which in turn increases or decreases the voltage sent to the PCM. Variable resistors do get bad "spots" on them over time from wear and tear and normal usage. In which case removing the fuel tank to replace it is your only option. so, to summarize, if it was mine, I would suspect the connection, if not that then the sending unit first. Like I said if it is possible to find the two wires and measure them when it is reading incorrectly versus when gauge is reading correctly this could tell you in which direction you need to go. If voltage or resistance is the same from unit and gauge is spiking then going to actual fuel level, i would suspect pcm or gauge cluster. If it gives 2 different readings, without the sending unit moving, then I would suspect connection or sending unit itself.

 

My fuel gauge does this every start. Hoping i dont have to drop the tank

 

run a c-code check by holding the tripometer and turning the ignition on.. watch the cluster for codes.. the first series is the code, the second series is how many key cycles since it last happened..

you'd be looking for a code for the instrument cluster being jacked up, or one for the sending unit wiring..

that may be one way to determine if you've got a rogue splayed wire, or if you've got sender or wiring issues..

it's without fortune that our years of trucks were plagued with wiring issues..

 

Awesome dodge! i just did this and have no c codes?

 

no chassis codes is a good thing.. it means your instruments and wiring checked out.. in my opinion, the next thing to hold suspect is where the gauges get there information- which is the sender or sensor.. in this case, the one hiding out on the fuel pump...

here is a fwiw:

a full tank of gas is hard to handle.. unbolting the bed isn't that big of a deal.. both reveal the top of the tank and the pump assembly.. I'd rec you guys do just that.. an extra set of hands to lift the bed and something to brace it on is just as easy, if not more so, than dropping the tank.

 

A friend and i did his fuel sender on his long bed cummins. We pulled the bed just the 2 of us, no big deal. Simple hand tools and a little know how is all you need. Dont forget to disconnect the ground from the bed to the frame and any wiring connected to the bed I.E. The taillights.