For 1990-2009 cars

### Circuit Description

The Vehicle Speed Sensor (VSS) (Transmission) provides the Powertrain Control Module (PCM) with an AC voltage input whenever the drive wheels rotate. The frequency of the AC voltage signal increases with wheel speed. The PCM calculates the vehicle speed from the frequency of the AC signal. The PCM uses the above information in order to determine shift points.

### Conditions for Setting the DTC

• | When the 4L switch is open, no VSS (transmission) signal is received when the VSS (speedometer) signal is received.

• | When the 4L switch is open, no VSS (transmission) signal is received when the engine runs at higher than torque converter stall speed in DRIVE range. Torque converter stall speed is 2300-2600 RPM.

• | Two driving cycle detection logic

### Action Taken When the DTC Sets

• | DTC P0720 stores.

• | The PCM enters fail-safe mode.

• | The Malfunction Indicator Lamp (MIL) turns ON.

The PCM uses the VSS (speedometer) for shift control during fail-safe mode operation.

### Diagnostic Aids

Inspect for the following conditions:

• | A faulty VSS (transmission)

• | Faulty electrical circuits between the VSS (transmission) and the PCM

• | A faulty PCM

The following conditions may cause an intermittent:

• | A reduced connection

• | Rubbed through wire insulation

• | A broken wire inside the insulation

Inspect the PCM harness connectors for the following conditions:

• | Backed out terminals

• | Improper mating

• | Broken locks

• | Improperly formed or damaged terminals

• | Reduced terminal-to-wire connections

If DTC P0720 cannot duplicate, the information included in the Freeze Frame data may be useful in determining the vehicle operating conditions when the DTC set.

Refer to Wiring Repairs in Wirng Systems when repairing any wiring.

### DTC Confirmation Procedure

1. Turn the ignition switch to the LOCK position.
2. Connect the scan tool to the Data Link Connector (DLC).
3. Turn the ignition switch ON.
4. Use the scan tool in order to clear the following items:

• | DTC P0720

• | Any pending DTCs

5. Start the engine.
6. Shift the transfer case to the 2H position.
7. Drive the vehicle to at least 40 km/h (25 mph) in Drive (D) range for at least 10 seconds.
8. Stop the vehicle.
9. Use the scan tool in order to test for a pending DTC.

### Test Description

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

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. The scan tool creates an electronic copy of the data taken when the fault occurred. The information then stores in the scan tool for later reference.

2. This step tests the internal resistance of the VSS.

3. This step tests for a faulty VSS (Transmission) (See the above figure).

4. This step tests for an open in the WHT wire.

5. This step tests for an open in the BLK wire.

6. This step tests for a faulty PCM.

7. This step measures the voltage output of the VSS (Transmission).

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

1. Turn the ignition switch to the LOCK position.
2. Disconnect the PCM connector C3.
3. Use J 39200 in order to measure the resistance between the PCM harness connector C3 terminal 1 and terminal 9.

Is the resistance within the specified range?

387-473 ohms

Go to Step 7

Go to Step 3

3 |

1. Disconnect the VSS (Transmission) electrical connector.
2. Use J 39200 in order to measure the resistance between the VSS (Transmission) electrical terminals (sensor side).

Is the resistance within the specified range?

387-473 ohms

Go to Step 4

Go to Step 8

4 | Use J 39200 in order to measure the resistance between the PCM harness connector C3 terminal 1 and the VSS (Transmission) harness connector 2. Is the resistance less than the specified value? | 2 ohms | Go to Step 5 | Go to Step 9
5 | Use J 39200 in order to measure the resistance between the PCM harness connector C3 terminal 9 and the VSS (Transmission) harness connector 1. Is the resistance less than the specified value? | 2 ohms | Go to Step 6 | Go to Step 10
6 |

1. Install a known-good PCM.
2. Reconnect all the connectors.
3. Perform the DTC Confirmation Procedure.

Does DTC P0720 reset?

--

Go to Step 12

Go to Step 11

7 |

1. Reconnect the PCM connector C3.
2. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
3. Start the engine.
4. Verify that the driving wheels lift from the ground in order to ensure proper wheel rotation during the test.
5. Press down the brake pedal.
6. Shift the manual selector to Drive (D) range.
7. Accelerate the vehicle to 40 km/h (25 mph).
8. Use J 39200 on an AC scale in order to measure the voltage between the PCM connector C3 terminal 1 and terminal 9.

Is the voltage within the specified range when the driving wheels rotate during the test?

0.5-2.3 V

Go to Step 14

Go to Step 13

8 | Replace the VSS (Transmission). Refer to Vehicle Speed Sensor Replacement . Is the procedure complete? | -- | Go to Step 15 | --
9 | Repair the open or the high resistance in the WHT wire between the VSS (Transmission) and the PCM. Is the repair complete? | -- | Go to Step 15 | --
10 | Repair the open or the high resistance in the BLK wire between the VSS (Transmission) and the PCM. Is the repair complete? | -- | Go to Step 15 | --
11 | Refer to Diagnostic Aids for a possible intermittent condition. If OK, replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls. Is the action complete? | -- | Go to Step 15 | --
12 | Refer to Diagnostic Aids for additional information about DTC P0720. Is the action complete? | -- | Go to Step 15 | --
13 | Inspect for an improperly installed VSS (Transmission). If the installation is OK, then replace the VSS (Transmission). Refer to Vehicle Speed Sensor Replacement . Is the action complete? | -- | Go to Step 15 | --
14 | Turn the ignition switch to the LOCK position. Is the action complete? | -- | Go to Step 6 | --
15 |

1. Turn the ignition switch to the LOCK position.
2. Reconnect all the connector(s).
3. Perform the DTC Confirmation Procedure.

Does the scan tool display any DTC(s)?

--

Refer to the applicable DTC table

System OK