viscous coupling locking center differential testTop
This video shows three BMW E30 3-series iX. The first one has a faulty viscous coupling and the center differential works as an open differential. During the jack test, the rear wheels spin freely and no torque is transferred to the front wheels. This is how any permanent four wheel drive car with an open center differential behaves.
The other two BMWs have a good viscous coupling unit and you can see that it locks and transfers torque to the front wheels.
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