RAM 1500 Truck 4WD V8-5.9L VIN Z (2002)
Fuel Gauge: Description and Operation
A fuel gauge is standard equipment on all instrument clusters. The fuel gauge is located in the lower left quadrant of the instrument cluster, below the
voltage gauge. The fuel gauge consists of a movable gauge needle or pointer controlled by the instrument cluster circuitry and a fixed 90 degree scale on
the cluster overlay that reads left-to-right from E (or Empty) to F (or Full). An International Control and Display Symbol icon for "Fuel" is located on the
cluster overlay directly below the "F" end of the gauge scale. An arrowhead pointed to the left side of the vehicle is imprinted on the cluster overlay next
to the "Fuel" icon in the fuel gauge to provide the driver with a reminder as to the location of the fuel filler access. The fuel gauge graphics are black
against a white field except for a single red graduation at the low end of the gauge scale, making them clearly visible within the instrument cluster in
daylight. When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the exterior lamps turned ON, the black
graphics appear blue and the red graphics still appear red. The orange gauge needle is internally illuminated. Gauge illumination is provided by
replaceable incandescent bulb and bulb holder units located on the instrument cluster electronic circuit board. The fuel gauge is serviced as a unit with
the instrument cluster.
The fuel gauge gives an indication to the vehicle operator of the level of fuel in the fuel tank. This gauge is controlled by the instrument cluster circuit
board based upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the
Programmable Communications Interface (PCI) data bus. The fuel gauge is an air core magnetic unit that receives battery current on the instrument
cluster electronic circuit board through the fused ignition switch output (run-start) circuit whenever the ignition switch is in the ON or Start positions.
The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition switch is turned to the OFF position. The
instrument cluster circuitry controls the gauge needle position and provides the following features:
-
Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating the percent tank full, the cluster programming
applies an algorithm to calculate the proper gauge needle position, then moves the gauge needle to the proper relative position on the gauge scale.
The algorithm is used to dampen gauge needle movement against the negative effect that fuel sloshing within the fuel tank can have on accurate
inputs from the fuel tank sending unit to the PCM.
-
Less Than Twenty Percent Tank Full Message - Each time the cluster receives messages from the PCM indicating the percent tank full is about
twenty percent or less for ten consecutive seconds and the vehicle speed is zero, or for sixty consecutive seconds and the vehicle speed is greater
than zero, the gauge needle is moved to the one-eighth graduation or below on the gauge scale, the low fuel indicator is illuminated, and a single
chime tone is sounded. The low fuel indicator remains illuminated until the cluster receives messages from the PCM indicating that the percent
tank full is greater than about twenty percent for ten consecutive seconds and the vehicle speed is zero, or for sixty consecutive seconds and the
vehicle speed is greater than zero, or until the ignition switch is turned to the OFF position, whichever occurs first. The chime tone feature will
only repeat during the same ignition cycle if the low fuel indicator is cycled OFF and then ON again by the appropriate percent tank full messages
from the PCM.
-
Less Than Empty Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating the percent tank full is less
than empty, the gauge needle is moved to the far left (low) end of the gauge scale and the low fuel indicator is illuminated immediately. This
message would indicate that the fuel tank sender input to the PCM is a short circuit.
-
More Than Full Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating the percent tank full is more
than full, the gauge needle is moved to the far left (low) end of the gauge scale and the low fuel indicator is illuminated immediately. This message
would indicate that the fuel tank sender input to the PCM is an open circuit.
-
Communication Error - If the cluster fails to receive a percent tank full message, it will hold the gauge needle at the last indication about five
seconds or until the ignition switch is turned to the OFF position, whichever occurs first. After five seconds, the cluster will move the gauge
needle to the low end of the gauge scale.
-
Actuator Test - Each time the cluster is put through the actuator test, the 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
The PCM continually monitors the fuel tank sending unit, then sends the proper messages to the instrument cluster. For further diagnosis of the fuel
gauge or the instrument cluster circuitry that controls the gauge, (Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING).
For proper diagnosis of the fuel tank sending unit, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the
fuel gauge, a DRB III scan tool is required Refer to the appropriate diagnostic information.
