RAM 2500 Truck 2WD L6-5.9L DSL Turbo VIN 7 (2001)
Tachometer: Description and Operation
A tachometer is standard equipment on all instrument clusters. The tachometer is located just to the left of the speedometer near the center of the
instrument 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 that reads left-to-right either from 0 to 4 for these engines. The text "RPM X 1000" imprinted on the cluster overlay directly
below the hub of the tachometer needle identifies that each number on the tachometer scale is to be multiplied times 1000 rpm. This engine tachometer
is red lined at 3375 rpm. This engine tachometer also includes text that specifies "DIESEL FUEL ONLY' located just above the hub of the tachometer
needle. The tachometer graphics are white and red against a black field, 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 white graphics appear
blue-green, while 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 tachometer is serviced as a unit with the
instrument cluster.
The tachometer gives an indication to the vehicle operator of the engine speed. This gauge 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 tachometer is an air core magnetic unit that 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. 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:
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Message Failure - If the cluster fails to receive an engine speed message, it will hold the gauge needle at the last indication for about four seconds,
or until the ignition switch is turned to the OFF position, whichever occurs first. If a new engine speed message is not received after about four
seconds, the gauge needle will return to the far left (low) end of the scale.
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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 crankshaft position sensor to determine the engine speed, then sends the proper engine speed messages to the
instrument cluster. For further diagnosis of the tachometer or the instrument cluster circuitry that controls the gauge, (Refer to INSTRUMENT
CLUSTER - DIAGNOSIS AND TESTING). For proper diagnosis of the crankshaft position sensor, the PCM, the CCD data bus, or the message inputs
to the instrument cluster that control the tachometer, a DRB III scan tool is required. Refer to the appropriate diagnostic information.
