Automatic Transmission/Transaxle: Description and Operation
DESCRIPTIONThe A4LD transmission is a fully automatic transmission with four forward and one reverse speed.
CONSTRUCTION
1. This transmission consists of a welded lockup torque converter assembly, a three unit planetary gear train and a hydraulic system to control gear selection, automatic shifts, and the locking torque converter.
2. The lockup torque converter is coupled to the engine crankshaft and transmits engine power into the gear train.
3. The output shaft drives the rear wheels through a conventional driveshaft and rear axle.
4. Gear reduction, necessary to match engine to axle, occurs in the planetary gear train and the locking torque converter.
5. Components from the intermediate brake drum rearward to the output shaft and extension housing seal are similar to the C3 automatic transmission.
6. Components forward of the intermediate brake drum are unique to the A4LD transmission. They are the following:
a. The center support assembly.
b. The overdrive band and drum.
c. The coast clutch.
d. The overdrive sun and planet gear assembly.
e. The center support assembly.
f. The converter and clutch/damper assembly.
7. The main control (valve body), besides supplying fluid for torque converter operation, piston plate clutch application, and pressure for applying bands and clutches, also contains an on-board computer controlled solenoid which electrically overrides the application of the piston plate clutch in the converter.
OPERATION
1. This 4 speed overdrive transmission is similar to the C3 automatic transmission. It uses electronic controls integrated in the on-board Electronic Engine Control-IV (EEC-IV) system.
2. These controls, along with hydraulic controls in the valve body, operate a piston plate clutch in the torque converter that eliminates torque converter slip when applied.
3. The electronic control of the converter clutch prevents its application in engine modes where noise, vibration, and harshness concerns are most evident.
4. These modes are tip-in/tip-out, closed throttle, cold engine, very heavy/wide open throttle, and low engine speed to minimize lugging boom.
FUNCTIONS OF MAIN COMPONENTS
Cross-Sectional View Of A4LD Transmission:
1. The torque converter (1) couples the engine to the gear train, provides torque multiplication, and absorbs shock of gear shifting.
2. The piston plate clutch and damper assembly (2) transmits engine power to the turbine from the converter cover.
3. The input shaft (3) transmits power out of the converter into the gear train.
4. The converter cover (4) transmits power from engine into the converter
5. The turbine (5) is splined to the input shaft and driven by fluid from the impeller.
6. The Idler (6) is driven by the cover and supplies torque multiplication. Its rear hub drives the transmission oil pump.
7. The reactor (7), also called the stator, contains a one-way clutch to hold it stationary only when reaction is required. It also causes hydraulic reaction during torque multiplication.
8. The overdrive clutch (8) couples the override sun gear to the override carrier to eliminate the free wheeling effect of the overdrive one-way clutch.
9. The overdrive one-way clutch (9) transmits power from the input shaft to the center shaft in all gears except 4th gear overdrive.
10. The reverse and high clutch (10) couples the center shaft to the gear train in reverse, 3rd, and 4th gears.
11. The forward clutch (11) couples the center shaft to the gear train in all forward gears.
12. The overdrive band (12) holds the overdrive gear train sun gear stationary in overdrive gear.
13. The intermediate band (13) holds the gear train sun gear stationary in 2nd gear.
14. The low and reverse band (14) holds the reverse planet carrier stationary in reverse gear and manual low.
15. The one-way clutch (15) holds the reverse planet carrier stationary in drive low (1st) gear.
16. The parking gear (16) holds the transmission output shaft to the case in P (Park) position by engagement with a parking pawl.
17. The reverse servo (17) applies the low reverse band.
18. The intermediate servo (18) applies the intermediate band for second gear.
19. The overdrive servo (19) applies the overdrive band.
20. The oil pump (20) provides a constant supply of oil under pressure to operate, lubricate, and cool the transmission.
21. The governor (21) provides a road speed signal to the hydraulic control system for upshift and downshift control.
22. The valve body (22) directs fluid under pressure to the converter, band servos, clutches, and governor to control transmission operation.
23. The vacuum diaphragm (23) provides an engine load (manifold vacuum) signal to the hydraulic system to control shifting and capacity for clutches and bands.
SHIFT PATTERNS
When operating in the OD position, the transmission will automatically shift gears from 1st to 2nd and from 2nd to 3rd as vehicle speed increases. After the shift into 3rd takes place, the converter clutch may apply. The shift into overdrive decreases RPM by 25 percent and occurs when the vehicle approaches the desired cruising speed and the driver eases up on the accelerator pedal. Dependent upon acceleration rate and vehicle speed, the converter clutch may apply before or after the shift into overdrive is accomplished. The converter clutch application is based on both engine speed and vehicle speed by combined transmission hydraulics and on-board computer electronic controls. It can be applied in 3rd gear as well as 4th. Because of this feature, the transmission may be sensed as executing as many shifts as would a 5 speed transmission.
