DTC P0502 Vehicle Speed Sensor (VSS) Circuit Low Input. Cadillac Concours 1997

For 1990-2009 cars
Automatic Transmission Components
Brake Switch, Solenoids, and VSS
Handling ESD Sensitive Parts Notice
OBD II Symbol Description Notice

### Circuit Description

The Vehicle Speed Sensor (VSS) is a Permanent Magnet (PM) generator attached to the transmission case. 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 the AC voltage into a digital signal. The PCM uses the vehicle speed in order to determine shift timing, Torque Converter Clutch (TCC) apply and release, and gear ratio. The VSS voltage will vary from 0.5 volts AC at 100 RPM to more than 100 volts AC at 8000 RPM. The VSS resistance value is 1300-1950 ohms.

If the PCM detects a low vehicle speed and a high Input Shaft Speed sensor signal while in a drive range, then DTC P0502 sets. DTC P0502 is a Type B DTC.

### Conditions for Setting the DTC

• | No MAF DTCs P0101, P0102 or P0103.

• | No TP sensor DTCs P0121, P0122 or P0123.

• | No A/T ISS sensor DTCs P0716 or P0717.

• | No TFP Val. Position Sw. DTC P1810.

• | The transmission is not in Park, Neutral, or Reverse.

• | The delivered torque is greater than 109 N·m (80 lb ft).

• | The TP angle is greater than 12.8°.

• | The vehicle speed is 8 km/h (5 mph) or less for 2 seconds.

• | The input shaft speed is 2000 RPM or greater.

### Action Taken When the DTC Sets

• | The PCM illuminates the Malfunction Indicator Lamp (MIL).

• | SERVICE ENGINE SOON, SPEED SENSOR FAULT displays on the Driver Information Center (DIC).

• | The PCM commands default line pressure.

• | The PCM disables steady state adapts.

• | The PCM disables upshift adapts.

• | The PCM inhibits TCC engagement.

• | The PCM calculates vehicle speed from the Automatic Transmission Input (Shaft) Speed sensor and commanded gear.

• | The PCM disables the cruise control.

• | DTC P0502 is stored in the PCM history.

### Conditions for Clearing the MIL/DTC

• | The PCM turns OFF the MIL after three consecutive trips 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

• | Inspect the wiring for poor electrical connections at the PCM. Inspect the wiring for poor electrical connections at the VSS 2-way connector. Inspect 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

- | Moisture intrusion

• | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

• | Check for Electromagnetic Interferences (EMI) induced on circuits 400 and 401 by a misrouted harness along spark plug wires.

### Test Description

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

1. This step checks for DTCs and for a vehicle speed signal. It is important to record Freeze Frame or Failure Record information before proceeding further.

2. This step has the scan tool verify that the correct calibration is in the PCM.

3. This step tests for a resistance value greater than 1950 ohms which would indicate a possible open in circuit 400 or circuit 401.

4. This step verifies that the ohmmeter displays a value less than or equal to the specified VSS value. Resistance less than 1300 implies that the circuits are shorted together. Resistance within specification implies that circuit 400 or circuit 401 is a possible short to ground.

5. This step performs a resistance check on the VSS, if no opens, shorts to ground or shorts together were found on circuit 400 or circuit 401.

#### DTC P0502 Vehicle Speed Sensor Circuit Low Input

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check

2

1. Install the Scan Tool ®.
2. With the engine OFF, turn the ignition switch to the RUN position.

Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame or Failure Records for reference. Using the Clear Info function will erase the stored Freeze Frame and Failure Records from the PCM.

3. Record the DTC Freeze Frame and Failure Records, then clear the DTC.
4. 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.

5. Start the engine and idle the engine.
6. Place the transmission in drive range.
7. Select Trans OSS RPM on the scan tool.

With the drive wheels rotating, does the OSS RPM increase when the wheel speed increases?

--

Go to Diagnostic Aids

Go to Step 3

3

1. Turn the ignition OFF.
2. Disconnect PCM connector C2.
3. Using the J 35616-A Connector Test Adapter Kit, Connect the J 39200 DMM from terminal 7 to terminal 8 of the PCM connector.
4. Select AC volts on the DMM.

Important: Place the gear select lever in the Neutral position before rotating the wheel.

5. Rotate the drive wheels and observe the display.

Is the voltage greater than the specified value?

0.5V

Go to Step 4

Go to Step 6

4 | Use the scan tool in order to check for the most current calibration ID. Refer to Service Bulletins. Does the ID number match? | -- | Go to Step 12 | Go to Step 5
5 | Update the PCM with the latest calibration. Refer to the PCM Replacement/Programming . Is the calibration update complete? | -- | Go to Step 13 | --
6 | Connect the DMM from terminal 7 to terminal 8 of the PCM connector and measure the resistance. Is the resistance greater than the specified value? | 1950ohms | Go to Step 7 | Go to Step 8

7

1. Inspect circuit 400 for an open.
2. Inspect circuit 401 for an open.

Refer to Electrical Diagnosis Section 8.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

8 | Is the resistance within (or less than) the specified range? | 1300-1950ohms | Go to Step 9 | Go to Step 10

9

1. Inspect circuit 400 or circuit 401 for a short to ground.
2. Inspect circuit 401 for a short to ground.
3. Inspect circuits 400 and 401 for a short together.

Refer to Electrical Diagnosis Section 8.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

10

1. Remove the VSS.
2. Measure the resistance from terminal A to terminal B of the VSS.

Is the resistance value within the specified range?

1300-1950ohms

Go to Step 12

Go to Step 11

11 | Replace the Vehicle Speed Sensor. Refer to VSS Replacement in On-Vehicle Service. Is the replacement complete? | -- | Go to Step 13 | --
12 | Replace the PCM. Refer to PCM Replacement/Programming in Section 6. Is the replacement complete? | -- | Go to Step 13 | --

13

In order to verify your repair, perform the following procedure:

1. Select DTC.
2. Select Clear Info.
3. Operate the vehicle under the following conditions:

• | Drive the vehicle in D4 with a throttle position greater than 13°.

• | The engine torque must be greater than 109 N·m (80 lb ft). The vehicle speed must be greater than 16 km/h (10 mph) for 2 seconds.

4. Select Specific DTC. Enter DTC P0502.

Has the test run and passed?

--

System OK

Begin the diagnosis again. Go to Step 1