2007 Liberty KJ Factory Service Manual 8 - ELECTRICAL

FUEL GAUGE

DESCRIPTION



A fuel gauge is standard equipment on all instrument clusters. This gauge is located in the lower left corner of the instrument cluster, to the left of the speedometer. This 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 above the hub of the gauge needle. 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 text and graphics are white against a black field, while the major and minor scale increments are black against a silver field, except for a pair of red zone increments at the far left (Empty) end of the gauge scale. All text, graphics and increments are 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 white text and graphics and the major black increments appear blue-green, while the major red increments still appear red. The minor increments are not illuminated. The red gauge needle has internal optical illumination. 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.

OPERATION

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 an electronic message from the PCM indicating the percent tank full, the cluster moves the gauge needle to the proper relative position on the gauge scale. The PCM applies an algorithm to the input from the fuel tank sending unit to dampen gauge needle movement against the negative effect that fuel sloshing within the fuel tank can have on accurate inputs to the PCM.
  • Less Than 15 Percent Tank Full Message - Each time the cluster receives electronic messages from the PCM indicating the percent tank full is less than 15, the gauge needle is moved to below the one-eighth position (red zone) on the gauge scale and the low fuel indicator is illuminated. The low fuel indicator remains illuminated until the cluster receives messages from the PCM indicating that the percent tank full is greater than 15 (one-eighth), or until the ignition switch is turned to the OFF position, whichever occurs first.
  • Less Than Empty Percent Tank Full Message - Each time the cluster receives an electronic 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 sending unit 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 sending unit input to the PCM is an open circuit.
  • 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.

The PCM continually monitors the fuel tank sending unit input to determine the level of fuel in the fuel tank. The PCM then applies an algorithm to the input and sends the proper percent tank full messages to the ElectroMechanical Instrument Cluster (EMIC).

For proper diagnosis of the fuel tank sending unit, the PCM, the EMIC, the PCI data bus or the electronic communication related to fuel gauge operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.