Transmission Electronic Control System
Transmission Electronic Control System
Electronic System Description
Transmission shift scheduling relies on the following:
- Accelerator Pedal Position (APP)
- Engine speed and acceleration
- Vehicle speed and acceleration
- Engine torque
- Converter state as defined by engine speed
- Turbine speed
- Transmission Range (TR) sensor position
- Brake status
- Speed control status
- Transmission Fluid Temperature (TFT)
- Engine Coolant Temperature (ECT)
The PCM and its input/output network control the following transmission operations:
- Variable force shift (shift feel)
- Shift timing
- Line pressure (engagement feel)
- Torque Converter Clutch (TCC) operation
The transmission control strategy 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 and driver related inputs, 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 Information for engine component diagnosis.
Using these input signals, the PCM can determine when the time and conditions are right for a shift, or when to apply or release the TCC. It will also determine the best line pressure needed to optimize shift feel. To accomplish this, the PCM controls the outputs to the transmission solenoids to control transmission operation.
The following provides a brief description of each of the input and output sensors and components used by the PCM for transmission operation.
Output Shaft Speed (OSS) Sensor
The OSS sensor is located on the extension housing. The is a Hall-effect type sensor. The OSS sensor reads the gear teeth on the output shaft park gear as it rotates. This input to the PCM is used for shift scheduling, shift timing, TCC operation and vehicle speed. The OSS has bi-directional capability and has a digital output.
Engine Coolant Temperature (ECT) Sensor (Diesel Engine Vehicles)
The ECT sensor is a 2 wire thermistor device in which resistance changes with temperature change. The resistance of the thermistor decreases as temperature increases, and the resistance increases as the temperature decreases. The varying resistance affects the voltage drop across the ECT sensor and provides an electrical signal to the PCM corresponding to temperature. The PCM uses this information to determine TCC operation.
Cylinder Head Temperature (CHT) Sensor (Gasoline Engine Vehicles)
The CHT sensor is a thermistor device in which resistance changes with temperature change. The resistance of the thermistor decreases as temperature increases, and the resistance increases as the temperature decreases. The varying resistance affects the voltage drop across the CHT sensor and provides an electrical signal to the PCM corresponding to temperature.
Intake Air Temperature (IAT) Sensor
The IAT sensor is a thermistor device in which resistance changes with temperature change. The resistance of the thermistor decreases as temperature increases, and the resistance increases as the temperature decreases. 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 mounted on the accelerator pedal on diesel applications. The APP detects the position of the accelerator pedal and sends this information as a voltage signal to the PCM. The PCM uses information to help determining line pressure, shift scheduling and TCC operation. Failure of this sensor will cause the transmission to operate at higher line pressure to avoid damage to the transmission. This higher line pressure causes harsh upshifts and harsh engagements.
Turbine Shaft Speed (TSS) Sensor and Intermediate Shaft Speed Sensor
The TSS sensor and intermediate shaft speed sensor are Hall-effect sensors requiring 12-volts for power and a ground source. The other 2 terminals at the sensor are for TSS and intermediate shaft speed sensor signal output. The TSS sensor detects teeth on the coast clutch input hub and the adjacent ring gear intermediate shaft speed sensors. Both read 30 teeth per revolution. The TSS and intermediate shaft speed sensors are mounted externally on the transmission case toward the top on the driver's side. The TSS and intermediate shaft speed sensors input to the PCM is a digital signal and is used to determine line pressure, shift scheduling, TCC timing and operation.
Brake Pedal Position (BPP) Switch
Brake status comes from the BPP switch. The BPP switch provides a voltage input 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).
PCM
Transmission operation is controlled by the PCM. Various sensors provide input information to the PCM. The PCM provides output signals to the transmission solenoids which determine transmission operation.
Transmission Control Switch (TCS)
Tow/Haul (Gasoline Engine Vehicles)
The Tow/Haul switch is a momentary contact switch located on the end of the selector lever. When pressed, the Tow/Haul switch sends a signal to the PCM. The PCM then makes changes in shift scheduling and TCC operation.
When Tow/Haul mode is activated the following occurs:
- All forward gears are available.
- Shifts occur at higher vehicle speeds to improve vehicle acceleration, reduce shift frequency and increase coast braking capability.
- TCC occurs at lower vehicle speeds to improve transmission cooling and efficiency.
- Upshifts are temporarily delayed during hill cresting to reduce transmission ratio hunting and to prepare for possible downshifts while descending a grade.
- Engine braking is provided in all forward gear positions, D, 3, 2 and 1.
- A PCM strategy, known as grade braking downshift automatically provides increased coast braking to assist the driver in maintaining a desired vehicle speed while descending a grade.
