I recently bought a '98 Cross Country, and I'm wondering how the AWD works with traction control.

As I understand it, the traction control works off a signal from the ABS sensors, and applies braking when it senses wheel spin. However, as far as I know, the viscous coupling in the AWD system will only engage when there is a difference in wheel speed between front and rear.

So, how does AWD work if the traction control is preventing wheel spin? Do I only have AWD when the traction control is turned off?

Thanks

 

AWD engages when the front wheels try to move faster than the rear wheels. Traction control engages when a wheel on one side tries to move significantly faster than a wheel on the opposite side.

 

Yeah, in theory that's how it works. I get the basic idea, and I can see it working just like that with computer controls in a car equiped with a Haldex system, which locks electronically. In the '98 though, it uses a viscous coupling, which works differently.

As far as I know, the viscous coupling relies on a series of vanes in fluid, with every other one being welded to either the front or rear part of the drive shaft. Vane 1, 3, 5, 7, etc, are attached to the front part of the drive shaft, and vane 2, 4, 6, 8, etc, are attached to the rear part of the drive shaft, and all of this is sealed in a unit filled with fluid.

When the front wheels start to slip, some of the vanes start to spin faster than the others, causing friction, which causes heat, which causes the fluid to thicken, which causes all the vanes to spin at the same speed, thereby engaging the rear differential.

So the question, still.... If the traction control limits wheel spin, then there can be no difference in speed between the front and rear parts of the driveshaft. As such, the viscous coupling will never engage. So how does the AWD ever engage?

 

Sorry, my previous reply wasn't sufficient, traction control (and ABS) ensures the speed of the front left wheel matches that of the rear right and the speed of the front right matches that of the rear left, thereby enabling AWD to operate. So if your front left starts spinning, torque is already being transferred to the rear right through the VC, traction control will try to control the spin but power will still reach the rear wheels due to the difference in VC input / output driveshaft rotation.