Allison - Automatic Transmission

DTC P0815, P0816, or P0826

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.



DTC Descriptors
DTC P0815
- Upshift Switch Circuit

DTC P0816
- Downshift Switch Circuit

DTC P0826
- Up and Down Shift Switch Circuit




Diagnostic Fault Information






Circuit/System Description

The range selection mode (RSM), also known as the TAP Shift, allows the driver to manually shift gears by pressing the Plus/Minus button and select the desired range of gears for current driving conditions by using the RSM shift switch located on the automatic transmission gear shift lever. Pushing the Plus switch will command an upshift and pushing the Minus switch will command a downshift. A 12-volt reference voltage flows from the body control module (BCM) through the RSM switch resistor array to the BCM. The RSM shift system is activated when the gear selector is in the Manual (M) position. RSM system is deactivated in all other range positions.


Conditions for Running the DTC

The engine speed is between 200-7500 RPM for at least 5 seconds.


P0815 or P0816
* No DTCs P0826 or P0708.
* The time since the last gear selector range change is greater than 6 seconds.



Conditions for Setting the DTC

P0815

The BCM detects an upshift request for 3 seconds in PARK, REVERSE, NEUTRAL, or D1 and for 600 seconds in PARK, REVERSE, NEUTRAL, D2, or D1.

P0816

The BCM detects a downshift request for 3 seconds in PARK, REVERSE, NEUTRAL, or D1 and for 600 seconds in PARK, REVERSE, NEUTRAL, D2, or D1.

P0826

The BCM detects an invalid voltage on the TAP up/down signal circuit for 5 seconds.


Action Taken When the DTC Sets

* DTCs P0815, P0816, P0826, and P1876 are Type C DTCs.
* The BCM disables TAP shift operation.



Conditions for Clearing the DTC

* DTCs P0815, P0816, P0826, and P1876 are Type C DTCs.
* A scan tool can clear the code from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without a failure.



Reference Information

Schematic Reference

Automatic Transmission Controls Schematics (Electrical Diagrams)

Connector End View Reference

Component Connector End Views


DTC Type Reference
* Diagnostic Trouble Code (DTC) Type Definitions (Diagnostic Trouble Code Descriptions)
* Diagnostic Trouble Code (DTC) List/Type (Diagnostic Trouble Code Descriptions)



Electrical Information Reference
* Circuit Testing
* Connector Repairs
* Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs



Circuit/System Verification

With the engine ON, engage the RSM Plus/Minus switch positions while observing the Driver Shift Request parameter in the body controls scan tool data list. The parameter should change to reflect the selected Up or Downshift switch position and with no Up or Downshift switch position selected the parameter should display None.


Circuit/System Testing


P0815
1. Select the Driver Shift Request parameter.
2. Disconnect the automatic transmission RSM shift switch connector.

• If the parameter status changes from Upshift to Invalid, replace the RSM shift switch.
• If the parameter status does not change, replace the BCM.




P0816
1. Select the Driver Shift Request parameter.
2. Disconnect the RSM shift switch connector.

• If the parameter status changes from Downshift to Invalid, replace the RSM shift switch.
• If the parameter status does not change, replace the BCM.




P0826
1. Test the 12-volt reference circuit and the tap up and tap down switch signal circuit for a short to ground.

• If the circuits test OK, test the 12-volt reference circuit and the tap up and tap down signal circuit for an open or short to voltage.

2. Test the RSM shift switch for correct resistance value, 8.11-8.39 ohms.

• If the resistance is not within specifications, replace the switch assembly.
• If the resistance is within specifications, replace the BCM.




Component Testing

Test the RSM shift switch for correct resistance value, 8.11-8.39 ohms.

If the resistance is not within specifications, replace the RSM switch.



Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

* Replace the RSM shift switch. Refer to Shift Lever Replacement (Shift Lever Replacement) .
* Replace the BCM. Refer to Control Module References for replacement, setup, and programming.