Concerned version - AT A540H - was installed until 2000 in various models of Corona, Vista, RAV4, Ipsum families.
Note, that many components of A540H 4WD system are similar to components of descripted above A241H.
AT A540H is based on the transmission A540E (electronic control). Center differential locking - by hydraulic multi-plate clutch. Center differential gears - bevel.
Operation modes: type 1 (early models) - "AUTO", during towing or spare wheel installed - "OFF"; type 2 (later models) - "AUTO". Service lock modes: early models - basic "FREE", at service "FREE" or "LOCK"; later models - not installed.
Fluid (AT and center differential locking) - Toyota ATF Type T (08886-00405). Transfer oil - Toyota transaxle oil E50 (08885-80206).
Lock control is performed in accordance with the difference of speed of front and rear wheels, type 1 - stepped, type 2 - continuously.
Center differential and transfer. 1 - front differential pinion gear, 2 - front differential pinion gear shaft, 3 - front differential right side gear, 4 - intermediate shaft, 5 - center differential pinion gear, 6 - center differential right side gear, 7 - to front right driveshaft, 8 - spline, 9 - transfer drive gear case, 10 - center differential left side gear, 11 - front differential right case, 12 - front differential left case, 13 - main gear case, 14 - front differential left side gear, 15 - to front left driveshaft.
2. Construction - executive part
1. Control switch in "AUTO", straight driving.
The power is transmitted from the transmission via main gear to the main gear case, then via splines to the front differential case. Here the power via the pinion gears split evenly between left and right side gears, which turn at the same speed as the center differential case (pinion gears do not rotate). The power from the left side gear is transmitted to the front differential case. Here, as in the center differential, the power split evenly between the left and right side gears and is further transmitted to the right and left driveshafts and wheels. The power from the right side gear of center differential, through the transfer gears and propeller shaft is transmitted to the rear differential.
2. Control switch in "AUTO", there is a difference between the speed of the front and rear wheels.
When speed difference arises between the front and rear wheels, the center differential operates, whereby the speed difference arises between main gear case and the front differential case. To reduce this difference the fluid pressure is applied to the pistons of hydraulic multi-clutch to compress the clutch pack. The frictional force depends on the driving conditions (throttle opening angle, vehicle speed, selector position) and is selected to provide optimum torque at the front and rear wheels.
"Auto" mode, the difference between the speed of the front and rear wheels. 1 - from AT, 2 - main gear mounting, 3 - hydraulic multi-plate clutch, 4 - splines, 5 - center differential case, 6 - center differential left side gear, 7 - to front right driveshaft, 8 - center differential right side gear, 9 - center differential pinion gear, 10 - center differential pinion gear shaft, 11 - transfer drive and driven gear, 12 - to rear differential, 13 - front differential case, 14 - front differential pinion gear, 15 - front differential pinion gear shaft, 16 - to front left driveshaft.
3. Control switch in "OFF", service maintenance / testing of brake system.
In this mode the solenoid valve is off and the pressure does not supplied to to the clutch pack. And, if the front or rear wheels rotate, while the other are stopped, speedometer indicates half of the wheel speed. For example, if the rear wheels are stopped, power is transmitted to the pinion gears via main gear case and center differential case, but since right side gear does not turn the center differential pinion gears rotate around its axis while moving around the right drive gear. Speed of front differential case is sum of these two rotation speeds, so it rotates twice as faster than the center differential. Speedometer drive gear is fitted to main gear case and rotates at the speed of center differential case.
"OFF" mode, service maintenance. 1 - from AT, 2 - main gear mounting, 3 - hydraulic multi-plate clutch, 4 - speedometer drive gear5 - to front right driveshaft, 6 - center differential right side gear, 7 - center differential pinion gear, 8 - center differential left side gear, 9 - speedometer driven gear, 10 - front differential case, 11 - to front left driveshaft.
Clutch . 1 - front differential, 2 - piston return spring, 3 - spline, 4 - main gear right case, 5 - piston 2, 6 - piston reaction sleeve, 7 - piston 1, 8 - clutch plate, 9 - clutch disc, 10 - flange, 11 - front differential case thrust washer, 12 - spline, 13 - main gear left case.
3. Hydraulic multi-plate clutch
|The clutch is installed between main gear case and front differential case, consists of a set of discs and plate, two pistons and other components. A540H clutch differs from A241H by large number of disks in the set (11 instead of 7).
Clutch . 1 - clutch plate, 2 - piston 1 & 2, 3 - front differential case, 4 - main gear case, 5 - clutch disc.
The control system limits the operation of the center differential, by creating friction between the clutch discs and plates when it compressed with moving pistons 1 and 2. The inner splines of clutch discs mesh with splines of the front differential case, the outer splines of clutch plates mesh with splines of inner surface of the main gear housing. Control pressure is regulated by the control unit by solenoid valves 1 and 2 (type 1) or by linear solenoid valve (type 2).
|Type 1||Type 2||Type 1||Type 2|
Control system. 1 - center differential control solenoid valve 1, 2 - center differential control solenoid valve 2, 3 - TPS ECU, 4 - ECT ECU, 5 - ABS ECU, 6 - CD control switch, 7 - stop light switch, 8 - TPS, 9 - vehicle speed sensor, 10 - lock-up solenoid, 11 - shift control solenoid valve 2, 12 - shift control solenoid valve 1, 13 - neutral start switch, 14 - rear speed sensor.
