GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2003 🢒 Blazer - 2WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 DTC P0502

Object Number: 728986 Size: MF
Master Electrical Component List
Manual Transmission Schematics
OBD II Symbol Description Notice

Circuit Description

Vehicle speed information is provided to the powertrain control module (PCM) by the vehicle speed sensor (VSS). The VSS is a permanent magnet generator that is mounted on the transmission and produces a pulsing voltage when vehicle speed is over 5 km/h (3 mph). The AC voltage amplitude and frequency increases with vehicle speed. The PCM converts the pulsing voltage into km/h (mph). The PCM supplies the necessary signal to the instrument panel for speedometer, odometer operation and to the cruise control and multi-function alarm module.

If the PCM detects no vehicle speed for a specified length of time while other sensors indicate that the vehicle is moving, DTC P0502 sets. DTC P0502 is a type B DTC.

Conditions for Running the DTC
    • No Manifold Absolute Pressure (MAP) Sensor DTCs P0106, P0107 or P0108.
    • No Throttle Position (TP) Sensor DTCs P0121, P0122 or P0123.
    • Engine speed is 1,700- 3,600 RPM.
    • Throttle position is less than 1 percent.
    • Engine vacuum is 70-80 kPa (10-12 psi).
    • The above conditions are met for 5 seconds.

Conditions for Setting the DTC

Vehicle speed is less than 3 km/h (2 mph).

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 disables Cruise Control.
    • The PCM commands the coolant fan ON.
    • The PCM records the operating conditions at the time when the Conditions for Setting the DTC are met. The PCM stores this information as Freeze Frame and Failure Records.
    • The PCM stores P0502 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.
    • The PCM cancels the DTC default actions when the fault no longer exists and the DTC passes.
    • The PCM clears the DTC from PCM history if the vehicle completes 40 warm-up cycles without an emission related diagnostic fault occurring.
    • A scan tool can clear the MIL/DTC.

Test Description

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

  1. The resistance measurement will not change if either the VSS high signal circuit or the VSS low signal circuit, but not both, is shorted to ground. The vehicle speed detector in the PCM and the VSS are matched in such a way that an open or a short to ground in the VSS low signal circuit will not usually cause a loss of speed signal or a DTC P0502 to set.

    2.

  2. This step isolates the short between the VSS and the wiring.

    3.

  3. The DMM will detect AC voltage if the VSS high signal circuit is shorted to ground.

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls in Engine Controls - 2.2L

2

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

  1. Install a scan tool.
  2. Turn ON the ignition with the engine OFF.

    3. Important: Record the Failure Records before clearing the DTCs. Using the Clear Info function erases the Failure Records from the PCM.

  3. Record the DTC Freeze Frame and Failure Records.
  4. Clear the DTC.
  5. Raise the drive wheels.
  6. Start the engine.
  7. Allow the engine to idle in gear.

Does the scan tool display vehicle speed above the specified value?

0 km/h (0 mph)

Go to Step 3

Go to Step 4

3
  1. Turn OFF the ignition.
  2. Turn ON the ignition with the engine OFF.
  3. Review the Freeze Frame data and note the parameters.
  4. Start the engine.
  5. Operate the vehicle within the Freeze Frame conditions, and the Conditions For Running The DTC.

Does the scan tool display vehicle speed above the specified value?

0 km/h (0 mph)

Go to Intermittent Conditions in Engine Controls - 2.2L

Go to Step 4

4
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Measure the resistance between terminal C1 63 and C2 62.

Is the resistance within the specified range?

966-2200 ohms

Go to Step 7

Go to Step 6

5

Test the VSS high signal and VSS low signal circuits for the following conditions:

    • An open circuit or high resistance condition.
    • VSS high signal and VSS low signal circuits shorted together.

Refer to Circuit Testing in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 14

Go to Step 11

6

Measure the resistance between terminal C1 63 and ground.

Is the resistance greater than the specified value?

50 K ohms

Go to Step 10

Go to Step 8

7
  1. Leave the DMM connected between terminal C1 63 and ground.
  2. Disconnect the VSS.
  3. Measure the resistance between terminal C1 63 and ground.

Is the resistance greater than the specified value?

50 K ohms

Go to Step 11

Go to Step 9

8

Repair the short to ground in the VSS high signal circuit.

Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 14

--

9
  1. Connect the DMM between terminal C1 63 and C2 62.
  2. Using the DMM, measure the AC voltage while rotating the drive wheels by hand.

Is the AC voltage equal to or greater than the specified value?

0.5 V

Go to Step 13

Go to Step 11

10

  1. Remove the VSS. Refer to Vehicle Speed Sensor Replacement .
  2. Inspect the VSS and the VSS rotor for the following conditions:
    3. • VSS damage
    • VSS rotor damage
    • Excessive air gap between the VSS rotor and the VSS
    • Incorrect alignment between the VSS and the VSS rotor
  3. Repair or replace any of the above items as necessary.

Did you find and correct a condition?

--

Go to Step 14

Go to Step 12

11

Replace the VSS. Refer to Vehicle Speed Sensor Replacement .

Did you complete the repair?

--

Go to Step 14

--

12

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls - 2.2L.

Is the action complete?

--

Go to Step 14

--

13

Perform the following procedure in order to verify the repair:

  1. Select DTC.
  2. Select Clear Info.
  3. Operate the vehicle and ensure that the vehicle speed sensor is greater than 3 km/h (2 mph) for 2 seconds.
  4. Select Specific DTC.
  5. Enter DTC P0502.

Has the test run and passed?

--

Go to Step 14

Go to Step 2

14

With the scan tool, observe the stored information, capture info and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List in Engine Controls - 2.2L

System OK