Transmission Control Systems: Description and Operation

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Electronic System Description
The Powertrain Control Module (PCM) and its input/output network control the following transmission operations:
^ Shift timing
^ Line pressure (shift feel)
^ Torque converter clutch
The transmission control is separate from the engine control strategy in the PCM, although some of the input signals are shared. When determining the best operating strategy for transmission operation, the powertrain control module uses input information from certain engine-related and driver-demand related sensors and switches.
In addition, the PCM receives input signals from certain transmission-related sensors and switches. The powertrain control module also uses these signals when determining transmission operating strategy.
Using all of these inputs signals, the PCM can determine when the time and conditions are right for a shift, or when to apply or release the torque converter clutch. It will also determine the best line pressure needed to optimize shift feel. To accomplish this the PCM uses hydraulic solenoids to control transmission operation.
The following provides a brief description of each of the sensors and actuators used to control transmission operation.

Engine Coolant Temperature (ECT) Sensor
The Engine Coolant Temperature (ECT) sensor detects the temperature of engine coolant and supplies the information to the powertrain control module. The ECT sensor is used to control Torque Converter Clutch (TCC) operation. The ECT is installed in the heater" outlet fitting or cooling passage on the engine. For engine control applications, the ECT signal is used to modify ignition timing, EGR flow and air-to-fuel ratio as a function of engine coolant temperature.

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 Powertrain Control Module (PCM). The TP sensor is used for shift scheduling, electronic pressure control and Torque Converter Clutch (TCC) control.

Mass Air Flow (MAF) Sensor
The Mass Air Flow Sensor (MAF) measures the mass of air flowing into the engine. The MAF sensor output signal is used by the powertrain control module to calculate injector pulse width For transmission strategies the mass air flow sensor is used to regulate Electronic Pressure Control (EPC) shift and torque converter clutch scheduling.

Powertrain Control Module (PCM)
The operation of the transmission is controlled by the Powertrain Control Module (PCM). Many input sensors provide information to the PCM. The PCM then controls actuators which determine transmission operation.

Air Conditioning Clutch (A/C)
An electromagnetic clutch is energized when the clutch cycling pressure switch closes. The switch is located on the suction accumulator/drier. The closing of the switch completes the circuit to the clutch and draws it into engagement with the compressor driveshaft. When the AJC clutch is engaged, Electronic Pressure Control (EPC) is adjusted by the Powertrain Control Module (PCM) to compensate for additional load on the engine.

Intake Air Temperature (IAT) Sensor
The Intake Air Temperature (IAT) sensor provides the sequential fuel injection (SFI) system mixture temperature information. The IAT sensor is used both as a density corrector for air flow calculation and to proportion cold enrichment fuel flow. The IAT sensor is installed in the air cleaner outlet tube. The IAT sensor is also used in determining Electronic Pressure Control (EPC) pressures.

Transmission Control Switch (TCS) and Transmission Control Indicator Lamp (TCIL)
The Transmission Control Switch (TCS) is a momentary contact switch. When the switch is pressed, a signal is sent to the powertrain control module to allow automatic shifts from first through fourth gears or first through third gears only. The Powertrain Control Module (PCM) energizes the Transmission Control Indicator Lamp (TCIL) when the switch is off. The TCIL indicates overdrive cancel mode activated (lamp on) and Electronic Pressure Control (EPC) circuit shorted (lamp flashing) or monitored sensor failure.

Electronic Ignition (EI) System
The electronic ignition consists of a crankshaft position sensor, two four tower ignition coils and the powertrain control module. The ignition control module operates by sending crankshaft position information from the crankshaft position sensor to the ignition control module. The ignition control module generates a Profile Ignition Pickup (PIP) signal (engine rpm) and sends it to the Powertrain Control Module (PCM). The PCM uses PIP signal in the transmission strategy, Wide-Open Throttle (WOT) shift control, torque converter clutch control and EPC pressure.

Brake Pedal Position (BPP) Switch
The Brake Pedal Position (BPP) switch tells the Powertrain Control Module (PCM) when the brakes are applied. The torque converter clutch disengages when the brakes are applied. The BPP switch closes when the brakes are applied and opens when they are released.

Output Shaft Speed (OSS) Sensor
The Output Shaft Speed (OSS) sensor is a magnetic pickup, located at the output shaft ring gear, that sends a signal to the powertrain control module to indicate transmission output shaft speed. The OSS is used for torque converter clutch control, shift scheduling and to determine electronic pressure control.

Digital Transmission Range (TR) Sensor
The digital Transmission Range (TR) sensor is located on the outside of the transmission at the manual lever. The digital sensor completes the start circuit in PARK and NEUTRAL, the back-up lamp circuit in REVERSE. The digital sensor also opens/closes a set of four switches that are monitored by the Powertrain Control Module (PCM) to determine the position of the manual lever (P, R, N, D, 2, 1).,

Anti-Lock Brake Speed (ABS) Sensor
The Programmable Speedometer/Odometer Module (PSOM) receives input from the rear brake anti-lock sensor. After processing the signal, the PSOM relays it to the Powertrain Control Module (PCM) and the speed control module.

Transmission Fluid Temperature (TFT) Sensor
The Transmission Fluid Temperature (TFT) sensor is located on the lead frame assembly near the shift solenoids on the main control valve body. It is a temperature sensitive device called a thermistor. It sends a voltage signal to the Powertrain Control Module (PCM). The voltage signal varies with transmission fluid temperature. The PCM uses this signal to determine whether a cold start shift schedule is necessary. The shift schedule is compensated when the transmission fluid temperature is cold. The PCM also inhibits Torque Converter Clutch (TCC) operation at low transmission fluid temperatures and corrects electronic pressure control.

Electronic Pressure Control (EPC) Solenoid
The Electronic Pressure Control (EPC) solenoid regulates transmission pressure. The EPC valve pressure is used to control line pressure.

Shift Solenoids - SSA, SSB
Two on/off shift solenoids provide gear selection of first through fourth gears by controlling the pressure to the three shift valves. One unit containing the two shift solenoids is located in the main control valve body. The shift solenoids are two-way, normally open style.

Solenoid Operation Chart - 1 Of 2:

Solenoid Operation Chart - 2 Of 2:

Torque Converter Clutch (TCC) Solenoid
The Torque Converter Clutch (TCC) solenoid is used to control the apply and release of the TCC.