Automatic Transmission/Transaxle: Description and Operation
DESCRIPTIONCross-Sectional View Of 4F20E Transaxle, (Part 1 Of 2):
Cross-Sectional View Of 4F20E Transaxle, (Part 2 Of 2):
The electronically-controlled automatic transaxle uses a combination of electronic and mechanical systems to control forward gear shifting, torque converter lock-up, and a self diagnosis capability to aid in troubleshooting procedures.
OPERATION
A power Electronic-Automatic Transmission (E-AT) switch allows for a selection of two shift schedules to tailor transaxle operation to specific driving conditions. (1) The Normal Schedule (E-AT switch released) provides increased fuel economy by providing earlier partial throttle upshifts. (2) The Power Schedule (E-AT switch depressed) provides delayed upshifts at part throttle, using more engine power. The Transaxle Control Module (TCM) has the capability to override the power schedule, depending on driving conditions. The electronic system controls transaxle shifting in forward speeds and controls torque converter lock-up by means of solenoid actuated valves. When energized, these shift solenoid valves actuate friction elements (clutches and band) to control shifting in the planetary gear. The shift timing and lock-up events are regulated by the TCM in programmed logic and in response to input sensors to produce optimum driveability.
Selector And Shift Patterns
1. The transaxle shift control selector has six positions; P, R, N, D, 2, and 1. Additionally, the O/D control switch allows overdrive to be canceled.
2. In the P (Park) position, there is no power flows through the transaxle. The parking pawl locks the output shaft to the transaxle case to prevent the vehicle from rolling. The engine can be started in this selector position. This is the only position in which the ignition key can be removed.
3. In the R (Reverse) position, the transaxle is in reverse at a reduced ratio. There is engine braking in R.
4. In the N (Neutral position), there is no power flow through the transaxle, but the wheels are free to move because the output shaft is not locked by the parking pawl. The engine may be started in neutral but the ignition key cannot be removed in this range.
5. The overdrive control (O/D) switch is used to inhibit fourth gear operation. The O/D OFF lamp on the instrument panel will illuminate when fourth gear has been locked out. If the O/D switch is depressed during fourth gear operation, a 4-3 downshift will occur. The O/D switch is located on the end of the shift control selector lever.
6. The D (Drive) position is the normal selector position for most forward driving conditions. This position provides all automatic shifts, application and release of the torque converter clutch, and maximum fuel economy during normal operation. Overdrive may be canceled by depressing the O/D control switch, allowing automatic shifts from first through third gear only. Overdrive shifting for all four gears may be activated by depressing the O/D control switch a second time. This should be done at closed throttle for best shift response.
7. The 2 (second gear) position allows the transaxle to shift up to second gear only. Engine braking occurs in second gear, making this position useful on steep grades.
8. The 1 (manual low) position allows first gear operation only (no upshifting). If this position is selected at normal road speeds, the transaxle will initially downshift to second gear, then downshift into first gear when vehicle speed falls below approximately 28 mph. The manual low position also provides engine braking useful for descending steep grades.
Forced Downshifts
Under certain conditions the transaxle will downshift automatically to a lower gear range. The three categories of automatic downshifts are (1) Coastdown, (2) Torque Demand, and (3) Kickdown.
1. The coastdown downshift occurs automatically when the vehicle is coasting down to a stop.
2. The torque demand downshift occurs 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 (if applied) to provide added acceleration.
3. The kickdown downshift occurs when the accelerator pedal is fully depressed. This forces an automatic downshift for maximum acceleration.
a. A forced downshift into second gear is possible below 55 mph.
b. Below 25 mph, a forced kickdown into first gear will occur.
4. For all shift speeds, specifications may vary due to tire size and engine calibration.
GEAR TRAIN
DESCRIPTION AND OPERATION
The transaxle uses two interconnected planetary gear sets to provide four forward speeds and reverse. The three basic components of the planetary gear sets are (1) the sun gear, (2) the planetary gear, and (3) the ring gear. The variety of ways in which these three components may be driven or held produces the different gear ratios as well as forward and reverse motion. The transaxle uses eight apply devices to drive or hold the planetary gear set components. These eight devices are as follows:
Exploded View Of 4F20E Transaxle (Part 1 Of 2):
Exploded View Of 4F20E Transaxle (Part 2 Of 2):
NOTE: Use the Exploded View Of 4F20E Transaxle image to reference the eight devices.
Forward Clutch
The forward clutch is applied in all forward gears. It connects the front planetary gear to the ring gear through the forward one-way clutch.
Coasting Clutch
The coasting clutch is applied in all forward gears only when the O/D control switch is OFF. The coasting clutch connects the front planetary gear to the ring gear bypassing the forward one-way clutch. The coasting clutch allows for engine braking upon acceleration.
Forward One-Way Clutch
The coasting clutch is applied in all forward gears only when the O/D control switch is OFF. The coasting clutch connects the front planetary gear to the ring gear bypassing the forward one-way clutch. The coasting clutch allows for engine braking upon acceleration.
2-4 Band
The 2-4 band is applied by the 2-4 servo in second and fourth gears. In third gear, the second gear apply pressure is overcome by the third gear release pressure in the 2-4 servo. The 2-4 band holds the primary sun gear stationary by locking it to the transaxle case.
Low One-Way Clutch
The low one-way clutch is engaged in first gear, except in manual low. The low one-way clutch prevents the front planetary gear from rotating counterclockwise. It overruns during deceleration in first gear.
Low/Reverse Clutch
The low/reverse clutch is applied in manual low and reverse gears only. It holds the front planetary gear stationary by locking it to the transaxle case.
3-4 Clutch
The 3-4 clutch is applied in third and fourth (overdrive) gears only. The 3-4 clutch drives the front planetary gear directly from the input shaft.
Reverse Clutch
The reverse clutch is applied in R only. It drives the primary sun gear directly from the input shaft.
Parking Pawl
The parking pawl locks the idler gear to the transaxle case.
Idler Gear
The idler gear is driven by the output shaft and attached to the reduction gear. The reduction gear drives the final ring gear, part of the differential. The idler gear also provides a gear for the parking pawl to lock the final drive from rotating.
Reduction Gear
The reduction gear connects the output shaft to the final drive ring gear and provides gear reduction.
Differential Assembly
The differential assembly drives the front wheel driveshaft and joints and allows the front wheel driveshaft and joints to turn at different speeds.