How to check if all 3 diffs, center & rear LSDs, trans & t-case are ok in AWD DSM?

[XC92]

I just read that in an AWD DSM with a VC LSD rear diff, which is what I have, if you raise the rear end and turn either wheel, the other wheel should turn in the same direction, and if it turns in the opposite direction then its VC LSD is shot and you either drive with an open diff or replace it.

Is this true, or is this only if say the trans is in gear, or in neutral, or all 4 wheels are raised? I tried this on my '92 Talon TSi AWD and the opposite wheel turned in the opposite direction when I turn each wheel. I just assumed that this was because with the front wheels down the prop shaft locked the rear diff so the opposite wheel had nowhere to go but the opposite direction.

Now I'm wondering if the rear diff and/or its LSD and maybe other driveline parts are shot. Is there a relatively simple way to check and confirm that all 3 diffs are ok, including the LSDs on the center & rear, along with the t-case and trans, at least in terms of basic power transfer?

E.g. by raising all 4 wheels on jack stands and putting the trans either in neutral or in gear and turning each wheel in either direction, or even running the car and putting it in gear? And is it necessary to detach the prop shaft per the FSM to overcome the resistance of the drivetrain even when in neutral, and any such test w/o detaching it is meaningless?

And yes, my Talon's rear diff has a VC LSD, with the orange sticker saying so and all, and none of the car's major driveline parts have been swapped, e.g. trans, t-case, rear diff--although come to think of it the trans was rebuilt under warranty a year or two after I bought it due to some gears being bad, but I don't believe they swapped its diffs let alone the rear diff.

I'm also assuming that ALL AWD DSMs had a VC LSD on the center diff.

 

[steve]

I'm also assuming that ALL AWD DSMs had a VC LSD on the center diff.

Correct!

When it comes to how things work you need to keep in mind that a VC LSD works by the shear between the two sets of plates acting on the fluid is what causes the coupling. Just turning the wheel by hand may not create enough shear to overcome friction and cause the wheels to turn in the same direction.

 

[XC92]

It's just that every online guide and video I've seen says if they turn in opposite directions, then either you don't have a VC LSD, or yours is shot.

But when I play out the mechanical flow in my head, it seems that even with the shifter in neutral, there's likely enough static resistance in the entire driveline, from the ring and pinion gears in the rear diff to the prop shaft bearings to the t-case bearings to the output, intermediate and input shafts and center and front diffs, to make it impossible for a slow hand-turning of one of the rear wheels to overcome that resistance, forcing the other wheel to turn the other way due to the design of all diffs, open or not.

That's probably why the FSM says to detach the prop shaft. Putting the car in neutral probably isn't enough to allow the rear diff to turn freely. If I was able to turn the wheel fast enough, the LSD would probably kick in and turn the other wheel in the same direction. Maybe I should try that, really wail on one of the wheels and see what happens.

Is detaching the prop shaft the only way to test the LSD, other than a burnout which I'd rather not try? I'm going to replace the fluid soon, and if it looks ok, the LSD is probably ok.

 

[Mech Addict]

What steve said, plus if you only had the rear wheels off the ground, the driveshaft would not turn, so both wheels would not be able to turn the same way. If you lift them all and shift into neutral, then you might get them all to spin the same way when one rear is rotated.

 

[XC92]

I considered that, but if the car's in neutral, what difference would it make if the front wheels were off the ground since they're detached from the power flow?

Oh, wait, they're detached from the power flow from the input shaft, but still connected to the output shaft, t-case & rear diff. They either all move of none move (assuming nothings broken).

And the front wheels are locked since they're on the ground, which locks the ring gear on the rear diff, so no wheels turning in same direction if turning by hand slowly.

Do I have this right? Neutral doesn't detach the 2 axles from each other. Nothing does, without detaching some part in that whole driveline subgroup (or the sort of selector found in trucks and newer higher-end 4WD cars, which the DSM doesn't have).

So the only direction for the opposite wheel on the rear to go is the other way.

 

[Mech Addict]

I think you’ve got it now. A lot of new awd cars have rear axle disconnects, but our old-school gsx are all connected all the time. The stock front diff is open, center is vc, and rear can be either (mine is lsd also).
Try the burnout on an icy day.

 

[XC92]

I've wondered, why is the front diff open? Because the center and rear do most of the heavy lifting? And, I assume that better recent AWD cars have digitally controlled active response diffs, part of their traction control system, where each wheel is effectively independently driven. Not to mention EVs, where there isn't really much of a transmission.

But, I still like my Talon. It has a road feel quality that I've never found in any other car I've driven (but then I haven't really driven that many cars, and fewer yet sports cars).

 

[Mech Addict]

The advantage of open diff on the front is that if too much power is applied, at least one front wheel will not loose traction, so steering is preserved to some extent. This makes sense on a street car especially, where drivers are not really prepared for 4 wheel drifting.
There are certainly aftermarket front lsd for 1g (Quaife, others) to address this. And yes, modern cars tend to have computers that ensure the car goes in the direction the steering wheel points it.

 

[benspilk09]

When a VC fails then it usually locks itself solid, or at least that has been my experience with full time 4wd Jeeps. You can tell it is bad when you make a tight turn in a parking lot and the tires chirp and the driveline gets all bound up and starts bucking.

To test the center diff, put the car in gear and jack the rear up until both tires are off the ground. Turn the driveshaft by hand. It should move, but with considerable resistance. If it is locked solid, then the center diff has welded itself together or someone has welded it solid. If it turns easily, then the fluid has probably leaked out of the VC or something broke and it needs replaced/rebuilt.

To test the rear LSD, turn the rear wheels in opposite directions while it is in gear. Again, it should move, but with a lot of resistance. Comparable to pushing a spoon through molasses.

 

[XC92]

By driveshaft I assume you mean prop shaft. I haven't tried this, but I assume that if I turn one of the rear wheels with the rear raised and this doesn't turn the prop shaft then it means that the center diff isn't turning easily, even if it would turn by hand manually. I'll try that.

And when I do turn a rear wheel, the other turns the opposite direction, but the prop shaft doesn't turn. But doesn't just just mean that the power applied to the side gear of the wheel being turned just follows the path of least resistance and turns the other side gear the other way, with the ring and pinion gears not budging?