Allison - Automatic Transmission
DTC P0502
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.
DTC Descriptor
DTC P0502
- Vehicle Speed Sensor (VSS) Absence of Signal
Diagnostic Fault Information
Circuit/System Description
The vehicle speed sensor (VSS) also called the output speed sensor (OSS) creates a AC voltage signal that increases in frequency as vehicle speed increases. The transmission control module (TCM) receives and process this signal. The TCM provides a digital VSS signal on the data communication line to the engine control module (ECM) and other controllers connected to the data line. The TCM also provides an identical VSS signal on a separate circuit to the ECM. The ECM will test and compare the two signals for being identical and valid.
Conditions for Running the DTC
1. DTCs P0717 or P0722 are not active.
2. The ECM indicates the transmission output speed signal is valid.
Conditions for Setting the DTC
The difference between the two output speed signals is greater than 8 km/h (5 mph) for more than 1 second.
Action Taken When the DTC Sets
P0502 is a Type A DTC.
Conditions for Clearing the DTC
P0502 is a Type A DTC.
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics (Allison - Automatic Transmission)
Connector End View Reference
* Component Connector End Views (Connector End Views By Name)
* Inline Harness Connector End Views (Connector End Views By Number)
Description and Operation
* Transmission General Description (Transmission General Description)
* Transmission Component and System Description (Transmission Component and System Description)
* Electronic Component Description (Electronic Component Description)
Electrical Information Reference
* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)
DTC Type Reference
Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle)
Scan Tool Reference
* Transmission Control Module Scan Tool Information (4L60-E) (Transmission Control Module Scan Tool Information (4L60-E))Transmission Control Module Scan Tool Information (6L45/6L50/6L80/6L90) (Transmission Control Module Scan Tool Information (6L45/6L50/6L80/6L90))Transmission Control Module Scan Tool Information (2ML70) (Transmission Control Module Scan Tool Information (2ML70))Transmission Control Module Scan Tool Information (Allison) (Transmission Control Module Scan Tool Information (Allison))
* Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle)
Circuit/System Verification
1. If DTCs P0722 or P0723 are set, diagnose those DTCs first.
2. Observe the TCM output speed signal parameters and the ECM speed signal parameters with a scan tool, while driving the vehicle. Both parameter values should be identical.
3. Operate the vehicle within the Conditions for Running the DTC, to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
Circuit/System Testing
1. Ignition OFF, disconnect the TCM harness connector.
2. Ignition ON, test for 4.8-5.2 volts between the signal circuit terminal 25 and ground.
• If less than the specified range, test the 5 volt reference circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the ECM.
• If greater than the specified range, test the 5 volt reference circuit for a short to voltage. If the circuit tests normal, replace the ECM.
3. If all the circuits test normal, replace the TCM.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Transmission Control Module Replacement (Allison - Automatic Transmission)
* Transmission Control Module Programming and Setup (Allison) (Allison - Automatic Transmission)Transmission Control Module Programming and Setup (4L60-E/4L65-E/4L70-E) (4L60-E/4L65-E/4L70-E - Automatic Transmission)