8HP50/850RE Automatic Transmission (Service Information): Operation: Notes

The 8HP50/850RE is an electronic eight-speed automatic transmission.

• The Transmission Control Module Assembly (TCMA), which is integrated into the valve body, provides fully synchronized clutch-to-clutch shifting through four planetary gear sets.
• The TCMA includes a mounting plate that holds the Transmission Control Module (TCM) and a molded wiring harness for connection to various transmission sensors and solenoids.
• The valve body assembly contains all the sensors and solenoids required for operation, completely inside the transmission. Eight speeds allow the engine to maintain its optimal rpm range, increasing fuel economy and performance.
• Transmission control is performed by the TCM based on hard-wired and CAN bus signals from sensors and modules.
• The TCM receives driveability data from the Powertrain Control Module (PCM) and other modules over the CAN-C bus.
• It also receives shift lever position information from the electronic shifter over a dedicated transmission CAN bus.
• The TCM processes this input data and controls operation of the torque converter clutch, park lock system, solenoid valves, and pressure regulating valve.
• The input and output speed sensors are Hall-effect sensors that measure shaft rotational speed.
• The input speed sensor is located at the top, near the center, of the of the TCMA and reads input shaft speed from the magnetic ring on the P2 carrier.
• The output speed sensor is located at the back of the TCMA and reads output shaft speed from the P4 carrier

FILTER SERVICE  The 8HP50/850RE has a conventional fluid sump design, however, the filter is integrated into the oil pan resulting in a lower profile for improved vehicle packaging. The oil pan gasket is reusable providing it is not damaged during removal.

FLUID CHECK AND FILL  A fluid fill tube and indicator are not provided. All work is performed under the vehicle while raised on a hoist. In the event of a transmission shift quality concern, a fluid leak, or in conjunction with a transmission repair, the transmission fluid level must be validated and topped off as necessary. The procedure involves the use of a scan tool to monitor transmission fluid temperature. Specific service procedures are necessary to check and fill the transmission with fluid. Refer to FLUID AND FILTER, STANDARD PROCEDURE .

SELECTABLE DRIVE MODES 

Tow / HAUL MODE 

When driving in hilly areas, towing a trailer, or carrying a heavy load and frequent transmission shifting occurs, pressing the TOW/HAUL switch to select TOW/HAUL mode will improve performance and reduce the potential for transmission overheating or failure due to excessive shifting. When operating in TOW/HAUL mode, transmission upshifts are delayed, 8th gear overdrive is prohibited, the Multiple Displacement System (MDS) system is prohibited from cylinder deactivation, and the transmission will automatically downshift (for engine braking) during steady braking maneuvers. The "TOW/HAUL Indicator Light" will illuminate in the instrument cluster to indicate that TOW/HAUL mode has been activated. Pressing the switch a second time restores normal operation. If TOW/HAUL mode is desired, the switch must be pressed each time the engine is started. The Radio serves as the user interface for the Drive Mode feature. When a vehicle is equipped with this feature the user will be able to select the TOW/HAUL mode in both run (engine not running/engine running) states. The Auxiliary Switch Bank Module (ASBM) houses the TOW/HAUL switch. The ASBM is responsible for performing any diagnostics necessary for the TOW/HAUL switch. When the switch is depressed, the ASBM module will update and send a LIN signal value over the LIN BUS as a command to engage the vehicle in Tow Haul mode.

The TOW/HAUL mode feature will be inhibited if:

• The 4 Wheel Drive Low mode is selected while the TOW/HAUL Mode is active the TCM will automatically disengage TOW/HAUL mode.
• The TOW/HAUL mode is requested while the 4 Wheel Drive Low mode is already active the TCM will ignore the TOW/HAUL Request.