Description And Operation: 948TE/9HP48
DESCRIPTION
The 948TE/9HP48 Transmission Control Module (TCM) (1) is mounted on the outside of the transmission case, over the differential.
The electronic control system consists of various components providing inputs to the TCM. The TCM monitors transmission sensors, shifter assembly, and bus messages to determine transmission shift strategy. After shift strategies are determined, the TCM controls the actuation of transmission solenoids, which controls the routing of hydraulic fluid within the transmission, by moving a sequence of four valves to make a shift occur.
The system performs its functions based on continuous real-time sensor feedback information. In addition the TCM receives information from the rest of the vehicle over the Controller Area Network (CAN)-C bus. Most messages are sent every 20 milliseconds.
The transmission control system automatically adapts to changes in engine performance, vehicle speed, and transmission temperature variations to provide consistent shift quality. The control system ensures that clutch operation during up-shifting and downshifting is more responsive without increased harshness. The TCM activates the solenoid valves and moves valves in the valve body to achieve the necessary gear changes. The required pressure level is calculated from the load condition, engine speed. Vehicle speed and transmission oil temperature, matched to the torque to be transmitted. Power for the transmission system is supplied through the shifter mechanism (no transmission control relay).
OPERATION
The TCM regulates the amount of hydraulic pressure used to engage the clutches and the Torque Converter Clutch (TCC), in addition to directing hydraulic pressure to engage or release any given clutch for any given required gear.
The TCM will actuate the valves via solenoids based on the position of the shifter, transmission fluid temperature, engine operating conditions, traction conditions and driver demands.
During a shift, the TCM will actuate the solenoids to match the gear ranges to the optimal torque range of the engine based on the position of the accelerator pedal, shifter and vehicle speed as determined by the Powertrain Control Module (PCM) based on input from the Vehicle Speed Sensor (VSS) and Anti-lock Brake System (ABS) module.
Due to the complexity of the 948TE transmission control system, always refer to the transmission electrical/electronic diagnostics in this service information when attempting to diagnose transmission problems.
Some examples of direct inputs to the TCM are:
- Battery (B+) voltage
- Ignition Status
- Transmission Range Sensor (TRS)
- Cruise Control Switch
- Throttle Position
- Body Control Module (BCM)
- Shifter Position
- Pressure Switches
- Transmission Temperature Sensor
- Input Shaft Speed Sensor
- Output Shaft Speed Sensor
- Line Pressure Sensor
Some examples of indirect inputs to the TCM are:
- Torque Reduction Confirmation
- Engine Coolant Temperature
- Scan Tool Communication
Based on the information received from these various inputs, the TCM determines the appropriate shift schedule and shift points, depending on the present operating conditions and driver demand. This is possible through the control of various direct and indirect outputs.
Some examples of TCM direct outputs are:
- Solenoids
- Torque Reduction Request
Some examples of TCM indirect outputs are:
- Transmission Temperature
- PRNDL Position
In addition to monitoring inputs and controlling outputs, the TCM has other important responsibilities and functions:
- Storing and Selecting Appropriate Shift Schedules
- System Self-diagnostics
- Diagnostic Capabilities (with scan tool)