TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instrument clusters. The tachometer is located to the right of center in the instrument cluster, between the speedometer and the coolant temperature gauge. 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. On all vehicles except SRT8, the tachometer reads left-to-right from 0 to 7. On SRT8 the tachometer reads left-to-right from 0 to 8. On all vehicles except SRT8, the text RPM X 1000 imprinted on the cluster overlay directly above the hub of the tachometer needle. On SRT8 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 vehicles except SRT8 the tachometer graphics are white against a black field except for several red graduations at the high end of the gauge scale. On SRT8 the tachometer graphics are gray against a black field. SRT8 also has a red zone that is designated by a red dot matrix on the dial face that radiates wedge-like from the hub of the gauge needle to cover from about the 6 to the 8 area at the high end of the scale, and each of the graduations within that area of the scale are red. The SRT8 tachometer also includes the text SRT 8 directly above the hub of the tachometer needle. In either case, the gauge graphics are clearly visible within the instrument cluster in daylight.
When illuminated from behind by the panel lamps dimmer controlled cluster general illumination lighting with the exterior lamps turned ON, the white text and graphics still appear white, the gray text and graphics still appear gray. The red gauge needle has internal optical illumination. Gauge illumination is provided by Light Emitting Diode (LED) units soldered onto 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 Controller Area Network (CAN) data bus.
The tachometer is driven by an electronic stepper motor 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 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 a speed message, it will hold the gauge needle at the last indication for about three seconds, or until the ignition switch is turned to the OFF position, whichever occurs first. After three 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 CAN data bus or the electronic communication related to tachometer operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.