Tachometer

OPERATION
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:
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.
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.
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 DIAGNOSIS AND TESTING - INSTRUMENT CLUSTER ). 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(R) scan tool is required. Refer to the appropriate diagnostic information.

PRELIMINARY DIAGNOSIS
WARNING:ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE SUPPLEMENTAL RESTRAINT SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, DRIVER AIRBAG, PASSENGER AIRBAG, SEAT BELT TENSIONER, SIDE CURTAIN AIRBAG, FRONT IMPACT SENSOR, SIDE IMPACT SENSOR, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE SUPPLEMENTAL RESTRAINT SYSTEM. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

WARNING:ON VEHICLES EQUIPPED WITH THE PREMIUM INSTRUMENT CLUSTER, THE CLUSTER CIRCUITRY PROVIDES AN ALTERNATING CURRENT TO SUPPLY POWER TO THE ELECTRO-LUMINESCENT ILLUMINATION LAMP THROUGH A PIGTAIL WIRE AND CONNECTOR THAT IS ACCESSIBLE AT THE BACK OF THE CLUSTER HOUSING. USE PROPER PRECAUTIONS WHEN HANDLING THIS UNIT DURING DIAGNOSIS OR SERVICE TO AVOID ELECTRICAL SHOCK AND POSSIBLE PERSONAL INJURY.

Check the fused B(+) fuse (Fuse 17 - 10 ampere) in the Junction Block (JB). If OK, go to Step 2. If not OK, repair the shorted circuit or component as required and replace the faulty fuse.
Check for battery voltage at the fused B(+) fuse (Fuse 17 - 10 ampere) in the JB. If OK, go to Step 3. If not OK, repair the open fused B(+) circuit between the JB and the Power Distribution Center (PDC) as required.
Check the fused ignition switch output (run-start) fuse (Fuse 22 - 10 ampere) in the JB. If OK, go to Step 4. If not OK, repair the shorted circuit or component as required and replace the faulty fuse.
Turn the ignition switch to the On position. Check for battery voltage at the fused ignition switch output (run-start) fuse (Fuse 22 - 10 ampere) in the JB. If OK, go to Step 5. If not OK, repair the open fused ignition switch output (run-start) circuit between the JB and the ignition switch as required.
Turn the ignition switch to the Off position. Disconnect and isolate the battery negative cable. Remove the instrument cluster. Reconnect the battery negative cable. Check for battery voltage at the fused B(+) circuit cavity of the instrument panel wire harness connector for the instrument cluster. If OK, go to Step 6. If not OK, repair the open fused B(+) circuit between the instrument cluster and the JB as required.
Turn the ignition switch to the On position. Check for battery voltage at the fused ignition switch output (run-start) circuit cavity of the instrument panel wire harness connector for the instrument cluster. If OK, go to Step 7. If not OK, repair the open fused ignition switch output (run-start) circuit between the instrument cluster and the JB as required.
Turn the ignition switch to the Off position. Disconnect and isolate the battery negative cable. Check for continuity between each of the ground circuit cavities of the instrument panel wire harness connector for the instrument cluster and a good ground. There should be continuity. If OK, refer to the ACTUATOR TEST. If not OK, repair the open ground circuit(s) between the instrument cluster and ground (G200) as required.

ACTUATOR TEST
WARNING:ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE SUPPLEMENTAL RESTRAINT SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, DRIVER AIRBAG, PASSENGER AIRBAG, SEAT BELT TENSIONER, SIDE CURTAIN AIRBAG, FRONT IMPACT SENSOR, SIDE IMPACT SENSOR, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE SUPPLEMENTAL RESTRAINT SYSTEM. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

WARNING:ON VEHICLES EQUIPPED WITH THE PREMIUM INSTRUMENT CLUSTER, THE CLUSTER CIRCUITRY PROVIDES AN ALTERNATING CURRENT TO SUPPLY POWER TO THE ELECTRO-LUMINESCENT ILLUMINATION LAMP THROUGH A PIGTAIL WIRE AND CONNECTOR THAT IS ACCESSIBLE AT THE BACK OF THE CLUSTER HOUSING. USE PROPER PRECAUTIONS WHEN HANDLING THIS UNIT DURING DIAGNOSIS OR SERVICE TO AVOID ELECTRICAL SHOCK AND POSSIBLE PERSONAL INJURY.

