Grand Cherokee 2WD V6-3.7L (2008)
Control Module: Description and Operation
Transmission Control Module - Operation
3.7L GAS/3.0L DIESEL VEHICLES
The transmission control module (TCM) determines the current operating conditions of the vehicle and controls the shifting process for shift comfort and
driving situations. It receives this operating data from sensors and broadcast messages from other modules.
The TCM uses inputs from several sensors that are directly hardwired to the controller and it uses several indirect inputs that are used to control shifts.
This information is used to actuate the proper solenoids in the valve body to achieve the desired gear.
The shift lever sensor assembly (SLSA) has sensors that are monitored by the TCM to calculate shift lever position. The reverse light switch, an integral
part of the SLSA, controls the reverse light relay control circuit. The Brake/Transmission Shift Interlock (BTSI) solenoid and the park lockout solenoid
(also part of the SLSA) are controlled by the TCM.
The ECM and ABS broadcast messages over the controller area network (CAN C) bus for use by the TCM. The TCM uses this information, with other
inputs, to determine the transmission operating conditions.
The TCM:
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determines the momentary operating conditions of the vehicle.
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controls all shift processes.
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considers shift comfort and the driving situation.
The TCM controls the solenoid valves for modulating shift pressures and gear changes. Relative to the torque being transmitted, the required pressures
are calculated from load conditions, engine rpm, vehicle speed, and ATF temperature.
The following functions are contained in the TCM:
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Shift Program.
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Downshift Safety.
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Torque Converter Lock-Up Clutch.
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Adaptation.
This transmission does not have a TCM relay. Power is supplied to the SLSA and the TCM directly from the ignition.
The TCM continuously checks for electrical problems, mechanical problems, and some hydraulic problems. When a problem is sensed, the TCM stores a
diagnostic trouble code (DTC). Some of these codes cause the transmission to go into "Limp-In" or "default" mode. Some DTCs cause permanent
Limp-In and others cause temporary Limp-In. The NAG1 defaults in the current gear position if a DTC is detected, then after a key cycle the
transmission will go into Limp-in, which is mechanical 2nd gear. Some DTCs may allow the transmission to resume normal operation (recover) if the
detected problem goes away. A permanent Limp-In DTC will recover when the key is cycled, but if the same DTC is detected for three key cycles the
system will not recover and the DTC must be cleared from the TCM with the scan tool.
TCM SIGNALS
The TCM registers one part of the input signals by direct inputs, the other part by CAN C bus. In addition to the direct control of the actuators, the TCM
sends various output signals by CAN C bus to other control modules.
Selector Lever Position
A series of 12 Hall-effect switches in the SLSA inform the TCM of the position of the selector lever.
The TCM monitors the SLSA for all shift lever positions through five position circuits. The SLSA provides a low-current 12-volt signal to the TCM. The
TCM compares the on/off signals to programmed combinations to determine the exact position of the shift lever.
ATF Temperature Sensor
The ATF temperature sensor is a positive temperature co-efficient (PTC) thermistor. It measures the temperature of the transmission fluid and is a direct
input signal for the TCM. The temperature of the ATF has an influence on the shifttime and resulting shift quality. As the temperature rises, resistance
rises, and therefore, the probing voltage is decreasing. Because of its registration, the shifting process can be optimized in all temperature ranges.
The ATF temperature sensor is wired in series with the park/neutral contact. The temperature signal is transmitted to the TCM only when the reed
contact of the park/neutral contact is closed because the TCM only reads ATF temperature while in any forward gear, or REVERSE. When the
transmission is in PARK or NEUTRAL, the TCM will substitute the engine temperature for the ATF temperature.
