Circuit Description
The speed sensor circuit consists of a magnetic induction type sensor, which is the Output Shaft Speed (OSS) Sensor, a Vehicle Speed Sensor (VSS) Buffer Module, and wiring. Gear teeth pressed onto the output shaft carrier assembly induce an alternating voltage into the sensor. This signal transmits to the VSS Buffer Module. The VSS Buffer Module compensates for various final drive ratios. The VSS Buffer Module also converts the AC OSS Sensor signal into a 40 pulse per revolution (PPR) 5 volt DC square wave form signal on circuit 437 to indicate transmission output speed.
When the Transmission Control Module (TCM) detects a low output speed when the vehicle has a high engine speed in a drive gear range, then DTC 24 sets.
Conditions for Setting the DTC
| • | No TP Sensor DTC 21 or DTC 22. |
| • | No TFP Val. Position Sw. DTC 28. |
| • | The TP Angle is 10-100%. |
| • | Circuit 437 voltage is constant. |
| • | Engine speed is greater than 3000 RPM. |
| • | The transmission is not in Park or Neutral. |
| • | All the above conditions are met for 3 seconds. |
| • | The OSS Sensor speed is less than 200 RPM. |
Action Taken When the DTC Sets
| • | The TCM does not illuminate the MIL. |
| • | The TCM commands maximum line pressure. |
| • | The TCM commands 2nd gear. |
| • | DTC 24 is stored in TCM history. |
Conditions for Clearing the DTC
| • | The TCM clears the DTC from Current to History when the fault condition(s) no longer exist and the ignition switch is cycled OFF and then ON. |
| • | A scan tool can clear the DTC from Current and History status. The TCM clears the DTC from the TCM history if the vehicle completes 40 ignition cycles without a failure reported. |
| • | The TCM cancels the DTC default actions when the fault no longer exists and the ignition is cycled OFF and then ON. |
Diagnostic Aids
| • | Inspect the wiring at the TCM, the A/T OSS and the VSS Buffer Module connectors and all other circuit connecting points for the following conditions: |
| - | A bent terminal |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Poor terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| - | Moisture intrusion |
| - | Corrosion |
| • | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to drive the vehicle. |
| • | Inspect the Speed Sensor wiring for contact with sharp metal edges. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step verifies the fault condition.
-
This step tests sensor integrity.
-
This step verifies power and ground to the VSS Buffer Module.
-
This step verifies the TCM input controlled by the Speed Buffer.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
Important:: Before clearing the DTC's, record the Current and History DTC(s) information for reference. The Clear Info function erases the stored Current and History DTC from the TCM. Does the Transmission OSS increase with the drive wheel speed? | -- | Cannot verify fault. Exit the DTC table and go to Diagnostic Aids | ||
Is the voltage greater than the specified value? | 2.0 volts at 2000 RPM | |||
4 |
Is the voltage greater than the specified value? | 2.0 volts AC at 2000 RPM | ||
5 |
Refer to General Electrical Diagnosis . Did you find the condition? | -- | ||
6 |
Refer to General Electrical Diagnosis . Did you find the condition? | -- | Refer to Diagnostic Aids | |
7 |
Is the voltage greater than the specified value? | 10.5 volts DC | ||
8 | Inspect the ignition feed circuit 439 for an open. Refer to General Electrical Diagnosis . Did you find and correct the condition? | -- | -- | |
With the key to the RUN position, measure the voltage between terminals 8 and 9 of the VSS Buffer connector. Is the voltage greater than the specified value? | 10.5 volts DC | |||
10 | Inspect the VSS Buffer Module ground circuit 451 for an open. Refer to General Electrical Diagnosis . Was the condition found and corrected? | -- | -- | |
11 | With the connector off of the Speed Buffer and the key in the RUN position, measure the voltage between terminal 13 of the VSS Buffer Harness Connector and a good ground. Is the voltage within the specified value? | 4.8 - 5.2 volts DC | ||
12 | Is the voltage in Step 11 greater than the specified voltage? | 5.2 volts DC | ||
13 | Inspect circuit 437 for a short to power. Refer to General Electrical Diagnosis . Did you find the condition? | -- | ||
14 | Inspect circuit 437 for a short to power. Refer to General Electrical Diagnosis . Did you find the condition? | -- | ||
Is the voltage within the specified values? | 1.5 - 3.5 volts DC | |||
16 |
Refer to General Electrical Diagnosis . Did you correct the condition? | -- | -- | |
17 | Inspect the TCM for faulty or intermittent connections. Did you find the condition? | -- | ||
18 | Replace the OSS Sensor. Refer to Vehicle Speed Sensor Replacement . Is the replacement complete? | -- | -- | |
19 | Replace the VSS Buffer Module. Refer to Vehicle Speed Signal Buffer Replacement . Is the replacement complete? | -- | -- | |
20 | Replace the TCM. Refer to DTC 51 PROM Error TCM Replacement. Is the replacement complete? | -- | -- | |
21 | In order to verify your repair, perform the following procedure:
Drive the vehicle under steady acceleration above 10% TP. Does the Scan Tool display an output speed greater than 500 RPM for 1 second? | -- | System OK | Begin the diagnosis again. |
