Tachometer Inaccurate or Inoperative

Tachometer Inaccurate or Inoperative

Diagnostic Instructions

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

Diagnostic Fault Information





Circuit/System 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 IPC converts the 2 pulses per engine revolution signal to a tachometer position indicating the engine speed. The engine speed is sent to the instrument panel cluster (IPC) over a discreet circuit from the ECM. The tachometer signal is pulled up to B+ in the instrument panel cluster (IPC). The ECM monitors the voltage at the engine speed signal circuit to determine when the voltage is out of the normal operating range.

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)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

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

Circuit/System Verification

Start the engine. Verify with the scan tool that the tachometer display matches the scan tool Engine Speed parameter in the ECM Engine Data List.

Circuit/System Testing

1. Ignition ON, with the scan tool installed, perform the IPC Gage Sweep Test in the IPC Output Controls. Verify the tachometer dial moves from the minimum to the maximum position and back to the tachometer minimum position.

• If the tachometer does not perform the IPC Gage Sweep Test, replace the IPC.

2. Ignition OFF, disconnect the X1 harness connector at the engine control module.
3. Ignition ON, test for 9-11 volts between the engine speed signal circuit terminal 25 and ground.

• If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the IPC.

• If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the IPC.

4. If all circuits test normal, test or replace the ECM.

Repair Instructions

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

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