rear wheels engage with a thunk

[weelliott]

The other day I decided to venture out in my 04 outback with an
automatic without shoveling my driveway. there was about 8 or 10
inches of snow. I made it out just fine. On the way back in I was
going slow, and got high centered. I waited a few minutes for the
exhaust to melt the snow down, then tried to back out. The car
struggled back a few feet, then once the back wheels had made it to
the cleared street I thought I'd be fine. they were on no snow, but
the front wheels wer spinning and throwing up snow and the car wasn't
moving. I revved the engine a little to see if I could dig down
through the snow to pavement, and I heard a thunk from the rear and
the car backed up. Was this a clutch pakc suddenly engaging or
something? Why hadn't it engaged before that?

I've owned two second gen legacies with manuals and have never ever
had problems in snow. I've heard the center diff is different between
the two transmissions. Is this the reason for my near stuckedness?

What's going on here? Is this normal, or is the 04 not quite working
properly?

 

[1 Lucky Texan]

The other day I decided to venture out in my 04 outback with an
automatic without shoveling my driveway. there was about 8 or 10
inches of snow. I made it out just fine. On the way back in I was
going slow, and got high centered. I waited a few minutes for the
exhaust to melt the snow down, then tried to back out. The car
struggled back a few feet, then once the back wheels had made it to
the cleared street I thought I'd be fine. they were on no snow, but
the front wheels wer spinning and throwing up snow and the car wasn't
moving. I revved the engine a little to see if I could dig down
through the snow to pavement, and I heard a thunk from the rear and
the car backed up. Was this a clutch pakc suddenly engaging or
something? Why hadn't it engaged before that?

I've owned two second gen legacies with manuals and have never ever
had problems in snow. I've heard the center diff is different between
the two transmissions. Is this the reason for my near stuckedness?

What's going on here? Is this normal, or is the 04 not quite working
properly?

this is very interesting. I want to read other responses, but have 2-3
comments myself. I'd want to check the U-joints for one. Also, the
transmission fluid level and condition.

I have a very vague memory of reading that, when in reverse(and maybe
when 1st is 'selected' with the shifter), the TCU/ECU/'computer'
always has AWD fully engaged - so, it DOES seem that something was
'stuck' related to the clutch pack.

curious

 

[Dave__67]

The other day I decided to venture out in my 04 outback with an
automatic without shoveling my driveway. there was about 8 or 10
inches of snow. I made it out just fine. On the way back in I was
going slow, and got high centered. I waited a few minutes for the
exhaust to melt the snow down, then tried to back out. The car
struggled back a few feet, then once the back wheels had made it to
the cleared street I thought I'd be fine. they were on no snow, but
the front wheels wer spinning and throwing up snow and the car wasn't
moving. I revved the engine a little to see if I could dig down
through the snow to pavement, and I heard a thunk from the rear and
the car backed up. Was this a clutch pakc suddenly engaging or
something? Why hadn't it engaged before that?

I've owned two second gen legacies with manuals and have never ever
had problems in snow. I've heard the center diff is different between
the two transmissions. Is this the reason for my near stuckedness?

What's going on here? Is this normal, or is the 04 not quite working
properly?

It does sound like the clutch pack did lag in engaging.
One of a few things that will cause this is a grooved clutch pack
carrier (happens to motorcycles, too).
The clutch plates get stuck 'together' or 'apart', then they shift
into place.

Failing Duty C solenoid will do this too.

Do some full-lock circles in a dry parking lot and see if you have any
torque-bind symptoms.

Dave

 

[weelliott]

It does sound like the clutch pack did lag in engaging.
One of a few things that will cause this is a grooved clutch pack
carrier (happens to motorcycles, too).
The clutch plates get stuck 'together' or 'apart', then they shift
into place.

Failing Duty C solenoid will do this too.

Do some full-lock circles in a dry parking lot and see if you have any
torque-bind symptoms.

Dave

I did some full lock turns with no apparent bind. Then again, I'm
assuming that bind would show up as either tire squeal or some
jerkiness to the drivetrain. Neither of those. Would the clutch pack
carrier be located in the housing for the rear diff? As in, could I
access this part by taking off the driveshaft, then dropping the
housing that holds on the rear diff? Then again, I'm not sure if the
auto has that separate housing like the manual of the previous
generation legacies does.

 

[StephenH]

This is normal. We often run Subaru's on a rack to 50-60 mph testing
wheel bearings; we have to rev them up and down to force all the
wheels to engage. The "clunk" can be quite noticeable when there is no
load on the tire(s) .