Automatic Transmission/Transaxle: Description and Operation
TRANSAXLE DESCRIPTIONThis automatic transaxle is a four-speed, front wheel drive automatic transaxle with electronic controls for:
^ Electronic pressure control (EPC) solenoid for shift quality.
^ Shift scheduling.
^ 3-2 shift timing.
^ Coast braking.
^ Torque converter clutch control.
The transaxle features a four element torque converter with a torque clutch and a geartrain that includes:
^ Compound planetary gearset.
^ Chain drive.
Planetary gearset final drive. Pinion and side gear differential.
The hydraulic control system of the transaxle has five solenoids that control:
^ Shift feel, through line pressure control.
^ Shift scheduling, through shift valve positioning control. Modulated application of the torque converter clutch.
^ Timing of 3-2 shifts.
^ Engine braking during coast operation.
IDENTIFICATION TAGS
When servicing the automatic transaxle, see the identification tag located on the case.
RANGE SELECTION
The transaxle range selector lever has six positions: P, R, N, DRIVE, 2,
1. In addition, a transaxle control switch (TCS) allows the driver to prevent a shift to fourth gear (Overdrive) and uses engine compression to help slow the vehicle (engine braking) in second and third gear.
Park
No power flow is transferred through the transaxle in PARK. A shift lever connected to a set of cams presses the parking pawl into the park gear on the driven sprocket. This locks the final drive and prevents the vehicle from rolling. For safety reasons, always apply the parking brake whenever the vehicle is parked.
Reverse
REVERSE allows the vehicle to be operated in a rearward direction, at a reduced gear ratio. Engine braking is provided in REVERSE.
Neutral
As in PARK, there is no power transferred through the transaxle in NEUTRAL. However, the final drive is not locked by the parking pawl, so the wheels are free to rotate. The vehicle may be started in NEUTRAL.
Drive
The DRIVE position provides all automatic shifts (first through fourth), apply and release of the torque converter clutch, and maximum fuel economy during normal operation. Engine braking is provided in the fourth gear. Fourth gear (Overdrive) may be canceled by depressing the transaxle control switch (TCS) that is located on the transaxle range selector. Engine braking is also provided in second and third gear with the TCS on.
Second Gear Position
SECOND provides a second gear hold position after automatic or manual upshift or downshift. When SECOND is selected from a stop, the transaxle will start in second gear for maximum traction on slippery surfaces. Engine braking is provided in second gear when in the SECOND position.
Low Position
MANUAL LOW provides a first gear hold after automatic or manual downshift. The transaxle is prevented from downshifting into first gear above a specific speed approximately 48 km/h (30 mph) to protect the powertrain from over speeding. Engine braking is provided in MANUAL LOW position making it especially useful for descending steep grades.
TRANSAXLE ELECTRONIC CONTROL SYSTEM
Shift timing, shift feel (line pressure) and torque converter clutch (TCC) control in the automatic transaxle are controlled electronically by the powertrain control module (PCM) and its input/output network. The transaxle control is separate from the engine control strategy in the PCM, although some of the input signals are shared. Some input signals come from the engine-related sensors, mass air flow (MAF) sensor, engine coolant temperature (ECT) sensor to give the PCM an idea of the load and climate in which the engine is operating under. Some other inputs are based on driver inputs, such as accelerator pedal position which is relayed to the PCM by the throttle position (TP) sensor. Still other inputs are provided by the transaxle itself, from sensors such as the transaxle range (TR) sensor (controlled by the placement of the transaxle range selector) and the transaxle fluid temperature (TFT) sensor. Using all of these input signals, the PCM can determine when the time and conditions are right for a shift or converter clutch application. The PCM can also determine the line pressure needed to optimize shift feel. To accomplish these functions, the PCM controls five electronic solenoids, two ON/OFF solenoids for shifting, one pulse width modulating (PWM) solenoid for torque converter clutch control, an electronic pressure control (EPC) solenoid for line pressure control, and a 3-2 timing/coast clutch solenoid to control the release of the coast clutch and the coordinated release of the direct clutch and the apply of the low and intermediate band, during a 3-2 downshift.
