RAM 3500 Truck 2WD L6-5.9L DSL Turbo VIN 7 (2002)
Low Fuel Lamp/Indicator: Description and Operation
LOW FUEL INDICATOR
A low fuel indicator is standard equipment on all instrument clusters. The low fuel indicator is located near the lower edge of the instrument cluster
overlay, to the right of center. The low fuel indicator consists of a stenciled cutout of the International Control and Display Symbol icon for "Fuel" in the
opaque layer of the instrument cluster overlay. The dark outer layer of the overlay prevents the indicator from being clearly visible when it is not
illuminated. 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 the indicator is illuminated from behind by a Light Emitting Diode (LED) soldered onto the instrument cluster electronic circuit board.
The low fuel indicator is serviced as a unit with the instrument cluster.
The low fuel indicator gives an indication to the vehicle operator when the level of fuel in the fuel tank becomes low. This indicator is controlled by a
transistor on the instrument cluster circuit board based upon 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 Light Emitting Diode (LED) 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 LED will always be OFF when the ignition switch is in any position except ON or Start. The LED only illuminates
when it is provided a path to ground by the instrument cluster transistor.
The instrument cluster will turn ON the low fuel indicator for the following reasons:
Bulb Test - Each time the ignition switch is turned to the ON position the indicator is illuminated for about two seconds as a bulb test.
Less Than 12.5 Percent Tank Full Message - Each time the cluster receives messages from the PCM indicating the percent tank full is 12.5
(one-eighth) or less for 10 consecutive seconds and the vehicle speed is zero, or for 60 consecutive seconds and the vehicle speed is greater than zero,
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 12.5 (one-eighth) for 10 consecutive seconds and the vehicle speed is zero, or for 60
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 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 low fuel indicator is illuminated immediately This message would indicate that the fuel tank sender input to the PCM is an open circuit.
Actuator Test - Each time the cluster is put through the actuator test, the indicator will be turned ON during the bulb check portion of the test to confirm
the functionality of the LED 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 low fuel
indicator or the instrument cluster circuitry that controls the LED, (Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING See: Instrument
Cluster / Carrier/Testing and Inspection). For proper diagnosis of the fuel tank sending unit, the PCM, the CCD data bus, or the message inputs to the
instrument cluster that control the low fuel indicator, a DRBIII scan tool is required. Refer to the appropriate diagnostic information.