4.1. Control system (type 1)
|Functions of components|
|1. Speed sensor||Detects vehicle speed by center differential ring gear revolution|
|2. Rear output shaft speed sensor||Detects transfer gear speed.|
|3. Throttle position sensor control unit||Detects throttle opening angle and sends it to ECT ECU|
|4. ABS control unit||Sends ABS operation signal|
|5. Center differential control switch||Switches differential control modes ("AUTO" and "OFF")|
|6. Engine control unit||Detects driving conditions by signals from the sensors and activates solenoid valves 1 and 2|
|7. Solenoid valve 1 and 2||Controls the operation of low-high shift valves 1 and 2 and clutch modulator valve|
Control system (type 2). 1 - pattern select switch, 2 - igniter, 3 - OD switch, 4 - combination meter, 5 - ECM, 6 - kick down switch, 7 - stop-light switch, 8 - solenoid valve SLD, 9 - front speed sensor, 10 - solenoid valve SL, 11 - solenoid valve 2, 12 - solenoid valve 1, 13 - neutral start switch, 14 - solenoid valve ST, 15 - rear speed sensor, 16 - water temperature sensor, 17 - distributor, 18 - diagnostic connector, 19 - TPS.
4.2. Control system (type 2)
1 - solenoid valve 1, 2 - solenoid valve 2, 3 - transfer upper valve body, 4 - transfer clutch modulator valve, 5 - transfer lower valve body, 6 - transaxle case, 7 - low-high shift valve 1, 8 - low-high shift valve 2.
5.1. Hydraulic system (type 1)
1 - line pressure (from oil pump), 2 - orifice, 3 - solenoid valve 2, 4 - solenoid valve 1, 5- to clutch, 6 - low-high shift valve 2, 7 - low-high shift valve 1, 8 - transfer clutch modulator valve.
Low-high shift valves 1 and 2 - open or close channels to supply line pressure to the modulator and clutch in accordance with the position of solenoid valves 1 and 2. Transfer clutch modulator valve - modulates line pressure supplied to the clutch in accordance with the movement of low-high shift valves 1 and 2. Hydraulic multi-plate clutch - limits operation of the center differential.
|Clutch pressures||Solenoid valve 1||Solenoid valve 2|
1 - to transfer valve body (line pressure), 2 - manual valve, 3 - ECM, 4 - solenoid valve ST (ON), 5 - reverse (line pressure), 6 - primary regulator valve, 7 - oil pump, 8- oil pressure.
5.2. Hydraulic system (type 2)
The control system increases the pressure applied to valve body when slipping. This increases the pressure in the clutch, increasing the limiting force. If the speed difference of front and rear wheels is large, and it occurs at low speed driving, at D, 2, L ranges solenoid valve ST is ON. Thus line pressure from the manual valve is supplied to the lower portion of the primary regulator valve, as when reversing. Accordingly, line pressure controlled by the primary regulator valve, as well as the line pressure supplied to valve body, is increased. Since the base line pressure increases, the clutch pressure, modulated in the valve body, also increases.
1 - oil pump, 2 - solenoid modulator valve, 3 - ECM, 4 - solenoid valve SLD, 5 - center differential clutch, 6 - center differential clutch control valve, 7 - modulated pressure, 8 - line pressure.
Using solenoid valve SLD, control system controls the pressure supplied to the clutch. Solenoid valve SLD maintains limited slip differential effect at the most appropriate level for current driving conditions.
1) Normal driving. Solenoid valve SLD opens and closes in accordance with the throttle position, controlling the pressure in clutch. During take-off in 1st gear, when wide throttle opening required, system modulates high pressure, to make smooth even for sudden start or slippery surfaces.
2) Slipping. At low speed, if there is a large difference between front and rear wheels speed, solenoid valve SLD is fully closed, so high pressure is applied, increases the limiting force.
3) Cornering. When cornering at low speed, if the turning radius cause the difference in speed of front and rear wheels, solenoid valve SLD is fully opened to reduce the limiting force and and improve turning smooth.
6. Service lock
Service mechanical locking of center differential was used on early models (type 1). This mechanism is used for certain types of inspections and adjustments, such as braking force check using a dynamometer with locked front or rear wheels. Service lock control lever is mounted on the transfer case.
Caution. Do not change the position of the lever except for the purpose of inspection and adjustment. To prevent damage of the transmission, driving with the lever in the "LOCK" position is prohibited.
There are two posotions of service lock:
"FREE" - normal automatic locking.
"LOCK" - center differential is mechanically locked, power is equally splitted between front and rear wheels.
When using the service lock should observe the following rules.
1. If the lever does not move smoothly, it is necessary to switch it while rotating the front wheel, but without applying excessive force.
2. Locking bolt must be tighten only with lever switched fully to one of positions.
3. After inspection the lever must be switched in "FREE".
Service lock operation
When "LOCK" mode is activated (turn anti-clockwise) the rod connected to the lever moves to the left and turns the shift fork shaft. The fork moves locking sleeve to the right and locks the center differential. The splines on the sleeve inner surface mesh with splines on the transfer drive gear case, whereby the right side gear if center differential is locked to the center differential case.
At "FREE" mode (turn clockwise) the rod moves to the right and rotates the fork shaft, the fork moves sleeve to the left and center differential is released.
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