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 other components for electronic control.

(b) Drive force appropriate to vehicle speed and condition is distributed to the front and rear wheels of the vehicle by ECU control of the hydraulic pressure of the center differential control limiting clutch to realize stable running.





(c) Main components and their functions.





2. FOUR WHEEL DRIVE CONTROL SYSTEM OUTLINE

(a) Four wheel drive control system is used as the full-time 4WD mechanism for 4WD vehicles.

(b) Four wheel control system is a full-time 4WD system that is capable of distributing optimum driving force to the front and rear wheels based on running conditions by electronically controlling the transfer center differential control limiting mechanism.

(c) This system adopts UF1AE that uses a planetary gear type center differential in the transfer section. A wet type multiple disc clutch in the differential control limiting mechanism and a silent chain are used to transfer driving force to the front wheels.

(d) Four wheel drive control system function.





3. REFERENCE MECHANISM

(a) Mechanism of the Center Differential.

(1) The center differential distributes power from the transmission to the front and rear wheels, and absorbs the rotational difference between the front and rear drive shafts generated when they rotate.

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

(3) Driving force from the transmission is input by the carrier. The carrier and sun gear are directly controlled by the differential control limiting clutch.

(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.





(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, and consists of discs, plates, and pistons of a wet type multiple disc clutch. The basic structure is the same as that of the automatic transmission clutch.

(2) The rotational difference generated between the plates and discs creates frictional resistance that transfers torque, and then distributes torque of the center differential control limiting mechanism and of 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 hydraulic pressure for transmission control is also used by the transfer to simplify the transfer hydraulic system.

(2) Line hydraulic 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.





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

(1) The valve body is positioned in the lower section of the transfer casing, and has a two-level structure consisting of the upper valve body and the valve body cover. A linear solenoid valve is attached to 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.