Default is Tow/Haul OFF.
Transmission Control Indicator Lamp (TCIL)
Tow/Haul (Diesel Engine Vehicles)
When the Tow/Haul switch has been pressed, the TCIL, located near the end of the selector lever, will illuminate the graphics "Tow/Haul ON." Operation of the TCIL is controlled by the PCM to alert the driver that Tow/Haul mode has been activated. Pressing the Tow/Haul switch again will turn off the TCIL. Turning off the engine will return the TCS to the default position (Tow/Haul OFF).
The vehicle PCM may command steady illumination of the if certain faults are detected in monitored sensors, solenoids or apply components.
Transmission Control Switch (TCS)
Overdrive (O/D) OFF (Diesel Engine Vehicles)
The O/D OFF TCS switch is a momentary contact switch located on the end of the selector lever which is used along with the O/D OFF indicator lamp TCIL. When pressed, the O/D OFF switch sends a signal to the PCM to only allow automatic upshifts that include 5th gear. O/D, or 6th gear, is disabled during O/D OFF mode. Pressing the O/D OFF switch again restores automatic upshifts from 1st through 6th gears.
Transmission Control Indicator Lamp (TCIL)
Overdrive (O/D) OFF (Gasoline Engine Vehicles)
When the O/D OFF switch is pressed, the TCIL, located near the end of the selector lever, close to the O/D OFF switch, will illuminate the graphics "Overdrive OFF." Operation of the TCIL is controlled by the PCM to alert the driver that the O/D OFF mode has been activated. Pressing the TCS switch again will turn off the TCIL and restore automatic upshifts, gears 1 through 6.
Each time the vehicle is started, the transmission will automatically turn off the TCS and automatic upshifts, all forward gears 1 through 6 will be available.
The vehicle PCM may command steady illumination of the TCIL if certain faults are detected in monitored sensors, solenoids or apply components.
Transmission Solenoid Body Assembly
The PCM controls the transmission operation through the following:
- Seven Variable Force Solenoid (VFS)
- A TFT sensor
- A manual valve
- An over-pressurization relief valve
All the above components are located on the solenoid body.
There is a solenoid dedicated to the function of each clutch. Line pressure and the TCC each have their own solenoid. Four solenoids are directly proportional; the pressure output is directly proportional to the applied DC current. Three solenoids are inversely proportional; the pressure output is inversely proportional to the applied DC current.
Solenoids are keyed to prevent misassembly. The tan wire connectors connect to the solenoids. There are separate connectors for the sensor and for the sensor. The 24-pin bulkhead connector completes the serviceable harness assembly and has serviceable O-ring seals.
All 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 sensor is a temperature-sensitive device called a thermistor. As the TFT rises, the TFT sensor resistance decreases. The PCM monitors the voltage across the TFT sensor to determine TFT. The PCM uses the TFT sensor signal as an input to determine cold and hot temperature shift and TFT operation.
Line Pressure Control Solenoid A (PCA)
The PCA is an inversely proportional 3-port device. The pressure output is inversely proportional to the applied DC current supplied through an electronically controlled driver, which varies the current between 0 and 1 amp from the PCM. 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 DC current supplied through an electronically controlled driver which varies the current between 0 and 1 amp from the PCM.
Shift Solenoid Pressure Control A (SSPCA), Shift Solenoid Pressure Control B (SSPCB), Shift Solenoid Pressure Control C (SSPCC), Shift Solenoid Pressure Control D (SSPCD) and Shift Solenoid Pressure Control E (SSPCE)
The intermediate (SSPCC), low/reverse (SSPCE) and (SSPCB) clutches are each controlled by a directly proportional VFS. The coast (SSPCA) and the direct clutch (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 solenoid and feedback to the pump main regulator valve. The LPC relief valve controls line pressure spikes when cold. If this valve fails, concerns may be seen with the filter found in the solenoid body assembly.
Transmission Range (TR) Sensor
The TR sensor is an internally mounted sensor that includes the detent spring, rooster comb and bracket, located next to the solenoid body. The TR sensor is non adjustable and is not serviced independently. The components of the TR sensor are factory adjusted to each other and a new sensor must be installed as a calibrated assembly. The TR sensor contains electronic circuitry that provides the PCM a fixed frequency for each of the various positions of the manual lever (PARK, REVERSE, NEUTRAL, DRIVE, M3, M2 and M1) to the PCM. The PCM uses the TR sensor signal for engine functions (start, reverse lamps) and for LPC, shift scheduling and TCC operation.
Power Take-Off (PTO)
Some vehicles may be 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 is operating at 1,200 rpm. The following chart shows the expected engine rpm and line pressure while driving with the PTO in use.
