I noticed on my 94 AWD that when I turn on the key I can see the AWD light illuminate. I never see it at any other time. I had my wife watch as I peeled out in the snow , and she said that just the back wheels seemed to be spinning. Is the awd always on?? When does the light go on?? How can I check to make sure it's working properly

 

As I remember AWD is always on (30% of torq. - to front wheels, 70% to rear) and when rear wheels skid t-case diff locks (light is on). Unfortunatly I used to drive my friend's blue aero 2.3 converted to awd with manual tranny of UAZ and there AWD ans Low Gear are controlled with special sticks.... Like in Jeep Wrangler. Just Imagine you have 3 shiftstiks inside....

 

woa - and Pablo is correct. The AWD doesn't kick in until certain conditions are met(and even then you have to be traveling at or less then 30 mph) - and burnouts in the snow are about as fun as burnouts in a parking lot after work after it's rained, but even then, it's all 2wd.

 

Pablo is right; the AWD is always engaged to provide 30/70 split between F/R. When the sensors detect more than about 5% difference between the F/R wheels, the computer locks the clutch in the center differential to make the AWD 50/50 split. The rear axle has limited slip clutches in the differential so the two rear wheel will lock up. The front differential is open so one side could be slipping while the other is tracking the ground. The real test to see if it's working is to jack up wheels from the ground, and see if all the wheel turn.

The 4wd light on your dashboard will come on when the controller detects something wrong with the system. Some versions will actually blink out error codes. Otherwise, the only other time you'll see it is when you turn the key on.

 

The AWD system has 3 open differentials (unless the rear has the option LSD), so it does always send torque to all wheels. However, the diff in the transfer case also has an electromagnetic clutch in parallel to the diff. The idea is that when the rear wheels slip (which will usually slip first, since there's more torque there), the clutch comes online briefly to lock the center up.

When the center is locked, it isn't a 50/50 split. Under perfect conditions, maybe, but in the real world that doesn't happen. With a locked center - and this also true for a typical 4x4 truck with locked transfer case - torque can flow freely from axle to axle, anywhere from 100/0 to 0/100. What that means, essentially, is that when the rear is slipping, it contributed 0% to moving the vehicle, and the front axle is doing 100% of the job or vice-versa. It does not mean that the front axle is getting 100% of what the engine has the potential to produce. The amount of torque the engine actually produces is only equal to how much the tires can use before slipping and never more then that.

But anyway, the clutch locks the center briefly till the slip stops and the allows the diff to revert back to being open. The common problem people see is with the control of the clutch unit. If the ECU that operates the clutch goes offline, then the clutch never actuates. Then the center diff simply is an open differential. If the rear spins, the center will allow it to do so. It will still send some torque to the front axle, but only the same amount that rear can handle, which isn't much when it's spinning.

If the van does have a rear LSD (mine has a Torsen), then the limited slip function of the diff will help prevent the rear from spinning in the first place, but only to a certain extent. The LSD does not truly lock the rear wheels together. If it did, it wouldn't be an LSD, it would be a locker (after market lockers are available, BTW). An LSD helps reduce spin, but they can't prevent it entirely. If the difference in traction from side to side is greater then what the LSD can help with, one wheel will still spin. On the other hand, if the traction under both rear wheels is very low, then the LSD isn't much help. It'll hold both wheels together enough that when they loose traction on an icy surface, they'll do so together. This compromises rear lateral traction, so the rear end can/will slide sideways. With an open diff in the rear end, usually one wheel will spin first, so other still has it's lateral traction and helps anchor the side-to-side movement.

Either way, raising all 4 tires off the pavement and putting it in drive to see if all 4 turn will only tell you that the center diff isn't broken. If it was, though, you'd already know since the van probably wouldn't be able to move. Or if it did, it would be accompanied by "bad" noises... Also, you might not see all 4 turn. If all diffs are open, uneven brake drag could prevent one tire from turning, since there's no actual load from the drivetrain to overcome it (again, you need traction to create torque). What you might able to do, I suppose, is let the wheels turn, then try to hold both fronts. The ECU should think that the rears are spinning as a result and kick in the clutch. But this is a relatively dangerous thing to do, since it involves trying to hold onto spinning tires that are under power. Probably not the safest thing to do. Another approach to see if the clutch is working is put the rear tires on a slippery surface (ice, wet grass, soapy concrete, etc) while the front is on dry, grippy pavement. Stomp the throttle and have an observer standing outside tell you whether the rear spins briefly and then the front pulls, or if the rear simply just spins for a duration.

Anyway, that turned out to be longer then I planned...