Refer to
DIESEL PCM, VSS, EBCM
Circuit Description
The speed sensor circuit consists of a magnetic induction type sensor, a vehicle speed sensor buffer module and wiring. Gear teeth pressed on the output shaft induce an alternating current in the sensor. This signal is transmitted to the buffer. The buffer compensates for various axle ratios and converts the signal into a square wave for use by the speedometer, cruise control, antilock brake and PCM. The buffer sends two different signals to the PCM.
Conditions for Setting the DTC
| • | The PCM performs this DTC diagnostic continuously. |
| • | The vehicle speed greater than 20 mph. |
| • | The four wheel low not selected. |
| • | The VSS buffer calculated speed is less than half the transmission calculated speed. |
| • | All of the diagnostic set conditions met for 2 seconds. |
or
| • | The PCM performs this DTC diagnostic continuously. |
| • | The VSS buffer calculated speed is greater than transmission calculated speed by 20 mph. |
| • | All of the diagnostic set conditions met for 2 seconds. |
Action Taken When the DTC Sets
| • | The PCM disallows all cruise control inputs. |
| • | The PCM will not illuminate the Malfunction Indicator Lamp (MIL). |
| • | The PCM records the operating conditions at the time the diagnostic fails. This information stores in the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | A History DTC will clear when forty consecutive warm-up cycles during which the diagnostic does not fail (coolant temperature has risen 5°C (40°F) from the start up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle. |
| • | Use of a Scan tool. |
Diagnostic Aids
Check connections at VSS buffer and PCM. Refer to 4L80E Diagnostic Trouble Codes in Transmission if DTC P0722 or DTC P0723 is also set.
Test Description
The numbers below refer to the step numbers on the Diagnostic Table.
-
This tests for B+ at VSS buffer.
-
This tests for proper ground path for vehicle speed sensor signal buffer.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing any DTCs, use the scan tool Capture Info to save freeze frame and failure records for reference, as the scan tool loses data when using the Clear Info function. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 |
With the drive wheels rotating, does the vehicle speed increase with drive wheel speed increase? | -- | ||
Is the test light ON? | -- | |||
Back probe the VSS buffer module ignition feed circuit to the ground circuit with a test light. Is the test light ON? | -- | |||
5 |
Does the voltage increase on the J 39200 with drive wheel increase? | -- | ||
6 | Does the scan tool display a trans output speed (MPH) increase with drive wheel increase? | -- | ||
7 | The DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If additional DTCs are stored refer to those table(s) first. Are any additional DTCs stored? | -- | Go to the Applicable DTC Table | Go to Diagnostic Aids |
8 | Repair the open in the ignition feed circuit. Is the action complete? | -- | -- | |
9 | Repair the open in the ground circuit. Is the action complete? | -- | -- | |
10 | Check the complete VSS input circuit for an open or short to ground. Was a repair performed? | -- | ||
11 |
Was a problem found? | -- | ||
12 |
Is there voltage present? | -- | ||
13 | Check the VSS output circuit for an open or short to ground. Was a repair performed? | -- | ||
14 | Check VSS output circuit for a proper connections at the PCM. Was a repair performed? | -- | ||
15 | Replace the Vehicle Speed Sensor. Is the action complete? | -- | -- | |
16 | Replace the VSS Buffer module. Refer to Vehicle Speed Signal Buffer Replacement . Is the action complete? | -- | -- | |
17 |
Important: The new PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Replace the PCM. Is the action complete? | -- | -- | |
18 |
Does the scan tool indicate the diagnostic Passed? | -- | ||
19 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the Applicable DTC Table | System OK |
