Dodge Ram Truck 1500-2500-3500. Manual - part 396
When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the exterior
lamps turned On, the black graphics appear blue-green and the blue or gray graphics appear light blue. The gauge
needle has internal optical illumination. Gauge illumination is provided by replaceable incandescent bulb and holder
units located on the instrument cluster electronic circuit board. The speedometer is serviced as a unit with the
instrument cluster.
OPERATION
The speedometer gives an indication to the vehicle operator of the relative vehicle road speed. This gauge is con-
trolled 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 speedometer is an air core magnetic unit that receives battery current on the instrument cluster electronic cir-
cuit 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:
•
Vehicle Speed Message - Each time the cluster receives a vehicle speed message from the PCM it will cal-
culate the correct vehicle speed reading and position the gauge needle at that relative speed position on the
gauge scale. The cluster will receive a new vehicle speed message and reposition the gauge pointer accord-
ingly about every 88 milliseconds. The gauge needle will continually be positioned at the relative vehicle speed
position on the gauge scale until the vehicle stops moving, or until the ignition switch is turned to the Off
position, whichever occurs first.
•
Communication Error - If the cluster fails to receive a speedometer message, it will hold the gauge needle at
the last indication for about ten seconds, or until the ignition switch is turned to the Off position, whichever
occurs first. After ten seconds, the gauge needle will return to the left end of the gauge scale.
•
Actuator Test - Each time the cluster is put through the actuator test, the speedometer 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 vehicle speed sensor or the Controller Anti-lock Brake (CAB) to determine the
vehicle road speed. The PCM then sends the proper vehicle speed messages to the instrument cluster. For further
diagnosis of the speedometer or the instrument cluster circuitry that controls the gauge, (Refer to 8 - ELECTRICAL/
INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING).
For proper diagnosis of the vehicle speed sensor, the CAB, the PCM, the PCI data bus, or the electronic message
inputs to the instrument cluster that control the speedometer, a diagnostic scan tool is required. Refer to the appro-
priate diagnostic information.
TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instrument
clusters. The tachometer is located to the left of the
speedometer, just to the left of center in the instru-
ment cluster. The tachometer consists of a movable
gauge needle or pointer controlled by the instrument
cluster circuitry and a fixed 210 degree scale on the gauge dial face. The tachometer reads left-to-right from “0” to
“7” for gasoline engines. On vehicles with a diesel engine, the scale reads from “0” to “5”.
On all models except SRT-10, the text “RPM X 1000” is imprinted on the cluster overlay directly below the hub of
the tachometer needle. On SRT-10 models the text “X 1000 RPM” is imprinted on the cluster overlay directly below
the hub of the tachometer needle. This text identifies that each number on the tachometer scale is to be multiplied
by 1000 rpm.
On all models except SRT-10 the tachometer graphics are black against a white field, making them clearly visible
within the instrument cluster in daylight. On SRT-10 the tachometer graphics are black against a silver field except
for red graduations beginning at the “6” increment and at each 250 rpm increment to the “7” at the high end of the
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INSTRUMENT CLUSTER -SERVICE INFORMATION
DR/DH