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electronic differential lock (edl)(Edit)

This video shows Audi's EDL (Electronic Differential Lock) in action. Both left wheels are placed on the rollers, which simulate slippery surface (ice, mud). Both right wheels are on a dry surface. To get the car moving in these conditions, a front or rear differential lock is needed to transfer torque from left to right. Because this Audi has open (non-lockable) differentials front and rear, EDL plays the role of a locking differential. The system must brake the wheel that spins (the left wheel) and transfer torque to the wheel with traction (the right wheel). However, as seen on the video, Audi's EDL is not as effective as a real mechanically locking differential. The system is not capable of bringing the spinning wheels to a full stop and transferring most of the torque to the right. If there was a real differential lock installed in either front or rear axle, the vehicle would have moved with little or no wheelspin. So EDL is a differential lock imitation.

Then, when wheel speed increases, the car finally takes off the rollers. EDL is defenetly a helpful traction device, but for on-road conditions only. If the vehicle was on a slope, it is doubtful EDL would make the vehicle move.

The next video shows the Volkswagen's EDL (Electronic Differential Lock) in action. This is a Volkswagen Bora/Jetta with Haldex Generation I automatic all wheel drive. On this model, the EDL is installed on the front wheels only. EDL helps some, but it is not a full replacement for a real differential lock.

This video shows a Subaru Legacy Outback 2.5 4AT VTD hill climbing with VDC off and VDC on. VDC is Subaru's electronic stability and traction control system that works on all four wheels. You can see that when VDC is turned on, the wheelspin is minimized.

The following video shows how VDC works on 2009 Forester 2.5XT. When wheels start spinning and it seems like the car won't move any further, do not release the throttle pedal. VDC will engage when excessive wheelspin is detected and brake the spinning wheel.

why only center locking differential is not enough(Edit)

These videos show a Subaru Forester (2.5 A/T) whose diagonally opposed wheels have lost traction. This is a typical off-road situation where most four-wheel drive vehicles fail.

The car does not have an optional rear Limited Slip Differential neither it has an electronic traction control system. One of these devices could have helped to move the car forward.

These videos show a Jeep Grand Cherokee whose diagonally opposed wheels have lost the ground contact. The front left wheel and the rear right wheels spin. This is a typical off-road situation where most of the four-wheel drive vehicles fail because they do not have a locking differential in either of the axles.

As a rule, neither electonic traction control systems nor limited slip differentials help here. This is because these traction devices cannot transfer enough torque to the wheels that are on the ground. A fully lockable differential installed in the rear axle is needed to get the car moving in these conditions.

viscous coupling locking center differential test(Edit)

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.