Tachometer Inaccurate or Inoperative

Tachometer Inaccurate or Inoperative

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using the diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

Circuit Description

The engine control module (ECM) creates the engine speed signal by pulsing the engine speed signal circuit to ground at a rate of 2 pulses per engine revolution. The instrument panel cluster (IPC) converts the engine speed signal to a tachometer position to indicate the engine speed. The IPC also receives from the ECM a serial data message indicating the engine speed. The IPC uses the serial data engine speed information from the ECM for diagnostic purposes.

Reference Information
Schematic Reference

Instrument Cluster Schematics (Instrument Cluster Schematics)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

* Instrument Cluster Description and Operation (Description and Operation)
* Engine Control Module Description (Engine Control Module Description) for the 4.2L engine
* Engine Control Module Description (Engine Control Module Description) for the 4.8L, 5.3L, 6.0L, 6.2L and 7.0L engines

Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Testing

1. Engine ON, verify the scan tool Engine Speed parameter matches the tachometer display.

• If the Engine Speed parameter does not match the tachometer display, test the signal circuit terminal A12 for an open/high resistance.

2. If the circuit tests normal, replace the IPC.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

Control Module References (Programming and Relearning) for the IPC replacement, setup, and programming