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Automatic AWD
Goal: Better on-road handling stability.
Automatic all wheel drive was created to save weight and money. Auto AWD can be used full time on all surfaces including pavement. AWD only briefly activates automatically when stability threatening conditions arise.
How does it do it?
Since ABS was introduced, cars have wheel speed sensors on all wheels. A central computer compares constantly the speed of all wheels.
In the case of ABS the computer tells the brakes to apply less pressure when one or more wheels want to lock up (rotate less).
In the case of automatic AWD the computer engages the other axle when the main powered axle sends signals of faster rotating wheels = indicating it has slipping wheels. The engagement usually happens right at the center differential or at a joining section of the front and rear drive shafts with a clutch pack or similar.
As soon as all 4 wheels are rotating at the same speed again, AWD is turned off by the computer.
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4x4 driving tips