Est (Electronic Shift Transfer): Introduction
EST system is a kind of part time 4 wheel drive system and its full name is "Electronic shift transfer". Instead of previous free wheel hub, FRRD (Free Running Differential) was adopted and this one is for SOTF (shift on the fly) while driving vehicle. When the vehicle runs with 2WD again, the front axle will be rotated idly due to the vehicle speed and this makes noise and vibration on the propeller shaft and ring gear set. Therefore FRRD will prevent these phenomena to get a driving stability, efficiency and improved NVH.
When 4WD is selected by driver, a FRRD air pump motor operates and a dog clutch in FRRD is engaged to pinion shaft, making front wheels drive. Therefore the front propeller shaft and the front drive shaft are coupled rotating together. Oppositely, if 2WD is selected by a driver, the dog clutch in a FRRD is disengaged, resulting in disconnection between the front propeller shaft and the front drive shaft.