Allison - Automatic Transmission

DTC P1761 or P1762

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 P1761
- Up and Down Shift Switch Signal Circuit

DTC P1762
- Transmission Mode Switch Signal Circuit

Circuit/System Description

The drivers shift request (DSR), also known as the TAP Shift, allows the driver to select the desired range of gears for current driving conditions. Pressing the plus (+) switch will increase the highest attainable gear and pressing the minus (-) switch will decrease the highest attainable gear. The TCM will not allow a downshift if the transmission is compromised by over-speeding. The DSR shift switch is a resistor array and is located on the automatic transmission gear shift lever. The BCM supplies a 12 volt reference circuit and signal circuit to the DSR. The DSR shift system is activated when the gear selector is in the Manual (M) position. The DSR system is deactivated in all other range positions. The BCM sends the DSR request to the transmission control module (TCM).

The tow/haul mode enables the operator to achieve enhanced shift performance when towing or hauling a load. When tow/haul mode switch is pressed and released, the tow/haul switch signal circuit to the BCM is momentarily toggled to zero volts. The BCM communicates the tow haul switch input to the TCM via GMLAN. When tow/haul mode is active, the TCM changes the shift pattern to generally raise the output speed that an upshift or downshift is made and also applies the TCC in second range. When the tow/haul mode switch is pressed and released again, the BCM communicates the request to the TCM and tow/haul mode becomes inactive. The transmission upshift and downshift speeds and TCC command return to a normal shift pattern.

The TCM is constantly monitoring the serial data sent from the BCM for the correct number and sequence of counts for Tap Up/Tap Down function and tow/haul mode function.

Conditions for Running the DTC

* Battery voltage is between 9-18 V.
* Engine speed is greater than 200 RPM for 5 seconds.

Conditions for Setting the DTC

P1761

The TCM detects the DSR information is not received or the rolling counter expected value is not met 5 times within a 10 second period.

P1762

The TCM detects the Tow/Haul Mode information is not received or the rolling counter expected value is not met 5 times within a 10 second period.

Action Taken When the DTC Sets

P1761
* DTC P1761 is a Type C DTC.
* The TCM disables TAP shift operation.

P1762
* DTC P1762 is a Type C DTC.
* The TCM disables Tow/Haul Mode operation.

Conditions for Clearing the DTC

DTCs P1761 and P1762 are Type C DTCs.

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)

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

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Powertrain Diagnostic Trouble Code (DTC) Type Definitions)
Scan Tool Reference

Control Module References (Control Module References) for scan tool information

Circuit/System Verification

Note: If other DTCs are set, diagnose those DTCs first.

1. Ignition ON, verify with a scan tool the Tap Up/Down Switch parameter displays On.
2. Ignition ON, cycle the automatic transmission manual shift switch up and down, while observing the Tap Up/Down parameter. The parameter should change to Up when the plus (+) switch is selected, and to Down when the minus (-) switch is selected.
3. Cycle the tow/haul switch while observing the Tow/Haul Mode parameter on the scan tool. The Tow/Haul Mode parameter should change states between Active and Inactive with each cycle of the switch.
4. Operate the vehicle within the Conditions for Running the DTC to verify that the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Circuit/System Testing

If no other DTCs are set, diagnosis is accomplished by testing for a condition on the CAN circuit. Refer to Scan Tool Does Not Communicate with High Speed GMLAN Device (Scan Tool Does Not Communicate with High Speed GMLAN Device).

Repair Verification

If another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle) and perform the appropriate diagnostic procedure.