Tachometer Inaccurate or Inoperative

Tachometer Inaccurate or Inoperative

Diagnostic Instructions

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

Circuit/System Verification

Start the engine with the scan tool installed. Verify 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 C1 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 9 volts, test the engine speed signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the IPC.

• If greater than 11 volts, test the engine speed 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 after completing the diagnostic procedure. Verification Tests

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