Low Fuel Lamp/Indicator: Description and Operation

A low fuel indicator lamp is used only on the high-line instrument clusters. The low fuel indicator lamp is located near the lower left corner of the instrument cluster overlay, adjacent to the "Empty" graduation of the fuel gauge. The low fuel indicator lamp consists of a International Control and Display Symbol icon for "Fuel". This icon is a stenciled cutout in the opaque layer of the instrument cluster overlay. The dark outer layer of the overlay prevents the icon from being clearly visible when the lamp is not illuminated. However, an amber lens behind the cutout in the opaque layer of the overlay causes the icon to appear in amber through the translucent outer layer of the overlay when it is illuminated from behind by a replaceable incandescent bulb and bulb holder unit located on the instrument cluster electronic circuit board. The low fuel indicator lamp is serviced as a unit with the instrument cluster.

The low fuel indicator lamp gives an indication to the vehicle operator when the level of fuel in the fuel tank becomes low. This lamp is controlled by the instrument cluster circuit board based upon the cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Chrysler Collision Detection (CCD) data bus. The low fuel indicator lamp bulb receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (st-run) circuit whenever the ignition switch is in the ON or Start positions; therefore, the lamp will always be OFF when the ignition switch is in any position except ON or Start. The lamp bulb only illuminates when it is switched to ground by the instrument cluster circuitry. The instrument cluster will turn ON the low fuel indicator lamp for the following reasons:
- Bulb Test - Each time the ignition switch is turned to the ON position the lamp is illuminated for about three seconds.
- Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating that the percent tank full of the fuel tank equals about one-eighth full, the instrument cluster low fuel circuitry monitors the incoming percent tank full messages for ten seconds. If no percent tank full messages above one-eighth full are received during that ten seconds, the lamp will be illuminated. If any above one-eighth full messages are received during that monitored period, the ten second delay counter returns to zero. Once the lamp is illuminated, each time the instrument cluster low fuel circuitry receives a message indicating that the percent tank full of the fuel tank is above one-eighth full, it monitors the incoming percent tank full messages from the PCM for twenty seconds. If no percent tank full messages of one-eighth or less are received during that twenty seconds, the lamp will be turned OFF. If any one-eighth full or below messages are received during that monitored period, the twenty second delay counter returns to zero. This strategy is intended to reduce the effect that fuel sloshing within the fuel tank can have on reliable lamp indications. The lamp is also turned OFF and the delay counters are returned to zero when the ignition switch is turned to the OFF position.
- Actuator Test - Each time the cluster is put through the actuator test, the lamp will be turned ON and OFF in a prescribed sequence to confirm the functionality of the lamp and the cluster circuitry.

The PCM continually monitors the fuel tank sending unit, then sends the proper messages to the instrument cluster. If the low fuel indicator fails to light during the bulb test, replace the bulb with a known good unit. For further diagnosis of the low fuel indicator lamp or the instrument cluster circuitry that controls the lamp, (Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING). For proper diagnosis of the PCM, the CCD data bus, or the message inputs to the instrument cluster that control the low fuel indicator, a DRB III scan tool and the appropriate diagnostic information are required.