Powertrain Control Module Controlled Tachometer Diagnosis

Powertrain Control Module Controlled Tachometer Diagnosis






Circuit Description

The tachometer supplies ignition positive voltage to the powertrain control module (PCM) by way of the tachometer control circuit. The crankshaft position (CKP) sensor signal indicates to the PCM that the crankshaft is rotating. The PCM controls the tachometer by momentarily grounding the tachometer control circuit and causing the voltage to pulse at the rate of two times per crank shaft revolution. The solid state circuits of the tachometer convert the voltage pulses in order to drive the tachometer pointer and display engine speed in RPM.

Diagnostic Aids

Inspect for the following conditions:

* Poor connection at the PCM or instrument cluster
Inspect harness connectors for the following conditions:

- Backed out terminals
- Improper mating
- Broken locks
- Improperly formed or damaged terminals
- Poor terminal to wire connection

* Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe a voltmeter connected to the tachometer control circuit at the PCM harness connector while moving connectors and wiring harnesses related to the tachometer control circuit. A change in voltage will indicate the location of the fault.

Test Description

Numbers below refer to the step numbers on the diagnostic table.
1. The tachometer diagnostic table will direct you back to this diagnostic table if a problem is not found elsewhere in the tachometer circuitry.
2. This test may help identify a CKP sensor problem by indicating a CKP DTC.
3. Normally, ignition positive voltage is present on the tachometer control circuit with the PCM disconnected and the ignition turned ON.