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P0721






P0721-OUTPUT SPEED SENSOR CIRCUIT PERFORMANCE





For a complete wiring diagram, refer to the Wiring Information.

Theory of Operation

1. The transmission system uses two speed sensors, one to measure input RPM and one to measure output RPM. These inputs are essential for proper transmission operation. Therefore, the integrity of this data is verified through the following checks:
- When in gear, if the gear ratio does not compare to a known gear ratio, the corresponding in-gear trouble code is set (DTCs P0731-36 and P1736).
- An excessive change in input or output speeds indicating signal intermittent will result in codes P0716 and/or P0721 being set.
- If the common speed sensor ground circuit is lost, both sensor inputs will read the signal from the input speed sensor at idle in neutral. Since the input speed sensor has 60 teeth and the output speed sensor has 30 teeth, this results in a an apparent speed ratio of 1:2 and may cause the DTC P1794 to set.

- When Monitored:
The transmission gear ratio is monitored continuously while the transmission is in gear.

- Set Condition:
If there is an excessive change in output RPM in any gear. This DTC can take up to five minutes of problem identification before illuminating the MIL.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure.

1. CHECK IF THE DTC IS CURRENT
1. Turn the ignition off to the lock position.

CAUTION: Properly support the vehicle and raise all drive wheels off the ground.

2. Release the parking brake.
3. Start the engine in park.
4. Place the transmission gear selector in drive, release foot from brake.

WARNING: Be sure to keep hands and feet clear of rotating wheels.

5. With the scan tool, monitor the Output Speed Sensor RPM.

Is the Output Speed Sensor RPM below 100 RPM?

Yes

- Go To 2

No

- Go To 9

2. CHECK THE OUTPUT SPEED SENSOR
1. Turn the ignition off to the lock position.
2. Remove the Starter Relay.

CAUTION: Removal of the Starter Relay will prevent the vehicle from being started in gear.

WARNING: The Starter Relay must be removed from the PDC. Failure to do so may result in possible serious or fatal injury.

NOTE: Failure to remove the Starter Relay can cause a Transmission - No Response condition.

3. Install the Transmission Simulator 8333.
4. Ignition on, engine not running.
5. With the Transmission Simulator, set the selector switch to 3000/1000 and the Input/Output switch to ON.
6. With the scan tool, monitor the Input and Output Speed Sensor RPM

Does the Input Speed read 3000 RPM and the Output Speed read 1000 RPM (± 50 RPM)?

Yes

- Replace the Output Speed Sensor.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Go To 3

3. CHECK THE (T14) OUTPUT SPEED SIGNAL CIRCUIT FOR AN OPEN




1. Turn the ignition off to the lock position.
2. Disconnect the Transmission Simulator 8333.
3. Disconnect the PCM harness connectors and connect the PCM Pinout Box 8815.

CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install the PCM Pinout Box 8815 to perform diagnosis.

4. Measure the resistance of the (T14) Output Speed Signal circuit from the Output Speed Sensor harness connector to the appropriate terminal of the PCM Pinout Box 8815.

Is the resistance above 5.0 Ohms?

Yes

- Repair the (T14) Output Speed Signal circuit for an open.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Go To 4

4. CHECK THE (T13) SENSOR GROUND CIRCUIT FOR AN OPEN




1. Measure the resistance of the (T13) Sensor Ground circuit between the Output Speed Sensor harness connector and the appropriate terminal of the PCM Pinout Box 8815.

Is the resistance above 5.0 Ohms?

Yes

- Repair the (T13) Sensor Ground circuit for an open.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Go To 5

5. CHECK THE (T14) OUTPUT SPEED SIGNAL CIRCUIT FOR A SHORT TO GROUND




1. Measure the resistance between ground and the (T14) Output Speed Signal circuit.

Is the resistance below 5.0 Ohms?

Yes

- Repair the (T14) Output Speed Signal circuit for a short to ground.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Go To 6

6. CHECK THE (T13) SENSOR GROUND CIRCUIT FOR A SHORT TO GROUND




1. Measure the resistance between ground and the (T13) Sensor Ground circuit.

Is the resistance below 5.0 Ohms?

Yes

- Repair the (T13) Sensor Ground circuit for a short to ground.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Go To 7

7. CHECK THE (T14) OUTPUT SPEED SIGNAL CIRCUIT FOR A SHORT TO VOLTAGE




1. Remove the Transmission Control Relay.
2. Place a jumper wire between the Fused B(+) circuit and the Transmission Relay Output circuit in the Transmission Control Relay connector.
3. Ignition on, engine not running.
4. Measure the voltage of the (T14) Output Speed Signal circuit.

Is the voltage above 0.5 of a volt?

Yes

- Repair the (T14) Output Speed Signal circuit for a short to voltage.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Go To 8

8. CHECK THE (T13) SENSOR GROUND CIRCUIT FOR A SHORT TO VOLTAGE




1. Measure the voltage of the (T13) Sensor Ground circuit.

Is the voltage above 0.5 of a volt?

Yes

- Repair the (T13) Sensor Ground circuit for a short to voltage.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Using the schematics as a guide, check the Powertrain Control Module (PCM) terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. If no problems are found, replace and program the PCM. With the scan tool, perform Quick Learn. Quick Learn Procedure
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

9. CHECK WIRING AND CONNECTORS
1. The conditions necessary to set this DTC are not present at this time.
2. Using the schematics as a guide, inspect the wiring and connectors specific to this circuit.
3. Wiggle the wires while checking for shorted and open circuits.
4. With the scan tool, check the Event Data to help identify the conditions in which the DTC was set.

Were there any problems found?

Yes

- Repair as necessary.
- Perform the 545RFE TRANSMISSION VERIFICATION TEST. . 545RFE Transmission Verification Test

No

- Test Complete.

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