4.0L AWD Viscous Coupling, Failure?

[JeffBoudah]

I have a question in regards to my AWD system, it is indeed AWD and not 4x4, no selectable transfer case.

It has recently begun to snow like a hooker here in MI, and I've begun to notice that my AWD does not feel like it once was last winter.

First it does not engage the front wheels in reverse. Maybe I just never noticed that it has never done this, and is intended to do so by design. (but I'm pretty sure it has and is supposed to)

I backed the ass up to a snow bank, let it bump and gently pushed into the throttle to get the tires to slip. Both rears light up and dig. (i have an open diff BTW...) No power to the fronts... no spinning, no brakes were applied either.

Second test, pull up to snow bank in drive, let it bump, gently roll into the throttle, and it begins to climb and spin the front tire, and one rear tire.

So, I've got power in drive, and none in reverse... is this a early sign of a viscous coupling failure?

No need to dig up and copy paste the only reference used for a viscous coupling test on the interwebs (the VW test). I understand the theory of the coupling too... I've searched.... i just want to know if it will take a total crap on me any time soon, and is this no power in reverse a early symptom of a potential failure?

Thanks in advance

 

[JeffBoudah]

Bump...

Any thoughts?

 

[Big6ft6]

I'm curious about this too. Only done a few minutes of research mostly on this site, but it seems like no bona-fide test method exists to check the function of the VC. I'm questioning mine...I seem to have binding when turning sharply in reverse (backing out of a parking stall/driveway). What is the way to know?

If there is no perfect way to test...my next step is to research the rebuild procedure.

 

[imp]

Jeff, Ford's viscous coupling is the source of just about every kind of information and MIS-information as any part of the drivetrain. Since you mention you understand the concept, skip to consideration of: a fluid-filled coupling much like a torque converter, acting to "absorb" the speed difference between front and rear axles, while they remain basically mechanically connected together.

How in the world could it function only in one direction? Perhaps I'm missing something crucial here. What I can't tell you, because I don't know, is whether or not (I suspect NOT) the coupling depends on the fluid fill of the transfer case for it's hydraulic function. Thus, it is likely a sealed unit, about the size of those used on washing machines, filled with an appropriate fluid, which given the possibility of it leaking out, could this be the mode of failure? Everything in the transfer case, given level ground under the vehicle, slopes downward towards the rear. So, logically, the fluid, if not full, congregates toward the rear. All hypothesis here.

How about taking this thing out on a fairly steep dirt or gravel hill, and have someone watch from the outside if indeed, front wheels are driving in both forward and reverse? If you face the vehicle forward up the hill, put it in reverse, if the hypothesis is right, gun that baby backwards and see if maybe the front wheels don't spin at all. Conversely, if facing downhill, and rev her up in reverse, do the fronts then aid in pulling it backwards uphill?

Just a thought. imp

 

[jrford]

i've got the AWD too. . .i saw this post but it took a while to test mine. NO SNOW here in SE Mi, but i did get enough. Careful you might get a face of slop, but roll your window down stick your head out (watch out, or have a passenger do it for laughs-just kidding), gun it in forward and reverse on slippery stuff. Mine-forward the front wheels spin, reverse same thing. It can fail open or closed, if you suspect a closed failure try the following test.

One test for a binding VC, is to do tight figure 8's, if the wheels bind, VC is mostly likely shot, (careful - if it does bind back off and don't force it). Next test is to pull the front drive shaft to disconnect the xfr case and repeat. If the binding goes away its most likely the VC. Note, the front axle will be unloaded and if there is a CV joint that's failing it might stop too. So the test is about 90% accurate, if everything else is OK i'd put it at 99%. ..

Open failure, you would feel no binding but you might get stuck. I'd put it on a hoist and put it in gear, on mine all 4 wheels spin, i'd expect only two to do so. Another test, but i not sure you can do it with a 3rd/4th gen as the diff is Alum. and you not supposed to put a jack under it, but the VW guys would jack the rear up with a rolling floor jack, put a 2x4 under the fronts wheels and see if it can 'climb' over it, if the VC is working it should. Or find a parking lot with a frozen puddle, put the rears in and see if it will climb out. If failed open it should just spin the rears.