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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 PCM 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 PCM 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 feed To accomplish this the PCM uses six output solenoids to control transmission operation.

The following provides a brief description of each of the sensors and actuators used to control transmission operation.

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.

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.

ENGINE COOLANT TEMPERATURE (ECT) SENSOR
The Engine Coolant Temperature (ECT) sensor detects temperature of engine coolant and supplies the information to the Powertrain Control Module (PCM). The ECT sensor is used to control Torque Converter Clutch (TCC) operation.

ANTI-LOCK BRAKE SPEED 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.
Information from the PSOM is used by the PCM to help determine shift scheduling torque converter clutch operation and Electronic Pressure Control (EPC).

TRANSMISSION CONTROL SWITCH (TSC) AND TRANSMISSION CONTROL INDICATOR LAMP (TCIL)
The Transmission Control Switch (TCS) is a momentary contact switch that allows the driver to cancel operation of 5th (overdrive) gear.

The Powertrain Control Module (PCM) uses the shift solenoids to disengage/disable 5th gear operation and activate the coast clutch.

At the same time the PCM illuminates the Transmission Control Indicator Lamp (TCIL), to notify the driver that 5th gear is canceled.

When the TCS is pressed again, 5th gear operation is enabled' the coast clutch is released and the TCIL is turned off.

Whenever the ignition is cycled (vehicle shut off then started again) the TCS Is turned off and 5th gear will be enabled, even if the TCS had been on when the ignition was shut off.

The Transmission Control Indicator Lamp (TCIL) Is located in the instrument panel and is labeled O/D OFF It is illuminated in conjunction with the Transmission Control Switch (TCS).

BRAKE PEDAL POSITION (BPP) SWITCH
The Brake On/Off (BOO) switch tells the powertrain control module (PCM) when the brakes are applied. The torque converter clutch disengages when the brakes are applied. The BOO switch closes when the brakes are applied and opens when they are released.

ELECTRONIC IGNITION (EI) SYSTEM
The electronic ignition consists of a crankshaft position sensor, two four tower ignition coils and the Powertrain Control Module (PCM). 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.

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 is engaged, Electronic Pressure Control (EPC) pressure is adjusted to compensate for additional load on the engine.

MASS AIR FLOW (MAF) SENSOR
The Mass Air Flow (MAF) sensor 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 pressure control, shift and torque converter clutch scheduling.

TURBINE SHAFT SPEED (TSS) SENSOR
The Turbine Shaft Speed (TSS) sensor is a magnetic pickup that sends the Powertrain Control Module (PCM) torque converter turbine speed information.

The TSS sensor is mounted internally on the center support.

The PCM uses TSS information to help determine Electronic Pressure Control (EPC) and Torque Converter Clutch (TCC) operation.

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 PCM then modifies the shift schedule for 4x4L transfer case gear ratio.

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 (PCM) 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 TR sensor completes the start circuit in Park and Neutral, the back-up lamp circuit in Reverse and the neutral sense circuit (4x4 only) in Neutral. The digital TR sensor also opens and closes a set of four switches that are monitored by the powertrain control module to determine the position of the manual lever (P, R, N, D, 2, 1).

TRANSMISSION FLUID TEMPERATURE (TFT) SENSOR
- The Transmission Fluid Temperature (TFT) sensor is a thermistor-type sensor that varies a reference voltage signal. The resistance in the TFT varies with temperature The Powertrain Control Module (PCM) monitors the voltage signal across the TFT, and uses this information to determine the transmission fluid temperature.
- The TFT is located on the main control body wiring harness assembly.
- The Powertrain Control Module (PCM) uses the TFT signal to help determine shift scheduling, torque converter clutch operation and Electronic Pressure Control (EPC).

SHIFT SOLENOIDS-1, 2, 3, AND 4
Four On/Off shift solenoids allow the Powertrain Control Module (PCM) to control shift scheduling.

The solenoids are two-way, normally open style.

The shift solenoids (SSA/SS1, SSB/SS2, SSC/SS3 and SSD/SS4) provide gear selection of 1st through 5th gears by controlling the pressures of the shift valves.

SSD/SS4 is also used to apply and release the coast clutch.

ELECTRONIC PRESSURE CONTROL (EPC) SOLENOID
The Electronic Pressure Control (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.

The Powertrain Control Module (PCM) varies the current to the EPC solenoid.

This action causes the solenoid to regulate transmission line pressure and line modulator pressure. This is done by producing resisting forces to the main regulator and line modulator circuits. These two pressures control clutch application pressures.

TORQUE CONVERTER CLUTCH (TCC) SOLENOID
The Torque Converter Clutch (TCC) solenoid is used to control the apply and release of the TCC.

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 (PCM). The PCM then controls actuators which determine transmission operation.
- Check the fluid levels and condition.
- Check for non-factory add-on items.
- Check shift linkages for proper adjustment.
- Check TSBs messages regarding the concern.

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