Section 126 (Instrument Cluster): Tachometer: Operation

WARNING: This page is about a different car, the 2004 Dodge Pickup and 2004 Dodge Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

The tachometer gives an indication to the vehicle operator of the engine speed measured in Revolutions Per Minute (RPM). This gauge is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) on vehicles with a gasoline engine, or from the Engine Control Module (ECM) on vehicles equipped with a diesel engine over the Programmable Communications Interface (PCI) 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 (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:

Engine Speed Message - Each time the cluster receives an engine speed message from the PCM or ECM it will calculate the correct engine speed reading and position the gauge needle at that relative engine speed position on the gauge scale. The cluster will receive a new engine speed message and reposition the gauge pointer accordingly about every 88 milliseconds. The gauge needle will continually be repositioned at the relative engine speed position on the gauge scale until the engine stops running, or until the ignition switch is turned to the Off position, whichever occurs first.
Communication Error - If the cluster fails to receive an engine speed message, it will hold the gauge needle at the last indication for a short delay period, or until the ignition switch is turned to the Off position, whichever occurs first. The delay is about three seconds for all diesel and gasoline engines except SRT-10, and about five seconds for SRT-10. After the appropriate delay, the cluster will move the gauge needle to the low end of the gauge scale.
Actuator Test - Each time the cluster is put through the actuator test, the tachometer 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.

On vehicles with a gasoline engine, the PCM continually monitors the crankshaft position sensor to determine the engine speed. On vehicles with a diesel engine, the ECM continually monitors the engine speed sensor to determine the engine speed. The PCM or ECM 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 DIAGNOSIS AND TESTING - INSTRUMENT CLUSTER ). For proper diagnosis of the crankshaft, position sensor, the engine speed sensor, the PCM, the ECM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the tachometer, a DRBIII(R) scan tool is required. Refer to ELECTRONIC CONTROL MODULES .