P0503

Circuit Description
The vehicle speed sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case extension, and a toothed vehicle speed sensor reluctor wheel, which is pressed onto the final drive carrier assembly. As the vehicle drives forward, the vehicle speed sensor reluctor wheel also rotates. This rotation produces a variable AC signal in the pickup coil that is proportional to vehicle speed.
The AC signal generated has a frequency and amplitude (voltage). The PCM uses the frequency portion of the AC signal to calculate the vehicle speed. The system is calibrated to produce 4000 pulses per mile and it is the amount of time it takes to produce those pulses that determines the vehicle speed. The voltage portion of the signal is used for diagnostic purposes only. The voltage will vary from 0.5 volts AC at 100 RPM to more than 200 volts AC at 6000 RPM. This information is then sent to the PCM to indicate how fast the vehicle is traveling and to develop the shift pattern for the transmission. Other systems also use VSS information:
^ TCC apply and release
^ Cruise control system
^ Fuel delivery Systems
If the PCM detects a large change in vehicle speed in a short period of time, then DTC P0503 sets. DTC P0503 is a Type B DTC.

Conditions for Running the DTC
^ The transmission is not in PARK or NEUTRAL.
^ The time since the last gear range change is more than 6 seconds.
^ The engine speed is more than 500 RPM for 5 seconds and not in fuel shut off.
^ No output shaft speed rise greater than 500 RPM in 2 seconds.

Conditions for Setting the DTC
The output shaft speed drops greater than 1500 RPM in 3 seconds.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the conditions for setting the DTC are met.
^ The PCM freezes shift adapts.
^ The PCM commands maximum line pressure.
^ The PCM calculates vehicle speed from the automatic transmission input shaft speed (A/T ISS) sensor and the commanded gear.
^ The PCM stores DTC P0503 in PCM history.

Conditions for Clearing the MIL/DTC
^ The PCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
^ A scan tool clears the DTC from the PCM history.
^ The PCM clears the DTC from the PCM history if the vehicle completes 40 consecutive warm-up cycles without an emission related diagnostic fault occurring.
^ The PCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the PCM.

Diagnostic Aids
^ Inspect the wiring at the PCM, the VSS connector and all Other circuit connecting points for the following conditions:
- A backed out terminal
- A damaged terminal
- Reduced terminal tension
- A chafed wire
- A broken wire inside the insulation
- Moisture intrusion
- Corrosion
^ Inspect the engine wiring harness for pinched wires which may cause an intermittent. When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ Check for electromagnetic interference (EMI) in circuits 400 (YEL) and 401 (PPL). EMI may be induced by a misrouted wiring harness along the spark plug wires.
^ Ensure that the VSS is tightened securely into the transmission case extension.
^ Too much runout in the final drive carrier assembly may set DTC P0503. It you cannot find a condition with the VSS or its circuitry, replace the final drive carrier assembly.

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 VSS.
3. This step tests the ability of the VSS to produce an AC voltage as well as from the VSS to the PCM connector.
4. This step performs a resistance test on the VSS circuits from the PCM connector.
10. A YES answer indicates an intermittent condition.
11. A NO answer indicates an intermittent condition.