For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC P0502: Vehicle Speed Sensor (VSS) Circuit Low Voltage
DTC P0503: Vehicle Speed Sensor (VSS) Circuit Intermittent

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
VSS Low Signal | -- | P0502, P0503 | -- | --
VSS High Signal | P0502, P0503 | P0502, P0503 | P0502, P0503 | --

### Circuit/System Description

The vehicle speed information is provided to the engine control module (ECM) by the vehicle speed sensor (VSS). The VSS is a permanent magnet generator that is mounted to the transmission and produces a pulsing voltage. The AC voltage amplitude and frequency increases with vehicle speed. The ECM converts the pulsing voltage into km/h (mph). The ECM supplies the VSS signal to the following components:

• | The instrument panel for speedometer and odometer operation

• | The cruise control module

### Conditions for Running the DTC

#### P0502

• | DTC P0120, P0121 or P0503 is not set.

• | The TP sensor angle is more than 12 percent.

• | The engine speed is 1,000-6,800 RPM for 8 seconds.

• | The system voltage is 10-18V.

• | Engine torque is 40-300 N·m (30-221 lb ft).

#### P0503

• | DTC P0502 is not set.

• | The engine speed is 450-6,800 RPM for 8 seconds.

• | The system voltage is 10-18V.

• | The time since the last gear change is 6 seconds or more.

• | The engine speed change is more than 500 RPM in 2 seconds.

### Conditions for Setting the DTC

#### P0502

The transmission output speed is 100 RPM or less for 3 seconds.

#### P0503

The transmission output speed drops 1,500 RPM in less than 0.5 seconds.

### Action Taken When the DTC Sets

• | P0502 and P0503 are Type B DTCs.

• | Cruise control is disabled.

### Conditions for Clearing the DTC

P0502 and P0503 are Type B DTCs.

### Reference Information

#### Schematic Reference

Manual Transmission Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Transmission System Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References

#### Special Tools

J 35616 GM-Approved Terminal Test Kit

### Circuit/System Verification

1. Raise and support the drive wheels.

Notice: Support the lower control arms in the normal horizontal position in order to avoid damage to the drive axles. Do not operate the vehicle in gear with the wheels hanging down at full travel.

2. Support the lower control arms so that the drive axles are in a horizontal position.
3. Start and idle the engine.
4. Place the transmission in second gear and release the clutch pedal.
5. Monitor VSS parameter on the scan tool. The VSS parameter should increase as the drive wheel speed increases.
6. Operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

### Circuit/System Testing

1. Disconnect the ECM harness connector X1.
2. Measure for 1,300-1,500 ohms at 25°C (77°F) between the VSS high signal circuit terminal 54 and the VSS low signal circuit terminal 34.

⇒ | If the value is greater than 1,500 ohms test the VSS high signal circuit and the VSS low signal circuit for an open/high resistance. If the circuits/connections test normal, replace the vehicle speed sensor.

⇒ | If the value is less than 1,300 ohms test the VSS high signal and the VSS low signal circuits for a shorted together condition. If the circuits/connections test normal, replace the vehicle speed sensor.

3. Measure for more than 10 ohms of resistance between the VSS high signal circuit terminal 54 and ground.

⇒ | If the value is less than the specified range, repair the short to ground.

4. Measure for more than 10 ohms of resistance between the VSS low signal circuit terminal 34 and ground.

⇒ | If the value is less than the specified range, repair the short to ground.

5. Reconnect the ECM connector. Disconnect the VSS connector.
6. Ignition OFF, test for less than 5 ohms between the low reference circuit at the VSS harness connector terminal A and ground.

⇒ | If greater than the specified value, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

7. Ignition ON, test for less than 6 V between the signal circuit at the VSS harness connector terminal B and ground.

⇒ | If greater than the specified value, test the signal circuit for a short to voltage. If the circuit test normal, replace the ECM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | Vehicle Speed Sensor Replacement

• | Control Module References for ECM replacement, setup and programming.