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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1997 🢒 Firefly (Canada) 🢒 DTC 🢒 DTC P0720 Output Speed Sensor Circuit

Object Number: 1558770 Size: MF
Automatic Transmission Controls Schematics

Circuit Description

The Powertrain Control Module (PCM) receives an AC voltage input from the Vehicle Speed Sensor (VSS) (Transaxle) whenever the drive wheels are rotating. Because the frequency of the AC voltage signal increases with wheel speed, the PCM can calculate vehicle speed from the frequency of the AC signal. The PCM uses this information in order to determine shift points.

Conditions for Setting the DTC

DTC P0720 will set if the following conditions are met:

    • No VSS (Transaxle) signal is inputted while the vehicle is running in Drive (D) or Second (2).
    • One driving cycle detection logic.

Action Taken When the DTC Sets

A DTC P0720 is stored and the Malfunction Indicator Lamp (MIL) is turned ON.

Diagnostic Aids

Check for the following conditions:

    • Open or shorted VSS (Transaxle)
    • Faulty electrical circuits between the VSS (Transaxle) and the PCM
    • A faulty PCM

An intermittent may be caused by the following conditions: a poor connection, rubbed through wire insulation or a wire broken inside the insulation.

    • A poor 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 of damaged terminals
    • Poor terminal-to-wire connections

Perform the following steps if DTC P0720 cannot be duplicated:

  1. Read the information in the freeze frame data.
  2. Use the data to determine the vehicle operating conditions when the DTC was set.

Inspect the wiring. Repair wiring as necessary.

DTC Confirmation Procedure
  1. Turn the ignition switch to LOCK.
  2. Connect the scan tool to Data Link Connector (DLC).
  3. Turn the ignition switch to ON.
  4. Use the scan tool to clear the following items from the PCM memory:
    5. • The DTC
    • The pending DTC
    • The freeze frame data
  5. Start the engine.
  6. Operate the vehicle in Drive (D). Obtain a speed of at least 40 km/h (25 mph) for at least 10 seconds.
  7. Stop the vehicle.
  8. Use the scan tool to inspect for DTC P0720.

Test Description

The number(s) below refer to the step number(s) 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. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

  2. This step inspects for internal resistance of the VSS.

    3.

  3. This step inspects for a faulty VSS (Transaxle).

    4.

  4. This step inspects for an open in LT GRN wire.

    5.

  5. This step inspects for an open in PPL wire.

    6.

  6. This step inspects for a faulty PCM.

    7.

  7. This step inspects the voltage output of the VSS (Transaxle).

Step

Action

Value(s)

Yes

No

1

Was the On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain OBD System Check in Engine Controls

2
  1. Install a scan tool.
  2. Turn the ignition ON, leaving the engine OFF.
  3. Perform the scan tool Clear DTC Information function.
  4. Operate the vehicle within the Freeze Frame conditions as observed.

Does the scan tool display any other DTCs in addition to DTC P0720?

--

Refer to the applicable DTC table.

Go to Step 3

3

  1. Turn the ignition switch to LOCK.
  2. Disconnect the PCM connector C3.
  3. Use the J 39200 to measure resistance between PCM harness connector C3 terminals 2 and 13.

Is resistance within the specified range?

100-300 ohms

Go to Step 8

Go to Step 4

4

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

Is resistance within the specified range?

100-300 ohms

Go to Step 5

Go to Step 9

5

Use the J 39200 to measure the resistance between the PCM harness connector C3 terminal 13 and VSS (Transaxle) harness connector terminal 2.

Is resistance less than the specified value?

2 ohms

Go to Step 6

Go to Step 10

6

Use the J 39200 to measure the resistance between the PCM harness connector C3 terminal 2 and VSS (Transaxle) harness connector 1.

Is the resistance less than the specified value?

2 ohms

Go to Step 7

Go to Step 11

7

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

Does the DTC reset?

--

Go to Step 12

Go to Step 13

8

  1. Reconnect the PCM connector C2.
  2. Raise and suitably support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  3. Start the engine.
  4. Ensure that the driving wheels are lifted from the ground. The driving wheels will rotate during this test.
  5. Step on the brake pedal.
  6. Shift the manual selector to the Drive (D) range and accelerate the driving wheels to 40 km/h (25 mph).
  7. Set the J 39200 to the A/C scale. Measure the voltage between PCM connector C3 terminals 2 and 13, while connected, by back probing.

Is the voltage within the specified range while the driving wheels rotate?

0.5-5.0 V

Go to Step 15

Go to Step 14

9

Replace the VSS (Transaxle). Refer to Vehicle Speed Sensor

Is the action complete?

--

Go to Step 16

--

10

Repair open or high resistance in the GRY/WHT wire or in the GRY wire between VSS (Transaxle) and PCM.

Is the repair complete?

--

Go to Step 16

--

11

Repair open or high resistance in the GRY wire or in the PPL wire between VSS (Transaxle) and PCM.

Is the repair complete?

--

Go to Step 16

--

12

Refer to Diagnostic Aids for a possible intermittent condition. If OK, replace the Powertrain Control Module (PCM). Refer to Powertrain Control Module Replacement

Is the action complete?

--

Go to Step 16

--

13

Refer to Diagnostic Aids for additional information regarding this DTC.

Is the action complete?

--

Go to Step 16

--

14
  1. Test for an improperly installed VSS (Transaxle).
  2. If the VSS is correctly installed, replace the VSS (Transaxle). Refer to Vehicle Speed Sensor

Is the test complete?

--

Go to Step 16

--

15
  1. Press the brake pedal in order to stop the driving wheels.
  2. Turn the manual selector to Park (P).
  3. Turn the ignition switch to Lock.

Is the action complete?

--

Go to Step 7

--

16

Perform the DTC Confirmation Procedure.

Are any DTCs displayed on the Scan Tool?

--

Go to the applicable DTC table

System OK