Tachometer: Description and Operation

A tachometer is only used in the high-line instrument clusters. The tachometer is located just to the left of center in 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 cluster overlay that reads left-to-right from 0 to 6000 rpm. The gauge needle and gauge scale are clearly visible on the face of the instrument cluster in daylight, and are illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting when the exterior lamps are turned ON. Cluster 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 gauge 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:
- 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.
- 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 to confirm the functionality of the gauge and the cluster circuitry.

The PCM continually monitors the crankshaft position sensor, then sends the proper 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 PCM, the CCD data bus, or the message inputs to the instrument cluster that control the tachometer, a DRB III scan tool and the appropriate diagnostic information are required.