P0608

DTC P0608

DTC DESCRIPTOR

DTC P0608: Vehicle Speed Output Circuit.




DIAGNOSTIC FAULT INFORMATION

IMPORTANT: Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Testing and Inspection

CIRCUIT/SYSTEM DESCRIPTION
The instrument panel cluster (IPC) displays the vehicle speed based on the information from the engine control module (ECM). The ECM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The ECM then sends the vehicle speed signal information via the GMLAN serial data circuit to the IPC. The IPC displays the vehicle speed based on the GMLAN serial data message from the ECM.

CONDITIONS FOR RUNNING THE DTC
- The ignition is ON, with the engine running.
- The system voltage is between 9-16 volts.
- The engine speed is more than 400 RPM.

CONDITIONS FOR SETTING THE DTC
- The ECM detects that the commanded state of the driver and the actual state of the signal do not match.
- The above condition is present for greater than 5 seconds.

ACTION TAKEN WHEN THE DTC SETS
The ECM records the operating conditions at the time that the diagnostic test fails. The ECM displays the failure information in the Failure Records on the scan tool.

CONDITIONS FOR CLEARING THE DTC
- The DTC becomes history when the conditions for setting the DTC are no longer present.
- The history DTC clears after 40 malfunction free warm-up cycles.
- The ECM receives a clear code command from the scan tool.

CIRCUIT/SYSTEM TESTING
1. Install a scan tool.
2. Raise the vehicles drive wheels.
3. Start the engine.
4. Place the transmission into drive.
5. With the scan tool, observe the Vehicle Speed Sensor (VSS) parameter in the ECM Engine Data list.
- If the vehicle speed sensor parameter matches the speedometer display, check for a poor connection at the harness connector of the ECM.
6. Turn OFF the ignition.
7. Disconnect the ECM connector.
8. With the ignition ON, measure the voltage from the vehicle speed signal circuit to a good ground.
- If the voltage measured is more than 9 volts, test the vehicle speed signal circuit for a short to battery voltage.
- If the vehicle speed circuit tests normal, replace the ECM.
- If the voltage measured is less than 9 volts, test the vehicle speed signal circuit for an open, a high resistance or for a short to ground.
- If the circuit tests normal, replace the IPC.

REPAIR INSTRUCTIONS

IMPORTANT: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for IPC and ECM replacement, setup, and programming