RAM 3500 Truck 4WD V8-5.9L VIN Z LDC (2000)
Fuel Level Sensor: Description and Operation
FUEL LEVEL SENSOR - PCM INPUT
The fuel level sensor (fuel gauge sending unit) 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).
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.
Fuel Gauge Operation
A constant current source of about 32 mA is supplied to the resistor track on the fuel gauge sending unit. This is fed directly from the Powertrain
Control Module (PCM). The resistor track is used to vary the voltage 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. Output voltages will vary from about 0.6 volts at FULL, to about 8.6 volts at EMPTY
(Jeep models), or, about 7.0 volts at EMPTY (Dodge Truck models).
NOTE: For diagnostic purposes, this voltage can only be verified with the fuel gauge sending unit circuit closed (i.e. having all of the sending
units electrical connectors connected).
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.
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.
