Viscous Coupler

It depends on how it fails. If the fluid leaks out, it will be like there is no rear drive shaft. You will have strictly front-wheel-drive.

If the coupler is binding internally, or locking up, you can have the front and rear drives locked together solidly, just like on a four-wheel-drive truck. With those, "walking tires" is caused when the rear tires make a smaller diameter circle when turning than the front ones do, but both axles are forced to turn at the same rate. This is why you never switch to four-wheel-drive when driving on clean, smooth roads. Four-wheel-drive is only used when snow, ice, or mud allows the tires to slip.

"All-wheel-drive" is different. The manufacturers specifically do not call your system "four-wheel-drive" because they want to differentiate between the two systems, and they do not want people to think you can go off-roading like you would with a truck. The viscous coupler contains two sets of rotating blades surrounded in a thick fluid. The fluid tries to make both sets of blades turn at the same rate, thereby transferring power to the rear wheels, but it allows some slippage to occur to allow the rear wheels to turn slower than the fronts, when cornering.

If something came apart and is allowing the two sets of blades to contact each other, the coupler could lock up solidly and cause the tire hopping when cornering, or the coupler might just chatter. How the failure progresses from there is hard to say.

You're correct, but that wasn't always the case. To check, raise the vehicle on a hoist with all four tires off the ground, then try to turn just one wheel by hand. With a viscous coupling, such as has been used since the early 1990s on Caravans, at least one wheel on the other axle will turn too, without the ignition switch on and with no electronic anything involved. If a computer is involved, the two axles are not joined when the ignition switch is off. I attended the Chrysler class on this system long before it became as common as it is today. At that time there were no computer controls. The reason for using an electronically-controlled coupler is to offer adjustability through a driver-controlled switch, or, in this case, when the computer sees a difference in road speed signals from the wheel speed sensors. Engagement can be done through a viscous fluid that has iron particles in it. More common is to use a set of clutch plates that can be engaged with varying force as needed. The advantage to Chrysler's system is it's simple and needs little attention. The advantage to Hyundai's system is the rear wheels can be totally disengaged to offer better fuel mileage, tire life, and less strain on driveline components, but at the cost of all the potential problems encountered with electronic controls.

Both systems use clutch plates, and they can both fail in similar ways, with similar symptoms. Hyundai's coupler uses fluid too, but not for torque transmission, just for lubrication.

Thank you for correcting me. Feel free to add more details on this system, if you wish.