Abstract: A control system for automatically selecting a two wheel drive mode or a four wheel drive mode has a rainfall sensor for measuring the amount of precipitation such as rain, an acceleration sensor for measuring acceleration or deceleration of the vehicle, a control unit connected with the sensors for generating a 2WD or 4WD command signal, and an actuator for changing the drive system of the vehicle between the two wheel drive mode and four wheel drive mode in response to the command signal of the control unit. The control unit such as a microcomputer determines a reference acceleration and a reference deceleration each of which is a function of the rainfall measured by the rainfall sensor, and generates the 2WD command signal if the actual acceleration or deceleration is smaller than the reference acceleration or deceleration, and the 4WD command signal if the actual acceleration or deceleration is greater than the reference acceleration or deceleration.

Abstract: A four-wheel-drive system for a vehicle, comprising a power unit having an output shaft in a lateral direction of the vehicle, a power transmission gear unit having input and output shafts parallel with the output shaft of the power unit, a speed reduction gear to be driven by the transmission output shaft, two final reduction gear units one of which is arranged for driving the front road wheels and the other of which is arranged to drive the rear road wheels of the vehicle, and a planetary gear assembly including a sun gear connected to one of the final reduction gear units, a ring gear connected to the other of the final reduction gear, and a pinion carrier connected to the speed reduction gear, the planetary gear assembly being capable of providing a differential action between the sun and ring gears thereof.

Abstract: A four-wheel-drive system for a vehicle, comprising a power unit having an output shaft in a lateral direction of the vehicle, a power transmission gear unit having input and output shafts parallel with the output shaft of the power unit, a speed reduction gear to be driven by the transmission output shaft, two final reduction gear units one of which is arranged for driving the front road wheels and the other of which is arranged to drive the rear road wheels of the vehicle, a series combination of two planetary gear assemblies one of which is capable of selectively producing two different gear ratios for low-speed and high-speed vehicle driving and the other of which is operatively connected between the two final reduction gear units so as to be capable of providing a differential gear action therebetween, and a low-and-high speed shifting clutch mechanism intervening between the speed reduction gear and the former planetary gear assembly.