The engine control module (ECM) creates the vehicle speed output signal by pulsing the circuit to ground at a rate of 4KPPM. The VSS input is pulled high to B+ in the instrument panel cluster (IPC). The IPC converts the 4KPPM vehicle speed signal to a speedometer position to indicate the vehicle speed. The ECM monitors the voltage at the vehicle speed output signal circuit to determine when the voltage is out of the normal operating range.

Schematic Reference

Connector End View Reference

Electrical Information Reference

Scan Tool Reference

Start the engine with the scan tool installed, the vehicle drive wheels raised and the transmission placed in drive or the first gear. Refer to Lifting and Jacking the Vehicle . Verify the speedometer display matches the scan tool Vehicle Speed parameter in the ECM Engine Data List.

Important: The drive wheels of the vehicle must be raised and the transmission must be placed in drive or first gear. Refer to Lifting and Jacking the Vehicle .

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

⇒	If the speedometer does not perform the IPC Gage Sweep Test, replace the IPC.

2. Ignition OFF, disconnect the C2 harness connector at the radio.
3. Start the engine, with the drive wheels raised and the vehicle placed in drive or the first gear. Verify that the speedometer value does not match the scan tool Vehicle Speed parameter in the ECM Engine Data List.

⇒	If the speedometer matches the scan tool Vehicle Speed parameter, test the vehicle speed signal circuit terminal D for a short to voltage, a short to ground or an open/high resistance. If the circuit tests normal, replace the radio.

4. Ignition OFF, disconnect the harness connector at the head-up display (HUD).
5. Start the engine, with the drive wheels raised and the vehicle placed in drive or the first gear. Verify that the speedometer value does not match the scan tool Vehicle Speed parameter in the ECM Engine Data List.

⇒	If the speedometer matches the scan tool Vehicle Speed parameter, test the vehicle speed signal circuit terminal 3 for a short to voltage, a short to ground or an open/high resistance. If circuit tests normal, replace the HUD.

6. Ignition OFF, disconnect the C1 harness connector at the engine control module.
7. Ignition ON, test for 9-11 volts between the vehicle speed signal circuit terminal 57 and ground.

⇒	If less than 9 volts, test the vehicle speed signal circuit terminal 57 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 vehicle speed signal circuit terminal 57 for a short to voltage. If the circuit tests normal, replace the IPC.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for the ECM, IPC, HUD and radio replacement, setup and programming