Powertrain Control Module (PCM)
The operation of the transaxle is controlled by the powertrain control module. Many input sensors provide information to the PCM. The PCM then controls the actuators which affect transaxle operation.
Air Conditioning (A/C) Clutch
The A/C cycling switch is located on the suction accumulator/drier of an original equipment manufacture (OEM) factory-installed air conditioning system. When the air conditioning clutch cycling switch contacts close, the PCM receives a signal voltage from the A/C clutch switch indicating that the A/C compressor clutch is engaged. The PCM uses the A/C clutch cycling switch signal to adjust line pressure to compensate for the additional engine load.
Brake Pedal Position (BPP) Switch
The BPP switch signals the PCM when the brakes are applied. The BPP switch is closed when the brakes are depressed and open when they are released. The BPP switch will also disengage torque converter clutch (TCC) when brake is applied.
Engine Coolant Temperature (ECT) Sensor
The ECT detects the temperature of the engine coolant and supplies the information to the PCM. The PCM uses the ECT sensor to control torque converter clutch (TCC) operation.
Electronic Ignition (EI) System
The electronic ignition system consists of the PCM, a crankshaft position (CKP) sensor and one multi-tower ignition coil. The CKP sensor sends a crankshaft position signal to the PCM. The PCM then sends the appropriate ignition signal to the ignition coil. The PCM uses this signal in the transaxle shift strategy, as well as torque converter clutch (TCC) control and electronic pressure control. Wide open throttle (WOT) shift control is also affected by the El system input.
Mass Air Flow (MAF) Sensor
The MAF directly measures the mass of the air flowing into the engine. The MAF sensor output is a D.C. (analog) signal ranging from about 0.5 volt to 5.0 volts used by the PCM to calculate the injector pulse width for air/fuel ratio. For transaxle strategies, the MAF sensor is used for electronic pressure control (EPC), shift and torque converter clutch (TCC) scheduling. Transmission Control Switch (TCS) The TCS is a momentary contact switch. When the TCS switch is pressed, a signal is sent to the PCM. The PCM which then energizes the transaxle control indicator lamp (TCIL) and engages or disengages fourth gear operation and provides coast braking in second and third.
Transmission Control Indicator Lamp (TCIL)
The TCIL is located in the instrument panel and is labeled O/D OFF. The transaxle control switch (TCS) controls the ON/OFF operation of the TCIL. When the driver initially presses the button of the TCS, the TCIL turns ON to indicate that transaxle operation in fourth gear is disabled. When the driver presses the TCS again, the TCIL turns OFF.
Throttle Position (TP) Sensor
The 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 PCM as a varying voltage signal. If a fault occurs in the TIP sensor circuit, the PCM will recognize that the TIP sensor signal is out of specification. The PCM will then operate the transaxle at a high EPC pressure to prevent transaxle damage. The PCM also uses this signal for shift scheduling, EPC and TCC control.
Transmission Fluid Temperature (TFT) Sensor
The TFT sensor is located on the solenoid valve body. It is a temperature-sensitive device called a thermistor. The resistance value of the TFT sensor will vary with temperature. The PCM monitors the voltage across the TFT sensor to determine the temperature of the transaxle fluid. The PCM uses this signal to determine shift scheduling and to control line pressure for cold and hot temperature operation. The PCM also inhibits torque converter Clutch (TCC) operation at low transaxle temperature and adjusts electronic pressure control (EPC) pressures for temperature.
Transmission Range (TR) Sensor
The PCM sends a voltage signal to the TR sensor. The TR sensor incorporates a series of step down resistors which act as a voltage divider. The PCM monitors this voltage which corresponds to the position of the transaxle range selector lever (P, R, N, DRIVE, 2, 1) to determine desired gear and electronic pressure control (EPC) pressure. The TR sensor is located on the top of the transaxle, and also contains the neutral/start, and backup lamp circuits.
