Valve body replacement to solve torque bind

[carfreak85]

22 hours ago, GeneralDisorder said:

It's not the solenoid or the clutch pack....

Best to replace the clutch pack, and the drums as a set?

Our '99 Forester has a slow-to-engage AWD, and while I'm not at all opposed to a wholesale R&R of the RWD transfer assembly, if you say it's a waste of money, I won't bother buying the clutch pack (if they're still available, that is...).

[feralcoder]

On 8/31/2021 at 1:56 PM, brus brother said:

could the clunk actually be coming from the drive shaft/u joint complex?

I'm not certain, but I think it's in the rear of the tranny.  I parked against a brick wall so the sound would bounce back to me, and it sounds pretty forward under the car.
 

On 9/1/2021 at 2:56 PM, GeneralDisorder said:

the clutch plates are jammed together because they aren't free to slide apart - they are wedged into the wear grooves in the drums

TY, GD, I came to the right place to learn!  I'm looking forward to seeing what's going on in there.

@idosubaru, mine's the H6 w/ VTD.

How reliable / long-term a fix is it to smooth the grooves?  Would I gain much reliability or life by going with new drums?

[idosubaru]

5 hours ago, feralcoder said:

 

@idosubaru, mine's the H6 w/ VTD.

How reliable / long-term a fix is it to smooth the grooves?  Would I gain much reliability or life by going with new drums?

Well I’m thinking 4EAT H4 vs H6 with VTD era. Im not sure what trans offerings and which years they apply to happened when the 5EAT was offered. 

No - smooth the grooves (new will also accomplish that), change fluid 30-60k, keep the tires matching and rotated. 

Properly maintained and driven and the grooves shouldn’t be problematic again. New is not necessary. 

[idosubaru]

7 hours ago, carfreak85 said:

Best to replace the clutch pack, and the drums as a set?

Our '99 Forester has a slow-to-engage AWD, and while I'm not at all opposed to a wholesale R&R of the RWD transfer assembly, if you say it's a waste of money, I won't bother buying the clutch pack (if they're still available, that is...).

Are “Slow to engage” and torque bind different failure modes?

[carfreak85]

It depends on how the Duty-C solenoid is deployed.  IIRC some years ground the circuit to engage AWD, other years apply voltage to engage AWD.  Which failure mode you get may depend on how your AWD is implemented.

My car doesn't have torque bind in tight corners, but the rear wheels seem to have a delayed engagement, almost a rubber band effect in snow.  If I'm anticipating needing AWD I have learned to "pre-condition" the transmission to shake the clutch packs loose, but if you're simply driving along in the snow and decide to punch the throttle, it'll just spin the front tires until you release the accelerator, then AWD engages with a loud BANG!

[GeneralDisorder]

Certainly could be that the grooves are hanging up the apply piston from engaging AWD - just as easily as the grooves could keep it from disengaging. Probably depends on driving habits that led to the grooves in the first place - the auto's are basically 90% front wheel drive till they sense wheel slip so one could envision a vehicle that is driven very carefully and rarely has occasion to engage AWD. The grooves would form in the unlocked position and may hang up trying to engage till the duty-c ramps down to a lower state where enough pressure is held on the piston to overcome the sticking in the grooves. 

In all cases I'm aware of the duty-C solenoid dumps line pressure away from the lockup piston - so that a failure of the solenoid will result in full locked 4WD operation and the driver will experience really severe torque bind. If installing the fuse alleviates the problem then the duty-C is working. In such cases the torque bind is almost universally drum wear groove related except in rare cases where the vehicle has been run with severely different tire sizes or amounts of wear which can result in a burnt up clutch pack. 

New drums can be bought for some models - we recently got some new ones for a 2006 Forester XT where the customer wanted new rather than dressing the grooves. Some others may be NLA. 

Typically do have pretty good luck just filing down the edges of the grooves - usually this results in a 90% reduction in toque bind symptoms. On the older models prone to Duty-C failure we typically will replace the solenoid while in there. I will usually buy a clutch pack just in case but 90% of the time I return it. 

Have also seen a few (90's cars) with Duty-C driver failure inside the TCU. This is rare but shouldn't be discounted. They may or may not throw a code, but in the few cases I've seen the Duty-C solenoid and wiring checks out but the code is present and changing the solenoid doesn't help. Swapping out the TCU fixes these cars. Haven't tried going board level and checking for cracked solder joints or bad driver MOSFET's but given the age it's certainly a possibility. 

GD

Edited September 3, 2021 by GeneralDisorder

[idosubaru]

3 hours ago, carfreak85 said:

It depends on how the Duty-C solenoid is deployed.  IIRC some years ground the circuit to engage AWD, other years apply voltage to engage AWD.  Which failure mode you get may depend on how your AWD is implemented.

My car doesn't have torque bind in tight corners, but the rear wheels seem to have a delayed engagement, almost a rubber band effect in snow.  If I'm anticipating needing AWD I have learned to "pre-condition" the transmission to shake the clutch packs loose, but if you're simply driving along in the snow and decide to punch the throttle, it'll just spin the front tires until you release the accelerator, then AWD engages with a loud BANG!

That would be unfortunate. I’ve heard of others having that but never seen it yet. 

Cookies to GD for that detail!  I feel like I’m having a dork out engineering mechanic shop party high fives and cheers all around! 

One day I want to show up in Oregon and bring him a steak for this juicy stuff he gives us.  33% chance of happening - got some work related stuff and anniversary id like to celebrate out there.