Transmission Electronic Control System - TorqShift(R)

Transmission Electronic Control System - TorqShift

Electronic System Description

Transmission shift scheduling relies on the following components:

- Accelerator Pedal Position (APP)
- Engine rpm and acceleration
- Vehicle speed and acceleration
- Engine torque
- Torque converter state as defined by engine rpm
- Turbine Shaft Speed (TSS)
- Transmission Range (TR) sensor position
- Brake Pedal Position (BPP) status
- Cruise control status
- Transmission Control Switch (TCS) status
- Transmission Fluid Temperature (TFT)
- Engine Coolant Temperature (ECT)
- Power Take-Off (PTO) engaged signal
- Four-Wheel Drive (4WD) Low status

The PCM and its input/output network control the following transmission operations:

- Variable Force Solenoid (VFS) (shift feel)
- Shift timing
- Line pressure (engagement feel)
- Torque Converter Clutch (TCC) operation

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 certain input information from engine and driver related sensors and switches.

The following components are used to determine engine torque information for the transmission control strategies:

- Crankshaft Position (CKP) sensor
- Camshaft Position (CMP) sensor
- Barometric Pressure (BARO) sensor
- Mass Air Flow (MAF) sensor
- Manifold Absolute Pressure (MAP) sensor
- Engine Oil Temperature (EOT) sensor
- A/C pressure switch
- EGR valve
- Injection Pressure Regulator (IPR)
- Intake Air Temperature (IAT) sensor

Any concerns with the engine sensors must be diagnosed and repaired before proceeding with diagnosis of the transmission components. Refer to Computers and Control Systems for engine component diagnosis.

Using all of these input signals, the PCM can determine when the time and conditions are right for a shift or when to apply or release the torque converter. It also determines the best line pressure needed to optimize shift feel. To accomplish this, the PCM uses 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.

Output Shaft Speed (OSS) Sensor

The transmission OSS sensor is located on the extension housing. The OSS sensor is a Hall-effect type sensor. The OSS sensor reads the gear teeth on the park gear, which are different than the teeth used for the park function. The OSS input to the PCM is used for shift scheduling and TCC operation. The OSS sensor has bi-directional capability and has a digital output.

Engine Coolant Temperature (ECT) Sensor

The ECT sensor is a 2-wire thermistor which changes resistance as temperature changes. The resistance of the sensor increases as engine temperature decreases and the voltage measured by the PCM increases. The PCM uses this information to determine TCC operation.

Intake Air Temperature (IAT) Sensor

The IAT sensor is a thermistor which changes resistance as temperature changes. The resistance of the sensor decreases as the air temperature increases. The IAT provides air temperature information to the PCM which is used to determine transmission line pressure and shift scheduling.

Accelerator Pedal Position (APP) Sensor

The APP sensor is part of the accelerator pedal assembly. The APP detects the position of the accelerator pedal and inputs this information as a voltage signal to the PCM. The PCM uses APP sensor information in determining line pressure, shift scheduling and TCC operation. Failure of this sensor will cause the transmission to operate at high line pressure to avoid damage to the transmission. This high line pressure causes harsh upshifts and harsh engagements.

TSS Sensor and Intermediate Shaft Speed Sensor

The TSS and intermediate shaft speed sensors are both Hall-effect sensors requiring ignition voltage and a ground. The two other terminals at the sensor are TSS/intermediate shaft speed sensor signal output. The TSS sensor detects the teeth on the coast clutch input hub and the intermediate shaft speed sensor on the adjacent overdrive ring gear teeth. Both read 30 teeth per revolution. The TSS/intermediate shaft speed sensor is mounted externally toward the top of the transmission case on the driver's side. The TSS/intermediate shaft speed sensor input to the PCM is digital and is used to determine line pressure, shift scheduling and TCC operation.

Brake Pedal Position (BPP) Switch

Brake pedal position status comes from the BPP switch. The BPP switch sends a voltage signal to the PCM when the brake pedal is applied. The PCM uses this input to disengage the TCC, speed control and auxiliary idle (if equipped).

Powertrain Control Module

The operation of the transmission is controlled by the PCM. Various sensors provide information to the PCM. The PCM controls actuators which determine transmission operation.

Transmission Control Switch (TCS)

The TCS or (O/D OFF switch) is a momentary contact switch located on the end of the selector lever. When pressed, the O/D OFF switch sends a signal to the PCM allowing automatic upshifts that include 5th gear. Overdrive or 6th gear, is disabled during O/D OFF mode. Pressing the O/D OFF button again restores automatic upshifts from 1st through 6th gears or the next ignition cycle.