Turbine Speed Shaft (TSS) Sensor
The TSS sensor is a magnetic pickup that sends a signal to the PCM that indicates turbine shaft input speed. The TSS sensor provides converter turbine shaft speed information for torque converter clutch (TCC) control strategy. Also used in determining static electronic pressure control (EPC) pressure setting during shifts.
Output Shaft Speed (OSS) Sensor
The OSS Sensor is a magnetic pick-up which detects the park gear teeth rotation and sends a signal to the powertrain control module as an indicator of the transaxle output shaft speed. The OSS signal is processed by the PCM for shift scheduling and inputs to other control modules such as: Electronic Speedometer, Trip Computer, Speed Control, Adaptive Damping, Auxiliary Warning and Radio CD.
Solenoid Valve Body Assembly
The solenoid valve body assembly contains the transaxle fluid temperature sensor (TFT) sensor, as well as five PCM controlled output devices:
^ Electronic pressure control (EPC) solenoid.
^ Shift solenoid A (SS1).
^ Shift solenoid B (SS2).
^ 3-2 timing/coast clutch solenoid (3-2 TCCS).
^ Torque converter clutch (TCC) solenoid.
Powertrain Control Module (PCM)
The PCM controls the operation of the transaxle through wiring to the solenoid valve body assembly mounted on the main control assembly. An electrical connector from the solenoid valve body assembly fastens to the top of the transaxle case, where it connects to PCM wiring. The solenoid valve body assembly is serviced as one part when any new components are installed.
Electronic Pressure Control (EPC) Solenoid
The 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. This solenoid provides electronic pressure control (EPC) which regulates line pressure (LP) and line modulator (LM) pressure. This is controlled by producing a resisting pressure to the main regulator and line modulator circuits. The LP and LM pressures control the clutch application pressures.
Torque Converter Clutch (TCC) Solenoid
The TCC solenoid is a pulse width modulating (PWM) style solenoid. The PWM solenoid is used to control the apply and release of the bypass clutch in the torque converter. By modulating the pulse width of the TCC solenoid the pressure in the S4 circuit varies modulating the apply and release of the bypass clutch in the torque convener.
3-2 Timing/Coast Clutch Solenoid (3-2 T/CCS)
The 3-2 T/CCS is a variable force (VFS) solenoid. The VFS type solenoid is an electro-hydraulic actuator combining a solenoid and a regulating valve. It supplies pressure to the S3 circuit to control the release of the direct clutch and to apply the intermediate and overdrive band during a 3-2 downshift. This solenoid also regulates the pressure in the S3 circuit to control the application and release of the coast clutch.
Shift Solenoid Assembly (SS1, SS2)
Shift solenoids A and B provide gear selection of first through fourth gears by providing ON/OFF pressure control to the shift valves.
SHIFT PATTERNS
Upshifts
The powertrain control module (PCM) has an adaptive learn strategy to electronically control the transaxle. The adaptive learn strategy will automatically adjust the shift feel to the driver demands. The first few hundred miles of operation the transaxle may have abrupt shifts, this is a normal operation and will correct itself. If the battery has been disconnected for longer than 20 minutes the shift tables will reset and need to be relearned. Upshifting is controlled by the PCM. The PCM receives inputs from various engine and vehicle sensors along with driver demands to control shift scheduling, shift feel, and torque converter clutch operation.
Downshifts
Under certain conditions the transaxle will downshift automatically to a lower gear range (without moving the transaxle range selector lever). There are three categories of automatic downshifts: coastdown, torque demand, and forced or kickdown shifts.
Coastdown
The coastdown downshift occurs as the name indicates, when the vehicle is coasting down to a stop.
Torque Demand
The torque demand downshift occurs automatically during part throttle acceleration when the demand for torque is greater than the engine can provide at that gear ratio.
The transaxle will disengage the torque converter clutch (TCC) to provide added acceleration, if applied.
Kickdown
For maximum acceleration, the driver can force a downshift by pressing the accelerator pedal to the floor. A forced downshift into second gear is possible below 88 km/h (55 mph). Below approximately 40 km/h (25 mph) a forced kickdown to first gear will occur. For all shift speeds, specifications are subject to variation due to tire size and engine calibration requirements.
