Transmission Control Systems: Description and Operation
Electronic System DescriptionShift timing, shift feel (line pressure) and torque converter clutch (TCC) control in the automatic transaxle are controlled electronically by the powertrain control module (PCM) and its input/output network. The transaxle control is separate from the engine control strategy in the PCM, although some of the input signals are shared. Some input signals come from the engine related sensors (mass air flow (MAF) sensor, engine coolant temperature (ECT) sensor, etc.) to give the PCM an idea of the load and climate in which the engine is operating under. Some other inputs are based on driver inputs, such as accelerator pedal position which is relayed to the PCM by the throttle position (TP) sensor. Still other inputs are provided by the transaxle itself, from sensors such as the vehicle speed sensor (VSS), transmission range (TR) sensor (controlled by the placement of the transaxle range selector) and the transmission fluid temperature (TFT) sensor.Using all of these input signals, the PCM can determine when the time and conditions are right for a shift or converter clutch application. The PCM can also determine the line pressure needed to optimize shift feel. To accomplish these functions, the PCM controls five electronic solenoids - two On/Off solenoids for shifting, one pulse width modulating (PWM) solenoid for torque converter clutch control, an electronic pressure control (EPC) solenoid for line pressure control, and a 3-2 timing/coast clutch solenoid to control the release of the coast clutch and the coordinated release of the direct clutch and the apply of the low and intermediate band, during a 3-2 downshift.
Powertrain Control Module (PCM) - Inputs
^ Transmission Fluid Temperature (TFT) Sensor
^ Transmission Range (TR) Sensor
^ Brake On Off (BOO) Switch
^ Electronic Ignition (EI) System
- Crankshaft Position (CKP) Sensor
- Ignition Control Module
^ Mass Air Flow (MAF) Sensor
^ Throttle Position (TP) Sensor
^ Vehicle Speed Sensor (VSS)
^ Transmission Control Switch (TCS)
^ Air Conditioning (A/C) Clutch
^ Engine Coolant Temperature (ECT) Sensor
^ Turbine Shaft Speed (TSS) Sensor
^ Output Shaft Speed (OSS) Sensor*
NOTE: * Some components are model dependent.
Powertrain Control Module (PCM) - Output
^ Transaxle solenoid body assembly
- Shift Solenoid A (SSA)
- Shift Solenoid B (SSB)
- Electronic Pressure Control (EPC) Solenoid
- Torque Converter Clutch (TCC) Solenoid
- 3-2 Timing/Coast Clutch Solenoid (3-2T/CCS)
^ Transmission control indicator lamp (TCIL).
Powertrain Control Module (PCM)
The operation of the transaxle is controlled by the powertrain control module. Many input sensors provide information to the PCM. The PCM then controls the actuators which affect transaxle operation.
Air Conditioning (A/C) Clutch
The A/C cycling switch is located on the suction accumulator/drier of an original equipment manufacture (OEM) factory-installed air conditioning system. When the air conditioning clutch cycling switch contacts close, the PCM receives a signal voltage from the A/C clutch switch indicating that the A/C compressor clutch is engaged. The PCM uses the A/C clutch cycling switch signal to adjust line pressure to compensate for the additional engine load.
Brake Pedal Position (BPP) Switch
The BPP switch signals the PCM when the brakes are applied. The BPP switch is closed when the brakes are depressed and open when they are released. The BPP switch will also disengage torque converter clutch (TCC) when brake is applied.
Engine Coolant Temperature (ECT) Sensor
The ECT detects the temperature of the engine coolant and supplies the information to the PCM. The PCM uses the ECT sensor to control torque converter clutch (TCC) operation.
Electronic Ignition (EI) System
The electronic ignition system consists of the PCM, a crankshaft position (CKP) sensor and one multi-tower ignition coil. The CKP sensor sends a crankshaft position signal to the PCM. The PCM then sends the appropriate ignition signal to the ignition coil. The PCM uses this signal in the transaxle shift strategy, as well as torque converter clutch (TCC) control and electronic pressure control. Wide open throttle (WOT) shift control is also affected by the EI system input.
Mass Air Flow (MAF) Sensor
The MAF directly measures the mass of the air flowing into the engine. The MAF sensor output is a D.C. (analog) signal ranging from about 0.5 volt to 5.0 volts used by the PCM to calculate the injector pulse width for air/fuel ratio. For transaxle strategies, the MAF sensor is used for electronic pressure control (EPC), shift and torque converter clutch (TCC) scheduling.
Transmission Control Switch (TCS)
The TCS is a momentary contact switch. When the TCS switch is pressed, a signal is sent to the PCM. The PCM which then energizes the transmission control indicator lamp(TCIL)and engages or disengages fourth gear operation and provides coast braking in second and third.
Transmission Control Indicator Lamp (TCIL)
The TCIL is located in the instrument panel and is labeled O/D OFF. The transmission control switch (TCS) controls the ON/OFF operation of the TCIL. When the driver initially presses the button of the TCS, the TCIL turns ON to indicate that transaxle operation in fourth gear is disabled. When the driver presses the TCS again, the TCIL turns OFF.
