Transmission Control Systems: Description and Operation

The electronic control system controls line pressure, transaxle shifting, and torque converter lockup by means of solenoid operation. The line pressure solenoid regulates transaxle line pressure, controls kickdown, and optimizes clutch and braking pressure during shifting. The shift solenoids actuate friction elements (clutches and bands) to control shifting in the transaxle. The torque converter clutch solenoid minimizes lockup shock and enables the powertrain to deliver a 1:1 drive ratio to maximize fuel economy.

Inputs
The following are electronic control system inputs:
^ Throttle position sensor (TPS) Engine speed (RPM)
^ Digital transmission range (TR) switch
^ Vehicle speed sensor (VSS)
^ Overdrive (O/D) control switch
^ Transmission fluid temperature (TFT) sensor
^ Output shaft speed (OSS) sensor

Outputs
The following are electronic control system outputs:
^ Electronic pressure control (EPC) solenoid signal
^ Torque converter clutch (TCC) solenoid signal
^ Coasting clutch solenoid signal
^ Shift solenoid A signal
^ Shift solenoid B signal
^ O/D off indicator

Throttle Position (TP) Sensor
The throttle position (TP) sensor is a potentiometer mounted on the throttle body. The TP sensor detects the position of the throttle plate and sends this information to the TCM as a varying voltage signal.
The transmission control module (TCM) uses the monitored voltage level of the TP sensor for control of EPC pressure, torque converter clutch operation, and shift scheduling.
If a malfunction occurs in the TP sensor circuit, the processor will recognize that the TP sensor signal is out of specification. The processor will then operate the transaxle in a high capacity mode to prevent transaxle damage.

Transmission Control Module (TCM)
The TCM controls transaxle shift timing, feel, and torque converter lockup. The module uses various sensory input to determine the optimum shift points.

Digital Transmission Range (TR) Sensor
The digital transmission range (TR) sensor is located on the outside of the transaxle (toward the front of the vehicle) at the manual lever. The digital sensor completes the start circuit in PARK and NEUTRAL, and the back-up lamp circuit in Reverse. The digital TR sensor also opens/closes a set of switches that are monitored by the TCM to determine the position of the manual lever.

Output Shaft Speed (OSS) Sensor
This sensor is a magnetic pickup that sends a signal to the TCM that indicates transaxle output shaft speed. The OSS provides a means for the TCM to calculate transaxle slippage.

Shift Solenoids
The shift solenoid assembly is located on the main control valve body and contains five solenoids used to control transaxle functions. Shift solenoids A and B control valves in the main control valve body that control various up and down shifts. The coasting clutch solenoid controls operation of the coasting clutch, which provides engine braking when O/D is turned off.

Transmission Fluid Temperature (TFT) Sensor
This sensor is part of the shift solenoid harness. The sensor is immersed in the transaxle fluid and is used by the TCM to help determine shift timing and feel. The sensor is a variable resistor, and the value of its resistance changes with temperature.

Electronic Pressure Control (EPC) Solenoid
The EPC solenoid controls the line pressure in the transaxle by operating the pressure regulator valve. The TCM will control the solenoid in order to match line pressure to current vehicle conditions and requirements.

Torque Converter Clutch (TCC) Solenoid
This solenoid is used in the transaxle control system to control the application, modulation and release of the torque converter clutch.

Vehicle Speed Sensor (VSS)
The VSS is a magnetic pickup that sends a signal to the TCM. The signal is based on vehicle speed and is used to help determine shift timing and feel.