TORQUE CONVERTER
The torque converter is a four element assembly. The torque converter contains an impeller, a turbine, a reactor, and a torque converter clutch (TCC) for increased fuel economy. It couples the engine to the turbine shaft assembly, provides torque multiplication and absorbs engine shock of gear shifting.
Impeller and Cover
The impeller and cover assembly drives the impeller blades and pump assembly. The impeller is primarily responsible for driving the turbine with hydraulic fluid by means of centrifugal force. The cover provides a mating surface for the torque converter clutch piston plate and damper assembly.
Turbine
The turbine is driven by centrifugal fluid force from the impeller. The turbine transmits input torque to the drive chain and driven sprocket through the turbine shaft.
Reactor
The reactor redirects fluid flow from the turbine back to the impeller so that fluid rotates in the same direction as the impeller. This action also assists in torque multiplication.
Torque Converter Clutch (TCC)
The torque converter clutch (TCC) provides a mechanical link or direct drive between the engine crankshaft and turbine shaft when applied. The application of the torque converter clutch is controlled by the powertrain control module (PCM). Under certain conditions, the PCM sends the appropriate signal to the TCC solenoid which allows fluid pressure within the torque converter to force the TCC piston plate and damper assembly against the cover creating a mechanical link between the engine and transaxle.
Turbine Shaft
The turbine shaft connects the torque converter stator with the forward/coast/direct clutch cylinder. When applied, the forward/coast/direct clutch cylinder transmits input torque to the reverse/overdrive ring gear assembly, which also acts as the drive sprocket. This allows input torque to be transmitted from the torque converter to the drive chain and driven sprocket.
APPLY COMPONENTS
Intermediate and Overdrive Band
The intermediate and overdrive band assembly holds the reverse clutch drum assembly to the transaxle case in second and fourth gear. This action causes the reverse/overdrive sun gear and shell assembly to be held stationary in second and fourth gears.
Intermediate and Overdrive Servo
The intermediate and overdrive servo is the hydraulic actuator which applies the intermediate and overdrive band assembly Line pressure is applied through the servo apply circuit where it works on one side of the servo piston. This forces the piston to move up the servo bore in the transaxle case, which moves the servo apply rod against one end of the intermediate and overdrive band assembly. Because the other end of the intermediate and overdrive band assembly is anchored to the transaxle case, the ends of the intermediate and overdrive band assembly are squeezed around the reverse clutch drum assembly, holding the reverse clutch drum assembly stationary.
Forward Clutch
The forward clutch connects the turbine shaft to the outer race of the forward one-way clutch, which in turn drives the low/intermediate sun gear in all forward gear ranges. However, in fourth gear, the forward clutch transmits no power because the forward one-way clutch is overrunning. The forward clutch is a multi-disc clutch and is contained in the forward/coast/direct clutch cylinder assembly.
Direct Clutch
The direct clutch connects the turbine shaft to the low/intermediate ring gear assembly when the vehicle is in third and fourth gears. It is a multi-disc clutch and is contained in the forward/coast/direct clutch cylinder assembly.
Reverse Clutch Coast Clutch
The reverse clutch connects the turbine shaft to the reverse/overdrive sun gear and shell assembly when it is in reverse gear. It is a multi-disc clutch, and is contained in the reverse clutch drum assembly.
The coast clutch is a multi-disc clutch located beneath the forward clutch in the forward/coast/direct clutch cylinder assembly. It connects the low/intermediate sun gear assembly to the turbine shaft assembly when the transaxle control switch is ON in the DRIVE position or when any manual gear is selected.
Low/Reverse Clutch
The low/reverse clutch holds the reverse/overdrive carrier assembly to the transaxle case when the transaxle is shifted into the R or 1 position. The low/reverse clutch is splined to the transaxle case and is a mufti-disc clutch.
