Circuit Description
The powertrain control module (PCM) creates the vehicle speed output signal by pulsing the circuit to ground. The PCM monitors the voltage on the vehicle speed output circuit. If the PCM determines that the voltage is out of the normal operating range, a DTC sets.
Various components apply a voltage to the vehicle speed output circuit. The PCM creates the vehicle speed output signal by rapidly grounding this circuit via an internal switch called a driver. The driver operates at the same rate as the vehicle speed sensor (VSS) signal input. The various components recognize the voltage being pulled to ground as an indication of vehicle speed. The PCM driver supplies the ground for the component being controlled. Each driver has a fault line which the PCM monitors. When the PCM commands a component ON, the voltage of the control circuit should be LOW (near 0 volts). When the PCM commands the control circuit to a component 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 DTC P0608 to set.
Conditions for Running the DTC
| • | The engine speed is more than 400 RPM. |
| • | The ignition voltage is between 6 volts and 18 volts. |
Conditions for Setting the DTC
| • | The PCM detects that the commanded state of the driver and the actual state of the signal circuit do not match. |
| • | The above condition must be present for a minimum of 5 seconds. |
Action Taken When the DTC Sets
| • | The PCM will not illuminate the malfunction indicator lamp (MIL). |
| • | The PCM records the operating conditions at the time the diagnostic fails. |
| • | The PCM stores this information in the Failure Records. |
| • | The multifunction display (MFD) in the instrument cluster will display a message at key-on and key-off. |
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
| • | Using the Failure Records data may help locate an intermittent condition. If you cannot duplicate the DTC, the information in the Failure Records can help in determining how many miles since the DTC set. The Fail Counter and the Pass Counter can help determine how many ignition cycles that the diagnostic test reported a pass and/or a fail. |
| • | The PCM 5-volt reference circuits are internally connected within the PCM. If all the Engine Oil sensor circuits check to be OK, inspect related 5-volt reference circuits. Refer to DTC P0641 |
| • | If there is a failure with the Check Oil indicator, more than likely it is an instrument cluster fault. |
| • | Using the Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Failure Records data can aid in determining the distance travelled since the DTC reported a pass and/or fail. Operate the vehicle within the same Failure Records conditions (RPM, load, vehicle speed, temperature, etc.) that you observed. This will isolate when the DTC failed. |
| • | For an intermittent, refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
| • | Disconnecting the cruise control module will isolate the cruise control system from the rest of the vehicle systems. |
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Instrument Cluster Schematics | |||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |
2 |
Does the Vehicle Speed parameter match the speedometer display? | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 3 |
3 |
Does the vehicle speedometer indicate approximately 55 MPH? | Go to Step 5 | Go to Step 4 |
4 | Test the vehicle speed signal circuit for an open, for a short to voltage, or for a short to ground. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 9 | Go to Step 6 |
5 | Inspect for poor connections at harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 9 | Go to Step 7 |
6 | Inspect for poor connections at the harness connector of the instrument panel cluster (IPC). Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 9 | Go to Step 8 |
7 |
Important: Program the replacement PCM. Replace the PCM. Refer to Engine Control Module Replacement . Did you complete the replacement? | Go to Step 9 | -- |
8 |
Important: Perform the set up procedure for the replacement IPC. Replace the IPC. Refer to Instrument Cluster Replacement . Did you complete the replacement? | Go to Step 9 | -- |
9 |
Does the DTC reset? | Go to Step 2 | System OK |