The instrument cluster actuator test will put the instrument cluster into its self-diagnostic mode. In this mode the instrument cluster can perform a self-diagnostic test that will confirm that the instrument cluster circuitry, the gauges, the PCI data bus message controlled indicators, and the electro-luminescent illumination lamp (if equipped) are capable of operating as designed. During the actuator test the instrument cluster circuitry will sweep each of the gauge needles across the gauge faces, illuminate each of the segments in the Vacuum-Fluorescent Display (VFD), turn all of the PCI data bus message-controlled indicators on and off again, and turn the electro-luminescent illumination lamp (if equipped) on and off again.
Successful completion of the actuator test will confirm that the instrument cluster is operational. However, there may still be a problem with the PCI data bus, the Powertrain Control Module, the Airbag Control Module (ACM), the Body Control Module (BCM), the Controller Anti-lock Brake (CAB), the Sentry Key Immobilizer Module (SKIM), or the inputs to one of these electronic control modules. Use a DRBIII(R) scan tool to diagnose these components. Refer to the appropriate diagnostic information.
If an individual indicator lamp or the electro-luminescent illumination lamp do not illuminate during the actuator test, the instrument cluster should be removed. However, check that the incandescent lamp bulb is not faulty, that the bulb holder is properly installed on the instrument cluster electronic circuit board, or that the electro-luminescent lamp pigtail wire connector is properly connected to the instrument cluster electronic circuit board before considering instrument cluster replacement. If the bulb and bulb holder, or the electro-luminescent lamp connection check OK, replace the faulty instrument cluster unit.
Begin the test with the ignition switch in the Off position.
Depress the odometer/trip odometer switch button.
While still holding the odometer/trip odometer switch button depressed, turn the ignition switch to the On position, but do not start the engine.
Release the odometer/trip odometer switch button.
The instrument cluster will automatically begin the actuator test sequence, as follows:
The cluster will turn on, then off again each of the PCI data bus message controlled indicators (except Airbag) to confirm the functionality of the indicator and the cluster control circuitry:
The cluster will sweep the needles for each of the gauges from minimum to maximum and back to minimum to confirm the functionality of the gauge and the cluster control circuitry:
Only on models with a premium version of the cluster, the cluster will illuminate the electroluminescent lamp and turn it off again to confirm the functionality of the lamp and the cluster control circuitry.
The cluster will sequentially step the odometer/trip odometer VFD display from all zeros (000000) through all nines (999999) to confirm the functionality of all VFD segments and their control circuitry, then display the software version number, followed by "DONE".
The actuator test is now completed. The instrument cluster will automatically exit the self-diagnostic mode and return to normal operation at the completion of the test, if the ignition switch is turned to the Off position during the test, or if a vehicle speed message indicating that the vehicle is moving is received from the PCM over the PCI data bus during the test.
Go back to Step 1 to repeat the test, if required.

Then it has to be the (VSS) Vehicle Speed Sensor

INPUT & OUTPUT SPEED SENSORS
Removal & Installation - 545RFE Transmission
Raise vehicle. Place drain pan under transmission. Disconnect appropriate sensor harness connector. See Fig. 24. Remove sensor mounting bolt and sensor. To install, reverse removal procedure. Tighten sensor mounting bolt to specification. See TORQUE SPECIFICATIONS. Lower vehicle. Fill transmission to proper level with appropriate fluid. See DRAINING & REFILLING under LUBRICATION.