Generally, a power steering system uses the power of the engine to drive the vane pump that generates hydraulic pressure. EHPS is a power steering system that uses a motor to generate the hydraulic pressure and reduces the power required to operate the steering wheel. Since this system reduces the load on the engine, it improves fuel economy. The motor rotation rate (pump discharge volume) is controlled by the ECU according to such data as the vehicle speed and steering wheel turning angle.

Construction

(1) Pump body

The pump is driven by the engine crank pulley and drive belt, and sends fluid, under pressure, to the gear housing. The discharge volume of the pump is in proportion to the engine speed, but the amount of fluid sent to the gear housing is regulated by a flow control valve, with excess fluid being returned to the suction side.

(2) Reservoir tank

The reservoir tank supplies the power steering fluid. It is installed either directly to the pump body or separately. If not installed to the pump body, it is connected to it by two hoses. Normally, the reservoir tank cap has a level gauge for checking the fluid level. If the fluid in the reservoir falls below the standard level, the pump will suck in air, causing faulty operation.

(3) Flow control valve

The flow control valve controls the flow volume of the fluid from the pump to the gear housing, maintaining a constant flow regardless of the pump speed (rpm).

(4) Idle-up device

The pump produces maximum fluid pressure when the steering wheel is turned fully to the right or left. At this time, there is maximum load on the pump, which causes a decrease in engine idle speed. To solve this problem, almost all vehicles are equipped with an idle-up device which acts to raise the engine idle speed whenever there is a heavy load on the pump. The idle-up device functions to raise engine idle speed when pump fluid pressure acts on the air control valve (installed on the pump body) to control the flow of air. On EFl engines, when the piston of the air control valve is pushed by fluid pressure, the air control valve opens and the volume of air bypassing the throttle valve is increased to regulate engine speed.

The clutches and brakes that operate the planetary gear unit are operated by hydraulic pressure. The hydraulic control unit generates and adjusts this hydraulic pressure and switches its passage. The illustration on the left shows the hydraulic circuitry for the A140E model. The hydraulic pressure operates through various hydraulic pressure passages.

HINT:

If the battery is dead, it is possible to start the engine of manual transaxle vehicles by push-starting it, but with automatic transaxle vehicles, this is not possible. During push-starting, since the oil pump is not operating, the operating hydraulic pressure of the planetary gear unit is not generated. In other words, the power from the tires is not transmitted to the engine. The hydraulic control unit has the following three functions.

1. To generate hydraulic pressure

The oil pump has the function of generating the hydraulic pressure. The oil pump generates the hydraulic pressure required for automatic transaxle operation by driving the torque converter case (engine).

2. To adjust hydraulic pressure

The hydraulic pressure compressed by the oil pump is adjusted with the primary regulator valve. Also, the throttle valve produces the hydraulic pressure appropriate to the engine output.

3. To shift gears (to make theclutches and brakes operate)

When the clutch and brake operations of the planetary gear unit are switched, the gears are shifted. The fluid passage is created in accordance with the shift position by the manual valve. When the vehicle speed rises, signals are sent to the solenoid valves from the Engine & ECT ECU (Electronic Control Unit). The solenoid valves operate each shift valve to shifting the gears. The major components of the hydraulic control unit are the following.

Oil pump

Valve body

Primary regulator valve

Manual valve

Shift valve

Solenoid valve

Throttle valve

Center Differential and Transfer

1. Transfer

The location of the center differential is different for the viscous coupling type and the mechanical locking type. The center differential is separated from the front differential. The front differential is located inside the ring gear mounting case. It can rotate freely inside the case. Also, the hydraulic multi-plate clutch, which is used to limit the differential action of the center differential, is housed in this case. There is no switching button in newer models, and differential limitation is controlled automatically at all times. Because hydraulic pressure is controlled by the Engine & ECT ECU.

2. Power transmission

(1) During straight-ahead driving

The power is divided in two at the center differential. One part of the power is transmitted to the rear differential via the center differential right side gear. The other part of the power is transmitted to the front drive shafts via the front differential.

(2) During cornering

When a difference in the rotational speed of the front and rear wheels arises, the center differential operates, creating a difference in rotational speed between the ring gear mounting and front differential case. When this occurs, hydraulic pressure is applied to the hydraulic multi-plate clutch, with fluid, which therefore operates to reduce this difference. The friction generated to reduce this rotational difference differs depending on the vehicle driving conditions and optimum driving force always distributed to the front differential and rear differential. (2/4)

3. Center differential

(1) Construction The center differential gear is the bevel gear type and the center differential limit control mechanism is the hydraulic multi-plate type. This type of center differential uses a hydraulic multiplate clutch as the differential limiting mechanism. The hydraulic multi-plate clutch consists of the inner side clutch discs, the outer side clutch plates and pushing pistons. The clutch is located beside the front differential.

(2) Principle of operation When hydraulic pressure is applied to the pistons, the clutch discs and plates are pushed tightly together, limiting the difference in rotational speed between the ring gear mounting case on the clutch plate side and the front differential case on the clutch disc side. If the front wheels get stuck in a mudhole and spin, the differential action of the center differential is limited by the clutch, so power is transmitted to the rear wheels, making it easy for the vehicle to get out of the mudhole. During normal driving in the “D” range, since the differential limit force is weak, driving is smooth when cornering at low speeds or when parking, etc., and the tight corner braking phenomenon does not occur. In the “L” and “R” ranges, since a higher differential limiting force is obtained the power obtained at the tires is effective in freeing the vehicle when it gets stuck.

(3) Features Excellent control of the differential action limit force is achieved in accordance with the automatic shift lever position, the throttle opening angle, and the vehicle’s speed. Newer models, a computer controls the differential limiting force in accordance with vehicle speed and with differences in front and rear wheel rotational speed, to ensure smooth driving under any conditions, whether on muddy roads, when cornering or when traveling straight ahead.

HINT: Older models had a center differential control switch. When driving in the “Off” mode, the condition is the same as when the mechanical locking type is run with the differential lock switch OFF.