Transmission Control Systems: Description and Operation

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The Powertrain Control Module (PCM) and its input/output network control the following transmission operations:
- shift timing
- line pressure (shift feel)
- torque converter clutch operation

The transmission control is separate from the engine control strategy in the powertrain control module, 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 powertrain control module 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 powertrain control module 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 powertrain control module uses six output solenoids to control transmission operation.

The following provides a brief description of each of the sensors and actuators used by the PCM for transmission operation.

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 MAF sensor is used to regulate Electronic Control Pressure (EPC), shift and torque converter clutch scheduling.

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

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.

POWERTRAIN CONTROL MODULE (PCM)
The operation of the transmission is controlled by the powertrain control module. Many input sensors provide information to the powertrain control module. The powertrain control module then controls actuators which determine transmission operation.

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 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.

VEHICLE SPEED SENSOR (VSS)
The Vehicle Speed Sensor (VSS) (model-dependent) is a variable reluctance sensor that sends an A/C frequency signal to the powertrain control module. The vehicle speed sensor signal is used by the powertrain control module to calculate vehicle speed in mph and determine shift scheduling and electronic pressure control.

TRANSMISSION SOLENOID BODY
The powertrain control module controls the E40D transmission operation through four on/off solenoids and one variable force solenoid. These solenoids and transmission fluid temperature sensor are housed in the transmission solenoid body assembly. All are part of the transmission solenoid body and are not replaced individually. Additionally, in 1995, the protection diodes that were on the solenoid body were moved to the powertrain control module.

TRANSMISSION FLUID TEMPERATURE (TFT) SENSOR
The transmission fluid temperature sensor is located on the solenoid body assembly in the transmission sump. It is a temperature-sensitive device called a thermistor. The resistance value of the transmission fluid temperature sensor will vary with temperature change.

The powertrain control module monitors voltage across the transmission fluid temperature sensor to determine the temperature of the transmission fluid.

The powertrain control module uses this signal to determine whether a cold start shift schedule is necessary. The cold start shift schedule lowers shift speeds to allow for the reduced performance of cold engine operation. The powertrain control module also uses the transmission fluid temperature sensor input to adjust electronic pressure control pressure for temperature effects and to inhibit torque converter clutch operation during the warm-up period.

COAST CLUTCH SOLENOID (CCS)
The coast clutch solenoid provides coast clutch control by shifting the coast clutch shift valve. The solenoid is activated by pressing the transmission control switch or by selecting the 1 or 2 range with the transmission range selector lever. In MANUAL 1 and 2, the coast clutch is controlled by the solenoid and also hydraulically as a fail-safe to ensure engine braking. In reverse, the coast clutch is controlled hydraulically and the solenoid is not on.

TORQUE CONVERTER CLUTCH (TCC) SOLENOID
The Torque Converter Clutch Solenoid (TCC solenoid) provides torque converter clutch control by shifting the converter clutch control valve to apply or release the torque converter clutch.

ELECTRONIC PRESSURE CONTROL (EPC) SOLENOID

CAUTION: The electronic pressure control pressure output from the variable force solenoid is NOT adjustable. Any modification to the electronic pressure control solenoid may void the transmission warranty.

The electronic pressure control solenoid is a variable force solenoid. The variable-force type solenoid is an electrohydraulic actuator combining a solenoid and a regulating valve. It supplies electronic pressure control that regulates transmission line pressure and line modulator pressure. This is done by producing resisting forces to the main regulator and the line modulator circuits. These two pressures control clutch application pressures.

SHIFT SOLENOIDS A (1) AND B (2)
Shift solenoids l and 2 provide gear selection of first through fourth gears by controlling the pressure to the three shift valves.

DIGITAL TRANSMISSION RANGE (TR) SENSOR
The digital transmission range sensor is located on the outside of the transmission at the manual lever. The sensor completes the start circuit in Park and Neutral, the back-up lamp circuit in Reverse and a neutral sense circuit for GEM control of 4 x 4 low engagement. The 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).

4X4 LOW (4X4L) SWITCH
The 4x4 low (4x4L) range switch is located on the transfer case cover. It provides an indication of when the 4x4 transfer case gear system is in the low range. The powertrain control module then modifies shift schedule for 4x4L operation.

BRAKE ON/OFF (BPP) SWITCH
The Brake Pedal Position Switch (BPP) tells the powertrain control module 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.

ELECTRONIC IGNITION (EL) 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 PCM. The PCM uses PIP signal in the transmission strategy, Wide-Open Throttle (WOT) shift control, torque converter clutch control and EPC pressure.

DISTRIBUTOR IGNITION SYSTEM
The profile ignition pickup sensor sends a signal to the powertrain control module indicating the engine rpm and the crankshaft position.

AIR CONDITIONING (A/C) CLUTCH
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 A/C clutch is engaged, Electronic Pressure Control (EPC) is adjusted by the PCM to compensate for additional load on the engine.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR-GASOLINE ENGINES
On gasoline engines, the Manifold Absolute Pressure Sensor (MAP sensor) senses atmospheric pressure to produce an electrical signal. The frequency of this signal varies with intake manifold pressure. The powertrain control module monitors this signal to determine altitude. The powertrain control module then adjusts the E40D shift schedule and EPC pressure for altitude. On diesel engines, the manifold absolute pressure sensor measures boost pressure. The powertrain control module monitors this signal and adjust EPC pressure.