Circuit Description
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 VSS signal input. The various components recognize the voltage being pulled to ground as an indication of vehicle speed.
The 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 the DTC to set.
Conditions for Running the DTC
| • | The engine speed is greater than 600 RPM. |
| • | Ignition voltage greater than 6.0 volts but less than 16 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. |
| • | All above conditions exist for a minimum of 10.0 seconds. |
Action Taken When the DTC Sets
| • | The powertrain control module (PCM) stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | A last test failed, or the current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if failures are not reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL/DTC. |
Diagnostic Aids
Important: Remove any debris from the PCM\TAC module connector surfaces before servicing the PCM\TAC module. Inspect the PCM\TAC module connector gaskets when diagnosing/replacing the modules. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM\TAC modules.
Using Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and/or Failure Records data can aid in determining how many miles since the DTC set. The Fail Counter and Pass Counter can also aid determining how many ignition cycles the diagnostic reported a pass and/or a fail. Operate the vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This will isolate when the DTC failed.
For an intermittent, refer to Symptoms .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Important:: Do Not perform this test without supporting the lower control arms so that the drive axles are in a normal horizontal position. Running the vehicle with the wheels hanging down at full travel may damage the drive axles.
Does the vehicles speedometer indicate vehicle speed? | -- | Go to Diagnostic Aids | |
3 |
Does the vehicle's speedometer indicate vehicle speed? | -- | Further diagnosis of circuit required. Go to Body and Accessories/Wiring Systems. | |
4 | Check for faulty connections at the PCM. Did you find and correct the condition? | -- | ||
5 |
Important: Program the replacement PCM. Refer to Powertrain Control Module/Throttle Actuator Control Module Replacement . Replace the PCM. Is the action complete? | -- | -- | |
6 |
Does the scan tool indicate that this test failed? | -- | ||
7 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
