Built Before 6/17/96

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION
The Vehicle Speed Sensor (VSS) is a Permanent Magnet (PM) generator attached to the transmission case extension. The final drive assembly has a toothed wheel (VSS rotor). As the final drive assembly rotates, an AC voltage is induced and transmitted to the Powertrain Control Module (PCM).

The PCM converts this AC voltage into a digital signal. The PCM uses the vehicle speed to determine the shift timing, the TCC apply and release, and the gear ratio calculations. The VSS voltage will vary from 0.5 volts AC at 100 RPM to more than 100 volts AC at 6000 RPM. The VSS resistance value is 981-1864 ohms.

If the PCM detects a large drop in the vehicle speed, then DTC P0503 sets. DTC P0503 is a type A DTC.

CONDITIONS FOR SETTING THE DTC

^ The engine speed is more than 500 RPM for 5 seconds.
^ The engine is not in fuel shut off.
^ The time since the last gear range change is greater than 6 seconds.
^ No output speed rise greater than 1000 RPM within 2 seconds.
^ All of the above must be true and one of the following fail cases must be true:

- Fail Case 1: The transmission is NOT in Park or Neutral, and the output speed drops more than 1500 RPM within 2 seconds.
- Fail Case 2: The transmission is in Park or Neutral and the output speed drops more than 2050 RPM within 4 seconds.

ACTION TAKEN WHEN THE DTC SETS

^ The transmission defaults to 3rd gear.
^ The PCM inhibits TCC operation.
^ The PCM illuminates the Malfunction Indicator Lamp (MIL).

Conditions for Clearing the MIL/DTC

^ The PCM turns OFF the MIL after three consecutive ignition cycles without a failure reported.
^ A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.
^ The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

DIAGNOSTIC AIDS

^ Check for PRNDL DTC P0705. DTC P0705 affects transmission operation. This may set DTC P0503 falsely.
^ Check for Electromagnetic Interference (EMI) in circuits 400 and 401. EMI may be induced if the harness is routed along the spark plug wires.
^ Inspect the wiring for poor electrical connections at the PCM. Inspect the wiring at the TCC brake switch. Look for the following problems:

- A bent terminal.
- A backed out terminal.
- A damaged terminal.
- Poor terminal tension.
- A chafed wire.
- A broken wire inside the insulation.

^ When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ Ensure that the VSS is securely tightened to the case extension.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.

2. This Step verifies that the PCM is receiving a signal from the vehicle speed sensor.
3. This Step verifies that there are no opens in the circuitry from the vehicle speed sensor to the PCM connector.
5. This Step performs a resistance test on the VSS circuits from the PCM connector.