FORCED DOWNSHIFTS
1. Two functions have been incorporated in the A4LD transmission for times when the driver requires acceleration or more power for negotiating grades or driving into very strong head winds. These functions are:
a. Disengagement of the converter clutch to achieve added acceleration from the resultant increase in engine speed.
b. A part throttle 4-3 downshift for further acceleration.
c. These events can also occur during cruise control operation when more power is required to maintain the selected speed.
2. While driving at highway speeds, four different engine speeds can be obtained by transmission operation:
a. In overdrive with the converter clutch engaged.
b. In overdrive with the converter clutch disengage.
c. In 3rd gear.
d. With full throttle in 2nd gear.
3. The transmission also provides wide open throttle forced downshifts.
a. When driving in excess of approximately 35 mph and with the accelerator fully depressed to the floor, the transmission will downshift to 2nd gear for maximum acceleration.
b. If the accelerator is depressed fully to the floor at speeds below 30-35 mph, the transmission will downshift to 1st gear.
GEAR TRAIN
1. The transmission case provides a rigid support housing for the gear train. Both shafts are supported individually in the case.
2. The input shaft is splined at both ends. It has bushing journals next to the splines. The front splines mate with the converter turbine hub, so that the shaft always turns at turbine speed.
3. The output shaft is supported by two bushings; one at the rear of the case, and the other at the extension slip yoke bushing. The slip yoke fits over the rear and splines of the shaft.
Direct Drive
Direct drive is accomplished in the planetary gear set by locking any two members together. Then, regardless of which member is driven, the complete set turns as a unit If, for example, the pinion carrier is splined to an output shaft, and the ring gear and sun gear are locked together, direct drive through the pinion carrier will occur.
Reduction
One way to obtain reduction in a planetary gear set is to hold the sun gear and drive the ring gear with the planet carrier as the output member. Rotation of the ring gear makes the planet pinions walk around the sun gear, in the same direction the ring gear is turning, but not as fast. Input torque is increased or multiplied since output speed is less than input.
Reverse
In a simple planetary gear set, a reverse output is obtained by holding the planet pinion carrier and driving another member. The planet pinions just turn on their axes and act as idler gears, reversing the direction of the input.
Thrust Washers
Thrust Washer Identification, (1988 Mustang And 1988-1989 Bronco II And Ranger):
Thrust Washer Identification, (1988 Thunderbird And 1988-1989 Aerostar):
Thrust Washer Identification, (1989-1991 Mustang):
Thrust Washer Identification, (1990 Bronco II And 1990-1991 Aerostar And Ranger, Except 4.0L Engines):
Thrust Washer Identification, (1990-1991 Aerostar And Ranger And 1991 Explorer W/4.0L Engine):
1. The gear train uses ten thrust washers and four needle bearing assemblies to provide bearing surfaces where they run together. Washers No. 1 and 4 are selective thickness to control endplay.
2. The torque converter also has thrust washers and a needle bearing between its components. These are not identified by number because they are not individually serviceable.
BAND AND CLUTCH APPLICATION
Band And Clutch Apply Chart:
Information on band and clutch application and gear ratios for all possible operating conditions is shown in the Band And Clutch Apply Chart.
HYDRAULIC SYSTEM
1. The hydraulic system of the has several functions: it supplies fluid for torque converter operation, directs fluid under pressure, lubricates the working parts of the transmission, controls up-shifts and downshifts as a function of engine load and road speed, and removes heat generated by flow in the torque converter and by other parts of the transmission.
2. The transmission fluid used is special. It is composed of mineral oil and additives. In the transmission it is used as a combination power-transmission medium, hydraulic control fluid, heat transfer medium, bearing surface lubricant, and gear lubricant. The manufacturer's recommendations should be followed when servicing transmission fluid.
3. This transmission has an internal gear pump which is driven by flats on the converter impeller hub. The pump operates whenever the engine is operating, to supply fluid to the rest of the system. As the pump drive gear is driven by the impeller, it causes the internal driven gear to rotate also. Where the gears separate from mesh, a vacuum is created, forming the pump inlet. Atmospheric pressure in the pump forces fluid into the inlet through the pump screen.
4. At the point of greatest separation, the gears are closely fitted to a crescent in the pump housing. The crescent and the space between the gear teeth form chambers in which the fluid is trapped as the gears rotate past the inlet. Beyond the crescent, the gears begin to come together again. Since the crescent seals the fluid from getting back to the inlet, it now must be squeezed out the outlet port and into the system.
5. A pump that has the outlet sealed from the inlet like this is called a positive displacement or positive delivery pump. This means that as long as the pump is turning and fluid is supplied to the inlet, the pump will deliver fluid with its volume being in proportion to the drive speed.
6. The pump is designed to deliver more fluid than the transmission needs, and excess pump delivery is recirculated to pump by the control pressure regulator valve.