Grand Cherokee 2WD V8-4.7L VIN J (2004)
Tachometer: Description and Operation
TACHOMETER
A tachometer is standard equipment on all instrument clusters. The tachometer is located to the left of the speedometer, 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 255 degree
scale on the gauge dial face that reads left-to-right from "0" to "7" for gasoline engines, or from "0" to "6" for diesel engines. The text "X 1000" (base
cluster) or "RPM X 1000" (premium cluster) imprinted on the gauge dial face directly below the hub of the tachometer needle identifies that each
number on the tachometer scale is to be multiplied by 1000 rpm. The gasoline engine tachometer has a red zone beginning at 5800 rpm, while the red
zone for the diesel engine tachometer begins at 4200 rpm. The tachometer for the diesel engine also includes text located in the center of the gauge
dial face just above the hub of the tachometer needle that specifies the diesel fuel requirement.
The tachometer graphics are either white, gray and orange against a black gauge dial face (base cluster) or black, gray and red against a taupe gauge
dial face (premium cluster), 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 base cluster white gauge graphics appear blue-green and the
orange graphics still appear orange, while the premium cluster taupe gauge dial face appears blue-green with the black graphics silhouetted against the
illuminated background and the red graphics still appear red. The gray gauge graphics for both versions of the cluster are not illuminated. The orange
gauge needle in the base cluster gauge is internally illuminated, while the black gauge needle in the premium cluster gauge is not.
Base cluster gauge illumination is provided by replaceable incandescent bulb and bulb holder units located on the instrument cluster electronic circuit
board. Premium cluster gauge illumination is provided by an integral electro-luminescent lamp that is serviced as a unit with the instrument cluster.
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 electronic 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:
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Engine Speed Message - Each time the cluster receives an 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 engine speed message and
reposition the gauge pointer accordingly about every 86 milliseconds. The gauge needle will continue to be positioned 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.
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Communication Error - If the cluster fails to receive an engine 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.
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Actuator Test - Each time the cluster is put through the actuator test, the tachometer needle will be swept across the entire gauge scale and back 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 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 PCI data bus, or the electronic message inputs to the
instrument cluster that control the tachometer, a DRBIII scan tool is required. Refer to the appropriate diagnostic information.
