PCM Controlled Tachometer Diagnosis

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Engine Controls Schematics: I/P, Generator and Driver Information Display:

CIRCUIT DESCRIPTION
The PCM supplies the ground to energize the Instrument Cluster tachometer. When the ignition switch is first turned ON the PCM grounds the tachometer circuit. The circuit will remain grounded until the PCM receives the first 24X reference pulse. Once the 24X reference pulse is received, the PCM controls the pulse frequency in relationship to engine RPM. The tachometer is controlled at a rate of two pulses per crankshaft revolution.

DIAGNOSTIC AIDS
Refer to Instrument Cluster System Check in Instrument Panel, Gauges and Console for additional diagnostic information.

Check for the following conditions:

IMPORTANT: Remove arty debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams for diagnosis and repair.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Diagrams for diagnosis and repair.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
Numbers below refer to the step numbers on the Diagnostic Table:
3. Normally, ignition teed voltage should be present on the output driver circuit with the PCM disconnected and the ignition turned on.
4. Test for a shorted component or a short to battery positive voltage on the output driver circuit. Either condition would result in a measured current of over 0.5 amps.
5. Tests for a faulty instrument cluster.
16. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.