Theory Of Operation
The Powertrain Control Module (PCM) controls the Park Lock Solenoid within the transmission. The Transmission Control Module (TCM) sends a request to the PCM via Controller Area Network (CAN). The PCM will follow the TCM request by energizing or de-energizing the Park Lock Solenoid with a Pulse Width Modulated (PWM) High Side Driver (HSD). The PCM also controls the Low Side Driver for the Park Lock Solenoid. The LSD can be toggled off in the event that the HSD is shorted to battery, thus allowing the Park Lock Solenoid to be de-energized and allowing the Park Pawl to be engaged. The PCM sends the status of the commanded Park Pawl position to the TCM. This allows the PCM to maintain power to the Park Lock Solenoid in the event of TCM failure, keeping the transmission out of Park until the operator requests Park, Auto Park or the ignition is turned Off. During transmission default (Limp-In), the transmission will shift to 6th gear, the KO clutch is opened to disconnect the engine from the transmission, and vehicle propulsion is provided with the P2 motor until vehicle speed is 0 MPH. Once at 0 MPH or ignition Off, the PCM will cut power to the Park Lock Solenoid causing the transmission to be placed into Park.
The PCM does NOT follow request from the Electronic Shift Module (ESM) to go to Park unless there is a loss of communication or invalid communication between the PCM and the TCM.
The TCM monitors the actual Park Pawl position with internal sensors. The TCM will set DTCs if there is discrepancy between the actual Park Pawl position and the commanded Park Pawl position.
Once current flow is removed from the Park Lock Solenoid, the transmission will allow a shift to Park when the vehicle speed is low enough to allow the shift to occur. The Park system is an electro-hydraulic circuit, requiring fluid pressure to disengage the Park Pawl. The Park Lock Solenoid, once engaged (energized, or toggled on) will mechanically inhibit the Park Pawl from engaging in the event of a loss of hydraulic pressure. In the event of an Park Lock Solenoid fault, the transmission can hold the Park Pawl out of Park hydraulically as long as there is sufficient line pressure. The vehicle is equipped with a Manual Park Release (MPR) that can be used to manually release the Park Pawl. The MPR, when activated, releases the Park Pawl placing the transmission into a Neutral state.