Throttle Position (TP) Sensor
The 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 PCM as a varying voltage signal. If a malfunction occurs in the TP sensor circuit, the PCM will recognize that the TP sensor signal is out of specification. The PCM will then operate the transaxle at a high EPC pressure to prevent transaxle damage. The PCM also uses this signal for shift scheduling, EPC and TCC control.
Vehicle Speed Sensor (VSS)
The VSS is a magnetic pickup that sends a signal to the PCM. This VSS signal tells the PCM the vehicle speed and is used to determine shift scheduling and electronic pressure control (EPC).
Transmission Fluid Temperature (TFT) Sensor
The TFT sensor is located on the solenoid valve body. I is a temperature-sensitive device called a thermistor. The resistance value of the TFT sensor will vary with temperature. The PCM monitors the voltage across the TFT sensor to determine the temperature of the transmission fluid. The PCM uses this signal to determine shift scheduling and to control line pressure for cold and hot temperature operation. The PCM also inhibits torque converter clutch (TCC) operation at low transmission temperature and adjusts electronic pressure control (EPC) pressures for temperature.
Transmission Range (TR) Sensor
The PCM sends a voltage signal to the TR sensor. The TR sensor incorporates a series of step down resistors which act as a voltage divider. The PCM monitors this voltage which corresponds to the position of the transmission range selector lever (P,R,N,D,2,1) to determine desired gear and electronic pressure control (EPC) pressure. The TR sensor is located on the top of the transaxle, and also contains the neutral/start, and backup lamp circuits.
Turbine Speed Shaft (TSS) Sensor
The TSS sensor is a magnetic pickup that sends a signal to the PCM that indicates turbine shaft input speed. The TSS sensor provides converter turbine shaft speed information for torque converter clutch (TCC) control strategy. Also used in determining static electronic pressure control (EPC) pressure setting during shifts.
Output Shaft Speed (OSS) Sensor
The OSS Sensor is a magnetic pick-up which detects the park gear teeth rotation and sends a signal to the powertrain control module as an indicator of the transaxle output shaft speed. The OSS signal is processed by the PCM for shift scheduling and inputs to other control modules such as: Electronic Speed, Trip Computer, Speed Control, Adaptive Damping, Auxiliary Warning and Radio CD
Solenoid Valve Body Assembly
The solenoid valve body assembly contains the transmission fluid temperature sensor (TFT) sensor, as well as five PCM controlled output devices:
^ Electronic pressure control (EPC) solenoid
^ Shift solenoid A (SSA)
^ Shift solenoid B (SSB)
^ 3-2 timing/coast clutch solenoid (3-2 T/CCS)
^ Torque converter clutch (TCC) solenoid
Powertrain Control Module (PCM)
The PCM controls the operation of the transaxle through wiring to the solenoid valve body assembly mounted on the main control assembly. An electrical connector from the solenoid valve body assembly fastens to the top of the transaxle case, where it connects to PCM wiring. The solenoid valve body assembly is serviced as one part when any of its components require replacement.
Electronic Pressure Control (EPC) Solenoid
The EPC solenoid is a variable force style (VFS) solenoid. The VFS type solenoid is an electro-hydraulic actuator combining a solenoid and a regulating valve. This solenoid provides electronic pressure control (EPC) which regulates line pressure (LP) and line modulator (LM) pressure. This is controlled by producing a resisting pressure to the main regulator and line modulator circuits. The LP and LM pressures control the clutch application pressures.
Torque Converter Clutch (TCC) Solenoid
The TCC solenoid is a pulse width modulating (PWM) style solenoid. The PWM solenoid is used to control the apply and release of the bypass clutch in the torque converter. By modulating the pulse width of the TCC solenoid the pressure in the S4 circuit varies modulating the apply and release of the bypass clutch in the torque converter.
3-2 Timing/Coast Clutch Solenoid (3-2 T/CCS)
The 3-2 T/CCS is a variable force (VFS) solenoid. The VFS type solenoid is an electro-hydraulic actuator combining a solenoid and a regulating valve. It supplies pressure to the S3 circuit to control the release of the direct clutch and to apply the intermediate and overdrive band during a 3-2 downshift. This solenoid also regulates the pressure in the S3 circuit to control the application and release of the coast clutch.
Shift Solenoid Assembly (SSA, SSB, SSC)
Shift solenoids A and B provide gear selection of first through fourth gears by providing ON/OFF pressure control to the shift valves.
Solenoid Operation
The following solenoid operation chart shows normal solenoid operation for the given operation modes.
REFERENCE: SOLENOID OPERATION CHART No. 701
* Not allowed by hydraulics.
** Powertrain control module (PCM) commanded.
*** When a manual pull-in occurs above a calibrated speed the transaxle will not downshift from the higher gear until the vehicle speed drops below this calibrated speed.