Circuit Description
The PCM creates the vehicle speed output signal by rapidly grounding the VSS output circuit via an internal switch called a driver. The driver operates at the same rate as the VSS signal input. The various modules recognize the voltage being pulled to ground as an indication of vehicle speed.
The driver supplies the ground for the module being controlled. Each driver has a fault line which the PCM monitors. When the PCM commands a module ON, the voltage of the control circuit should be low (near 0 volts). When the PCM commands the control circuit to a module OFF, the voltage potential of the circuit should be high (near battery voltage). If the fault detection circuit senses a voltage other than what is expected, the fault line status changes causing the DTC to set.
Conditions for Running the DTC
| • | The engine speed is more than 400 RPM. |
| • | The ignition voltage is between 6.0 volts and 18.0 volts. |
Conditions for Setting the DTC
| • | The PCM detects that the commanded state of the driver and the actual state of the control circuit do not match. |
| • | The condition must be present for a minimum of 5 seconds. |
Action Taken When the DTC Sets
| • | The PCM stores the DTC information into memory when the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM displays the failure information in the Failure Records on the scan tool. |
Conditions for Clearing the MIL/DTC
| • | The history DTC clears after 40 malfunction free warm-up cycles. |
| • | The DTC becomes history when the conditions for setting the DTC are no longer present. |
| • | The PCM receives the clear code command from the scan tool. |
Diagnostic Aids
| • | Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion into the PCM. |
| • | Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Schematic Reference: Instrument Cluster Schematics | ||||
1 | Did you perform the IPC Diagnostic System Check? | -- | Go to Step 2 | |
2 |
Does the vehicle speedometer indicate a vehicle speed? | -- | Go to Diagnostic Aids | Go to Step 3 |
Does the voltage measure greater than the specified value? | 9.6 V | Go to Step 6 | Go to Step 4 | |
4 | Test the vehicle speed signal circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 5 |
Test the vehicle speed signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | -- | |
6 | Inspect for poor connections at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 7 |
7 |
Important: Perform the programming procedure for the replacement PCM. Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls-5.7L. Did you complete the replacement? | -- | Go to Step 8 | -- |
8 |
Does the DTC reset? | -- | Go to Step 2 | System OK |
