Transfer Case: Description and Operation

UF1AE TRANSFER: FOUR WHEEL DRIVE CONTROL SYSTEM: SYSTEM DIAGRAM

SYSTEM DESCRIPTION

1. GENERAL DESCRIPTION

(a) The Four Wheel Drive Control System (4WD Control System) consists of the hydraulic control mechanism of the transfer unit, steering angle sensor, wheel speed sensor, skid control ECU, and 4WD control ECU.

(b) The 4WD Control ECU controls the fluid pressure in the center differential limiting clutch to distribute the drive force to the front and rear wheels in accordance with the vehicle speed and vehicle behavior. Thus, it helps create a stable driving performance.

2. FOUR WHEEL DRIVE CONTROL SYSTEM OUTLINE

(a) The four wheel drive control system (4WD control system) is a full-time 4WD system that electronically controls the center differential limiting clutch of the transfer to distribute optimal drive forces to the front and rear wheels in accordance with the driving conditions.

(b) This system adopts a UF1A transfer case that uses a planetary gear type center differential in the transfer section. A wet type multiple disc clutch in the differential control limiting clutch and a silent chain are used to transfer driving force to the front wheels.

(c) Four wheel drive control system function.

3. SYSTEM OPERATION

Vehicle Start up and Low-speed Turn Control

The 4WD control ECU controls the transfer control solenoid based on signals from the throttle position sensor and steering angle sensor. The transfer control solenoid controls the fluid pressure applied to the center differential limiting clutch. Thus, vehicle start up and low-speed turn control will be performed.

The 4WD control ECU controls the center differential coupling it directly before starting the vehicle in order to prevent slipping at vehicle initial acceleration or during acceleration on a grade or a low friction road.

The 4WD control ECU decreases the fluid pressure applied to the center differential limiting clutch during a low-speed turn based on the steering angle to prevent tight corner braking.

Slip Control at Vehicle Start up

The 4WD control ECU controls the transfer control solenoid based on signals from the throttle position sensor and speed sensor. The transfer control solenoid controls the fluid pressure applied to the center differential limiting clutch. Thus, slip control at vehicle initial acceleration will be performed.

If the vehicle starts moving with the steering wheel turned, the fluid pressure applied to the center differential limiting clutch is decreased by low-speed turn control. An excessive driving force may be applied to the rear wheels on a low friction road, causing the vehicle rear to move sideways. To prevent this, the 4WD control ECU detects differences between the front wheel speed and the rear wheel speed based on signals from the speed sensors to directly couple the center differential immediately.

Slip Control

The 4WD control ECU controls the transfer control solenoid based on signals from the speed sensor. The transfer control solenoid controls the fluid pressure applied to the center differential limiting clutch. Thus, slip control will be performed.

The 4WD control ECU detects differences between the front wheel speed and the rear wheel speed while driving at a medium or high speed based on signals from the speed sensors. Then, the 4WD control ECU calculates the fluid pressure required based on the difference in the wheel speeds and the vehicle speed and continuously applies the appropriate fluid pressure to the center differential limiting clutch, which stabilizes the vehicle.

Acceleration Control

The 4WD control ECU controls the transfer control solenoid based on the throttle position sensor and speed sensors. The transfer control solenoid controls the fluid pressure applied to the center differential limiting clutch. Thus, acceleration control will be performed.

If the difference between the front wheel speed and the rear wheel speed exceeds the specified value while driving at a medium or high speed, the 4WD control ECU, in accordance with the throttle valve angle, increases the fluid pressure applied to the center differential limiting clutch to directly couple the center differential. This prevents the rear wheels from slipping and stabilizes the vehicle.

4. MECHANISM REFERENCE

(a) Mechanism of the Center Differential.

(1) The center differential distributes the power from the transmission to the front and rear wheels. In addition, it absorbs the rotational difference between the front and rear drive shafts that is created during cornering.

(2) A planetary gear is used for the center differential, which unevenly distributes the driving force to the front and rear wheels.

(3) The center differential receives the drive force that is input by the transmission by way of the planetary carrier. The planetary carrier and the sun gear are controlled by the differential limiting clutch until they are in the lockup state.

(4) A compact, lightweight and extremely silent chain is used for the front wheel drive transmission, and the driving force is transferred to the front wheel drive shaft by the sun gear of the center differential.

(5) The drive force to the rear wheel drive shaft is output in a straight line with the transmission.

(b) Mechanism of the Center Differential Control Limiting Clutch.

(1) The center differential control limiting clutch is located between the planetary gear and the front wheel driving force distribution section. It consists of wet type clutch discs, plates, and piston. Its basic structure is the same as the clutch of an automatic transmission.

(2) This clutch utilizes the friction resistance, which is created by the speed difference that occurs between the plates and discs, to transmit torque and distribute torque to the center differential limiting clutch and the front and rear wheels.

(3) Adjustment of differential control limiting force is controlled by changing the hydraulic pressure that functions in the piston.

(c) Mechanism of the Hydraulic Control (ATF Pressure Control).

(1) Line pressure for transmission control is also used by the transfer to simplify the transfer hydraulic system.

(2) Line pressure from the transmission is controlled by a modulator valve so that it does not rise above a specific pressure. This stabilizes hydraulic control of the linear solenoid valve.

(3) Hydraulic pressure controlled by the linear solenoid valve operates the control valve. It also controls the slip status of the center differential control limiting clutch by modulating the line hydraulic pressure from the transmission in the linear solenoid (transfer control solenoid).

(d) Mechanism of the Valve body & Linear Solenoid (Transfer Control Solenoid).

(1) The valve body, which is located in the bottom of the transfer case, has a two-stage composition consisting of an upper valve body and a valve body cover. The transfer control solenoid is installed in the upper valve body.

(2) The linear solenoid is integrated with electromagnetic section and the pressure modulating section. The spool pushes the pressure modulator valve according to the electric current value that makes the electromagnetic section function. The pressure modulating valve then generates hydraulic pressure to resist the pushing force of the spool, so it is possible to obtain hydraulic pressure proportional to the electric current value.