Transmission Electronic Control System
Transmission Electronic Control System
Electronic System Description
Transmission shift scheduling relies on the following:
- 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
- Speed control status
- Tow/haul switch 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 input information from certain engine-related and driver demand-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 Information 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 TCC. It will also determine 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 Output Shaft Speed (OSS) sensor is located on the extension housing. The OSS is a Hall-effect type sensor. The OSS reads the gear teeth on the park gear, which are different than the teeth used for park function. The OSS input to the PCM is used for shift scheduling, TCC operation and vehicle speed. The OSS 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 mounted on the accelerator pedal on diesel applications. 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 other 2 terminals at the sensor are for TSS/intermediate shaft speed sensor signal output. The TSS sensor detects teeth on the coast clutch input hub and the intermediate shaft speed sensor detects teeth on the adjacent Overdrive (O/D) ring gear. 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 digital and is used to determine line pressure, shift scheduling, timing and TCC operation.
Brake Pedal Position (BPP) Switch
Brake status comes from the BPP switch. The BPP sensor applies battery voltage 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)
Tow/Haul (Diesel 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 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 is applied 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 is pressed, the Transmission Control Indicator Lamp (TCIL), located in the Instrument Cluster (IC), 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 TCS again will turn off the TCIL. Turning off the engine will return the TCIL to the default position (Tow/Haul OFF).
The PCM may command steady illumination of the TCIL if certain faults are detected in monitored sensors, solenoids or apply components.
Transmission Control Switch (TCS)
Overdrive (O/D) OFF (Gasoline Engine Vehicles)
The O/D OFF Transmission Control Switch (TCS) is a momentary contact switch located on the end of the selector lever which is used along with the O/D OFF indicator lamp. 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 button 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 in the IC, will illuminate the graphics O/D OFF. Operation of the TCIL is controlled by the PCM to alert the driver that O/D OFF mode has been activated. Pressing the TCS again will turn off the TCIL and restore automatic upshifts, gears 1 through 6.
Turning off the engine will return the TCIL to the default position and automatic upshifts, all forward gears 1 through 6 will be available.
The PCM may command steady illumination of the TCIL if certain faults are detected in monitored sensors, solenoids or apply components.
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 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 the 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. The tan wire connectors connect to the solenoids. There are separate connectors for the TFT sensor and for the TR switch. The 24-pin connector completes the serviceable harness assembly and has serviceable O-ring seals.
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 TFT rises, the TFT sensor resistance decreases. The PCM monitors the voltage across the TFT sensor to determine TFT. The PCM uses the TFT signal as an input to determine cold and hot 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 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 direct current supplied through an electronically controlled driver which varies the current between 0 and 1 amp from the PCM.
Shift Solenoid Pressure Control Solenoids
The intermediate Shift Solenoid Pressure Control C (SSPCC), low/reverse Shift Solenoid Pressure Control E (SSPCE) and O/D 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 to each other 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)
Some of these vehicle 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 operates at 1200 rpm. The following schedule shows expected engine rpm and line pressure while driving with the PTO in use.
