GM Service Manual Online
For 1990-2009 cars only

Circuit Description

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 in the transaxle and produces a pulsing voltage whenever vehicle speed is over 5 km/h (3 mph). The alternating current (AC) voltage level and the number of pulses increases with vehicle speed. The ECM converts the pulsing voltage into km/h (mph) and then supplies the necessary signal to the instrument panel for speedometer/odometer operation and to the cruise control module and multi-function alarm module operation. This diagnostic trouble code (DTC) will detect if vehicle speed is reasonable according to engine RPM and load.

DTC Descriptor

This diagnostic supports the following DTC:

DTC P0502 Vehicle Speed Sensor (VSS) Circuit Low Voltage

Conditions for Running the DTC

    • DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0201, P0202, P0203, P0204, P0300, P0351, P0352, P0402, P0404, P0405, P0406, and P1404 are not set.
    • The engine is running.
    • The engine coolant temperature (ECT) is more than 60°C (140°F).
    • The ignition 1 voltage is between 11-16 volts.
    • The following are additional Conditions for Running for the applicable test:

Power Test

       - The engine speed is between 1,200-4,000 RPM.
       - The throttle position (TP) sensor is between 25-60 percent.
       - The manifold absolute pressure (MAP) is more than 60 kPa.

Deceleration Test

       - The MAP is less than 30 kPa.
       - The change in engine speed per cycle is less than or equal to 50 RPM/cycle.
       - The throttle position (TP) sensor is less than or equal to 0.8 percent.
       - The engine speed is between 1,800-6,000.
    • DTC P0502 runs continuously when the above conditions are met.

Conditions for Setting the DTC

The ECM detects that the vehicle speed is less than 5 km/h (3.1 mph) for more than 35 seconds.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the third consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

    • An intermittent condition may be caused by a poor connection, rubbed-through wire insulation, or a wire that is broken inside the insulation.
    • Ensure the VSS is correctly torqued to the transmission housing.
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

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

  1. The permanent magnet generator only produces a signal if the drive wheels are turning greater than 8 km/h (5 mph). This step determines if DTC P0502 is the result of a hard failure or an intermittent condition.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Turn ON the ignition, with the engine OFF.
  2. Clear the DTCs with a scan tool.
  3. Raise and support the drive wheels.
  4. Start and idle the engine.
  5. Engage the transmission into 2nd gear.
  6. Observe the Vehicle Speed Sensor parameter with a scan tool.

Is the vehicle speed more than the specified value?

0 km/h (0 mph)

Go to Step 3

Go to Step 4

3

  1. Observe the Freeze Frame/Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn OFF the ignition.
  2. Disconnect the vehicle speed sensor (VSS) connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage at the signal circuit of the VSS with a DMM.

Is the voltage within the specified range?

9-11 V

Go to Step 5

Go to Step 7

5

Measure the voltage at the ignition 1 voltage circuit of the VSS with a DMM.

Is the voltage within the specified range?

11-14 V

Go to Step 6

Go to Step 11

6

Measure the resistance between the ground circuit of the VSS and battery ground with a DMM.

Is the resistance less than the specified value?

5 ohms

Go to Step 8

Go to Step 12

7

Test the VSS signal circuit for the following conditions:

    • An open
    • A short to ground
    • A high resistance

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 10

8

  1. Remove the VSS.
  2. Inspect the VSS and the transmission for the following conditions:
  3. • Incorrect VSS
    • VSS damage
    • Excessive VSS to speed sensor rotor gap
    • Incorrect VSS rotor alignment
    • VSS rotor damage
  4. Repair any of the above conditions as necessary.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 9

9

Test for shorted terminals and poor connections at the VSS. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

10

Test for shorted terminals and poor connections at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

11

Repair the ignition 1 voltage circuit between the VSS connector and the fuse for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 15

--

12

Repair the VSS ground circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 15

--

13

Replace the VSS.

Did you complete the replacement?

--

Go to Step 15

--

14

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

15

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 16

16

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK