GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1999 🢒 Firefly (Canada) 🢒 DTC 🢒 DTC P0720 Output Speed Sensor Circuit

Object Number: 215390 Size: MF

Circuit Description

The Powertrain Control Module (PCM) receives an AC voltage input from the Vehicle Speed Sensor (VSS) when the drive wheels are rotating. Because the frequency of the AC voltage signal increases with the 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 signal is inputted while the vehicle is running in the Drive (D) gear or in the Second (2) gear.
    • 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:

    • An open VSS or A shorted VSS
    • Faulty electrical circuits between the VSS 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
    • Incorrect mating
    • Broken locks
    • Incorrectly formed terminals or damaged terminals
    • Poor terminal-to-wire connections

Perform the following steps if DTC P0720 cannot be duplicated:

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

Inspect the wiring. If any wiring repairs are necessary, Refer to Wiring Repairs in Wiring Systems.

DTC Confirmation Procedure
  1. Turn the ignition switch to the LOCK position.
  2. Connect the scan tool to Data Link Connector (DLC).
  3. Turn the ignition switch to ON.
  4. Use the scan tool in order 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 (2) Range. Obtain a engine speed of at least 3500 RPM for at least 10 seconds.
  7. Stop the vehicle.
  8. Use the scan tool in order 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 to store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. This information is 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.

    4.

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

    5.

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

    6.

  6. This step inspects for a faulty PCM.

    7.

  7. This step inspects the voltage output of the VSS.

Step

Action

Value(s)

Yes

No

\x941\x95

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 to ON, leaving the engine to OFF.
  3. Perform the Clear DTC Information function of the scan tool.
  4. Operate the vehicle within the Freeze Frame conditions as observed.

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

--

Go to the applicable DTC table

Go to Step 3

\x943\x95

  1. Turn the ignition switch to the LOCK position.
  2. Disconnect the PCM connector C2.
  3. Use the J 39200 in order to measure the resistance between the PCM harness and the connector C2 terminals 4 and 12.

Is resistance within the specified range?

100-300 ohms

Go to Step 8

Go to Step 4

\x944\x95

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

Is resistance within the specified range?

100-300 ohms

Go to Step 5

Go to Step 9

\x945\x95

Use the J 39200 in order to measure the resistance between the PCM harness connector C2 terminal 12 and the VSS terminal 2 electrical terminals of the sensor side.

Is resistance less than the specified value?

2 ohms

Go to Step 6

Go to Step 10

6

Use the J 39200 in order to measure the resistance between the PCM harness connector C2 terminal 4 and the VSS harness connector 1.

Is the resistance less than the specified value?

2 ohms

Go to Step 7

Go to Step 11

\x947\x95

  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

\x948\x95

  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. Verify that the driving wheels do not touch 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 an AC scale.

    8. Use the J 39200 in order to measure the voltage between the PCM connector C2 terminals 4 and 12 by backprobing.

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. Refer to Vehicle Speed Sensor .

Is the action complete?

--

Go to Step 16

--

10

Perform one of the following actions:

    • Repair the open in the GRY/WHT wire between the VSS and the PCM.
    • Repair the high resistance in the GRY/WHT wire between VSS and PCM.
    • Repair the open in the GRY wire between the VSS and the PCM.
    • Repair the high resistance in the GRY wire between the VSS and the PCM.

Is the repair complete?

-

Go to Step 16

--

11

Perform one of the following actions:

    • Repair the open in the GRY wire between the VSS and the PCM.
    • Repair the open in the PPL wire between the VSS and the PCM.
    • Repair the high resistance in the GRY wire between the VSS and the PCM.
    • Repair the high resistance in the PPL wire between the VSS and the 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 in Electrical Controls.

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 incorrectly installed VSS.
  2. If the VSS is correctly installed, replace the VSS. 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) position.
  3. Turn the ignition switch to the Lock position.

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