4WD system with original designation "i-Four" was first installed in Toyota vehicles in 1992. Since then, it has been used on Mark II and Crown families, and from the mid-2000s - also on related Lexus GS and IS models.
The scheme was used in the automatic transmission A340H (2nd generation) and A341H, then in more modern A750H, A760H, A761H, combined with UF1AE transfer.
Type - full-time all wheel drive.
Center differential - cylindrical, asymmetrical (torque distribution between front and rear wheels - 30:70).
i12 = -z2/z1
Locking - by multi-plate wet clutch, electronically controlled wtih variable lock rate.
Control - by ABS control unit, based on signals from wheel speed sensors, steering wheel position sensor and throttle position sensor. Direct control - by linear solenoid valve, modulator valve and spool valve in the transfer valve block.