Transmission Control Indicator Lamp (TCIL)

The TCIL, located in the Instrument Cluster (IC), will illuminate the graphic O/D OFF when the O/D OFF switch is pressed. Operation of the TCIL is controlled by the PCM to alert the driver that the O/D OFF mode is active. Pressing the O/D OFF switch again will turn off the TCIL and restore automatic upshifts from 1st through 6th gears or the next ignition cycle.

The PCM may command the TCIL on if certain transmission DTCs are detected.

Four-Wheel Drive (4WD) Low Switch

The 4WD Low switch sends a ground signal to the IC when the vehicle is in 4WD Low. The PCM then receives a 4WD Low status from the IC and adjusts the transmission shift schedule accordingly.

Transmission Solenoid Body Assembly

The PCM controls the transmission operation through:

- seven VFS.
- a TFT sensor.
- a manual valve.
- an over-pressurization relief valve.

All of the above components are located on the solenoid body.

There is a solenoid dedicated to the function of each clutch. Line pressure and TCC each have their own solenoid. Four solenoids are directly proportional; the pressure output is directly proportional to the applied direct current. Three solenoids are inversely proportional; the pressure output is inversely proportional to the applied direct current.

Solenoids are keyed to prevent misassembly. There are separate connectors for the TFT sensor and the TR switch. The 24-pin connector completes the serviceable harness assembly and has serviceable O-rings.

All of the solenoids except the line pressure solenoid can be serviced without removing the solenoid assembly from the transmission case. The solenoid assembly, which holds the solenoids, is aligned to the transmission case with permanent dowel pins. There is a filter-type gasket between the solenoid assembly and the transmission case.

Transmission Fluid Temperature (TFT) Sensor

The TFT sensor twist-locks into the solenoid body. The TFT is a temperature-sensitive device called a thermistor. As the transmission fluid temperature rises, the TFT sensor resistance decreases. The PCM monitors the voltage across the TFT sensor to determine transmission fluid temperature. The PCM uses the TFT signal to determine hot and cold temperature shift scheduling and TCC operation.

Pressure Control Solenoid A (PCA)

The Pressure Control Solenoid A (PCA) is an inversely proportional 3-port device. The pressure output is inversely proportional to the applied direct current supplied from the PCM through an electronically controlled driver, which varies the current between 0 and 1 amp. The PCA solenoid controls the line pressure circuits.

Torque Converter Clutch (TCC) Solenoid

The TCC solenoid is a directly proportional VFS. The pressure output of this 3-port device is proportional to the applied direct current supplied from the PCM through an electronically controlled driver which varies the current between 0 and 1 amp.

Shift Solenoid Pressure Control Solenoids

The intermediate Shift Solenoid Pressure Control C (SSPCC), low/reverse Shift Solenoid Pressure Control E (SSPCE) and overdrive Shift Solenoid Pressure Control B (SSPCB) clutches are each controlled by a directly proportional VFS. The coast Shift Solenoid Pressure Control A (SSPCA) and the direct clutch Shift Solenoid Pressure Control D (SSPCD) are each controlled by an inversely proportional VFS. All shift pressure solenoids are electronically controlled by the PCM which varies the current from 0 to 1 amp (direct proportional) or 1 to 0 amp (indirectly proportional).

Line Pressure Relief Valve

The solenoid body assembly contains an over-pressurization relief valve that will limit the line pressure through the PCA and feedback to the pump main regulator valve. The line pressure relief valve controls line pressure spikes when cold. If this valve fails, concerns with the filter found in the solenoid body assembly may be seen.

Transmission Range (TR) Sensor Assembly

The TR sensor assembly is an internally mounted sensor that includes the detent spring, rooster comb and bracket, located next to the solenoid body. The components of the TR sensor are factory adjusted and the TR sensor must be installed as a calibrated assembly. The TR sensor contains electronic circuitry that provides the PCM a fixed frequency duty cycle for each of the various positions of the manual lever (PARK, REVERSE, NEUTRAL, (D), M3, M2 and M1) to the PCM. The PCM uses the TR sensor signal for engine start, reverse lamps and for line pressure control, shift scheduling and TCC operation.

Power Take-Off (PTO)

Vehicles may come equipped with a PTO. During stationary operation in PARK or NEUTRAL, the torque converter is locked up when the PTO is ON and the engine operates at 1200 rpm. The following schedule shows expected engine rpm and line pressure while driving with the PTO in use.