AWD malfunctioning ?

[John Eyles]

I got my car stuck last night, in a shallow ditch
filled with ice/snow from the recent winter storm.
Living in N.Carolina, I'm not the most experienced
winter driver, but we've had more than our share
recently, so I'm not a total novice either.
Anyhow, it simply did not seem to me like my car
should have been stuck (I should have just been
able to drive out of the ditch). A former Subaru
owner who was present had the same feeling.

I'm wondering if maybe my AWD system is malfunctioning -
like maybe the thing is simply behaving as a FWD car.
We noticed that the front wheels were spinning but at
least one of the rear wheels seemed to be doing nothing.

So, two questions:

(1) is there a simple way to verify that the AWD system
is functioning properly ?

(2) is there a simple fault that can cause it to malfunction ?

Thanks, John

 

[Jim Stewart]

I got my car stuck last night, in a shallow ditch
filled with ice/snow from the recent winter storm.
Living in N.Carolina, I'm not the most experienced
winter driver, but we've had more than our share
recently, so I'm not a total novice either.
Anyhow, it simply did not seem to me like my car
should have been stuck (I should have just been
able to drive out of the ditch). A former Subaru
owner who was present had the same feeling.

I'm wondering if maybe my AWD system is malfunctioning -
like maybe the thing is simply behaving as a FWD car.
We noticed that the front wheels were spinning but at
least one of the rear wheels seemed to be doing nothing.

If you had one front wheel and one back
wheel spinning, the AWD was working.

 

[David]

John Eyles wrote:

If you had one front wheel and one back
wheel spinning, the AWD was working.

What he said. Only some Subes have an limited-slip differential at
either end. An STi would have spun all 4 tires. Forester XS should
spin 3. A VDC Outback should have applied the brakes to the spinning
wheels, diverting power to the others. Speaking of which, applying the
brakes with your foot can help in that situation too. Not nearly as good
as a VDC setup, but sometimes it works.

 

[John Emdall]

What he said. Only some Subes have an limited-slip differential at
either end. An STi would have spun all 4 tires. Forester XS should
spin 3. A VDC Outback should have applied the brakes to the spinning
wheels, diverting power to the others. Speaking of which, applying the
brakes with your foot can help in that situation too. Not nearly as good
as a VDC setup, but sometimes it works.

FYI my USDM 04 STi will only spin three with the DCCD in "LOCK"
position. Both rears, and the right front. This is logical since it
carried only the driver (me) at the time. This is on relatively flat,
dry packed sand/dirt, from a standing start.

As I recall the front and rear diff are
of different design. I believe the rear is a clutch type and the
front is a gear type, but I am not positive about this. I do not
know how the center diff slip is limited other than electromagnetism
is involved, or to what degree it actually "locks."

I did not do an immediate followup test with the DCCD in minimum
position. However when drifting on pavement it seems make no
difference how it is set

Finally to reiterate, to test your AWD, stop on a flat, low traction
surface, preferably dirt, and nail the gas for less than a car
length of motion. If in dirt it should be easy to tell what happened
by the tracks.

 

[Ken Gilbert]

Finally to reiterate, to test your AWD, stop on a flat, low traction

surface, preferably dirt, and nail the gas for less than a car
length of motion. If in dirt it should be easy to tell what happened
by the tracks.

of course, this tells you nothing of the limited slip characteristics
of your differentials. that test is a bit more difficult (but not
impossible).

ken gilbert

 

[Dominic Richens]

I'm wondering if maybe my AWD system is malfunctioning -
like maybe the thing is simply behaving as a FWD car.
We noticed that the front wheels were spinning but at
least one of the rear wheels seemed to be doing nothing.

What Subaru? For most models/years, what you describe is normal. The front
and rear differentials are open; that is if one wheel has to traction, the
other wheel as no power (same torque = 0). Only the center differentials
are locking/limited slip. So at least one of the axels needs to have
traction on both wheels.

Some high-end Imprezas or Outbacks have a limited slip diff on either the
front or rear axel, that would have helped you in this situation.

cheers!

 

[John Eyles]

What Subaru? For most models/years, what you describe is normal.

Sorry, it is a 1997 Legacy Outback 5-speed.

So at least one of the axels needs to have
traction on both wheels.

Ok, I can believe that maybe there was no traction on at least
one wheel per axle. Guess I've gotten a little complacent, the
thing has always been so amazing in the snow ! Also, my tires
(Bridgestone Turanzas) have 60K miles and need replacement soon,
so that is undoubtably contributing to the problem.

Some high-end Imprezas or Outbacks have a limited slip diff on either the
front or rear axel, that would have helped you in this situation.

I do not have a clue as to whether I have that.

Thanks, John

 

[David]

I do not have a clue as to whether I have that.

I'm guessing US model from your email, and I doubt very much you do.
60k miles on all season tires sounds like the real problem though.
Powering another rear wheel won't help if it doesn't have any traction
either. I think you can buy an LS rear diff to retrofit for ~US$1000
new (less from a junkyard), but I'd expect money on tires to be better
spent.