TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instrument clusters. The tachometer is located next to the speedometer, just to the right 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 255 degree scale on the gauge dial face that reads left-to-right from 0 to 7 for gasoline engines. On vehicles with a diesel engine, the scale reads from 0 to 5. The text RPM X 1000 imprinted on the cluster overlay directly above the hub of the tachometer needle identifies that each number on the tachometer scale is to be multiplied by 1000 rpm.
The tachometer text is white against a black field, while the major and minor scale increments are black against a silver field. A light gray inner secondary scale provides a balanced appearance between the tachometer and speedometer, but is purely cosmetic. All text and increments are clearly visible within the instrument cluster in daylight. The gasoline engine tachometer has red increments designating the engine red line beginning at 5800 rpm, while the red increments for the diesel engine tachometer begin at 4000 rpm. When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the exterior lamps turned ON, all text and major increments appear blue-green, while the red major increments still appear red. The minor increments and the secondary scale are not illuminated. The red gauge needle has internal optical illumination. 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.
OPERATION
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 cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) 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 electronic engine speed message from the PCM it will calculate the correct engine speed reading and position the gauge needle at that relative speed position on the gauge scale. The cluster will receive a new message and reposition the gauge pointer accordingly about every 86 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 a speed message, it will hold the gauge needle at the last indication for about six seconds, or until the ignition switch is turned to the OFF position, whichever occurs first. After six 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 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.
The PCM continually monitors the crankshaft position sensor to determine the engine speed. The PCM then sends the proper engine speed messages to the ElectroMechanical Instrument Cluster (EMIC).
For proper diagnosis of the crankshaft position sensor, the PCM, the EMIC, the PCI data bus or the electronic communication related to tachometer operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.