Forward One-Way Clutch
The forward one-way clutch is a sprag type, one-way clutch. The forward one-way clutch combines with the forward clutch to connect the turbine shaft assembly to the low/intermediate sun gear assembly in first, second, and third gears. The forward one-way clutch overruns during all coasting operations and is always overrunning in fourth gear.
Low One-Way Clutch
The low one-way clutch is a roller type one-way clutch which allows the transaxle case to hold the reverse/overdrive carrier stationary when the transaxle is in first gear. The low one-way clutch will only hold the reverse/overdrive carrier assembly during acceleration. When coasting, the low one-way clutch will overrun, disconnecting the final drive from the compound planetary gearset.
GEARTRAIN
The geartrain includes the planetary gearsets, apply components, and final drive gearset and differential.
Planetary Gearsets
The automatic transaxle has two planetary gearsets to provide operation in reverse and four forward speeds. The gearsets are comprised of the following components:
^ Low/intermediate sun gear assembly.
^ Low/intermediate carrier assembly.
^ Low/intermediate ring gear assembly.
^ Reverse/overdrive sun gear and shell assembly.
^ Reverse/overdrive carrier assembly.
^ Reverse/overdrive ring gear assembly.
Drive Chain Assembly
Drive Chain and Sprockets
A chain drive transfers torque from the torque converter turbine to the planetary gearsets. The chain drive is composed of the following components:
^ Reverse/overdrive ring gear assembly which acts as a drive sprocket.
^ Driven sprocket.
Drive chain
The drive chain connects the reverse/overdrive sun gear with the driven sprocket. The final drive sun gear located on top of the driven sprocket meshes with the final drive gearset.
The final drive consists of a planetary gearset that transfers and multiplies torque from the planetary gearsets to the differential. The final drive consists of the following components:
^ Final drive sun gear: is chain driven by the reverse/overdrive ring gear assembly and transfers torque to the final drive carrier.
^ Final drive carrier: the final drive carrier acts as the driven member and is part of the differential case.
^ Final drive ring gear: is held by the converter housing and is always the held member of the final drive planetary gearset.
Differential
The differential assembly drives the differential pinion gears and the differential side gears which in turn drive the front wheel driveshaft and joints, and provides differential action if driving wheels are turning at different speeds. The differential assembly consists of the following components:
^ Differential case (part of the final drive carrier).
^ Two pinion gears supported by a pinion shaft.
^ Two side gears supported by the differential case and halfshafts.
HYDRAULIC SYSTEM
The hydraulic system uses transaxle fluid to cool, lubricate, and provide hydraulic pressure to the hydraulic circuits within the transaxle.
Fluid Pump Assembly Components
Fluid Pump
The pump provides the volume of fluid required to charge the torque converter, main control valve body, cooling system, lubrication system, and hydraulic apply devices. The transaxle uses a positive displacement gear and crescent-type pump which is shaft-driven by the torque converter.
Fluid Level and Filter
Fluid from the sump area (formed by the transaxle case and converter housing) flows through a filter to the pump. The filter has a recirculation port connected to the main control area of the case and receives fluid from the main regulator exhaust. This provides fluid under pressure to aid the pump in higher efficiency operation. A thermostatic fluid control valve prevents foaming of fluid by maintaining a sump level below the rotating components. The two-piece chain drive cover prevents foaming by not allowing the chain to rotate in the fluid. A magnet on the underside of the chain cover collects unwanted magnetic material.
Main Control
The main control houses the hydraulic valves and solenoid valves. These valves direct fluid flow, restrict fluid flow and change fluid pressure. The main control receives signals from the solenoid valve body and changes those signals into hydraulic actions. These actions control the operation of the hydraulic clutches and intermediate and overdrive band assembly, and supply lubrication to the transaxle.
Low/Reverse Accumulator
The low/reverse accumulator cushions the application of the low/reverse clutch when the transaxle is shifted to reverse gear.
2-4 Accumulator
The 2-4 accumulator cushions the shift feel during intermediate and overdrive band applications.
Forward Accumulator
The forward accumulator cushions the application of the forward clutch when the transaxle is shifted into DRIVE, 2 or 1.