AWD System Clutch Pack Wear/Longevity?

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I may be scoping around for an AWD vehicle for a family member soon and will be looking at used vehicles in the $5000 range. I've always been hesitant on used AWD vehicles since it just seems like more parts that could break.

I'm noticing a lot of these modern AWD systems use clutch packs in their transfer cases (and sometimes in the differentials too) to shift power. My understanding is that these are wet clutches of which in theory the wear is minimal since it relies on hydrodynamic lubrication meaning there's no physical contact of the clutch plates. Is this typically true of all AWD systems, and how is the long term reliability on this? Is the fluid the only part that breaks down and needs servicing?

Would like to hear your thoughts and experiences on these AWD systems. Which manufacturers are known to be more reliable and which to stay away from?
 
The clutch packs with which I’ve got experience fall into two types:

Viscous coupling - which can wear out quickly if mismatched (2/32” or greater tread difference) tires are used.

Electronic switch and pump - which require regular fluid and filter (yep!) changes to ensure long term reliability.

It would help to know what specific models you’re considering.

For a lot of AWD vehicles, $5,000 gets you a much older model that is going to require some work. Some brands more than others.
 
Subaru uses a few different systems - the most common is a viscous coupling system that’s trouble-free as long you’re not being stupid - and care is taken to make sure all tires are within a difference of 2/32nds of a inch. No special fluids - ATF/CVTF for automatic model, and gear oil for the front diff and manual models.

Honda’s Real-Time 4WD and VTM-4(now called SH-AWD by Acura) use clutches but differently. Regular(every 30K) fluid changes with OE Honda DPSF or VTM-4 fluid will keep the rear diff happy and the front PTO is serviced with GL-6 gear oil.

The Haldex system used by VW/Audi, Volvo and GM does need regular fluid and filter changes with the proper stuff.
 
the pump and clutch pack types are even more prone to wear in case of mismatched tyres... more than 2mm difference in thread depth or mismatched tyres means no warranty in case of Hyundai AWD. If the clutch pack survives, the transfer case can die in it's place aswell
 
Also, Toyota’s system on the RAV4 can grenade on the V6 models if the tires are mismatched and if the brakes are wailed on.
 
I think the typical failure mode for many of these AWD / 4x4 couplings is bearing failure. They're sealed units and the bearings cannot be greased. I've seen some videos online where techs have figured out how to disassemble them and replace the bearings. (Eric O from SMA for example)

Our 2019 Nissan Pathfinder has this type of coupling. It basically has an electro-magnetic clutch attached to the input of the rear differential. The prop shaft to the E-clutch is live anytime the vehicle is rolling. The vehicle is FWD or full 4x4. There's nothing other than a 50/50 split that kicks in if wheel slip is detected.

People generally experience a grinding noise or vibrations when they go bad and of course the fix is to replace the coupling which is about a $3000 dealer job. You can find mail-order replacement couplings for around $500.
 
The VAST majority of Subarus do NOT have viscous coupling center differentials - only MT Subarus utilize a viscous coupling. You likely won't find a Subaru on the road still that has a viscous rear LSD, either.

ALL OTHER AWD Subarus come with open front and rear differentials, except the STI, which has a helical front LSD, DCCD and a Torsen rear LSD.

In your price range, you'll only find Subarus with Active AWD, which has open front and rear differentials and a plate-type, clutch pack center differential. Unlike other AWD systems, there is some preload that allows for a mostly FWD bias, but not fully FWD, meaning there is no wait time for at least some power to be distributed to the rear. As wheel slip occurs, the clutch pack duty cycle regulates how much to transfer to the rear.

Maintaining equal tread depth is important for this type of center differential; however and anecdotally, I had a '97 Legacy GT that had fully functioning AWD up until I gave it away at well over 200k miles. I wouldn't call the system fragile, so long as you rotate your tires every so often.

Finally, I thought viscous couplings usually failed in the locked condition, meaning you wouldn't have slip from front to rear? That's a lookup for me.
 
I think the typical failure mode for many of these AWD / 4x4 couplings is bearing failure. They're sealed units and the bearings cannot be greased. I've seen some videos online where techs have figured out how to disassemble them and replace the bearings. (Eric O from SMA for example)

Our 2019 Nissan Pathfinder has this type of coupling. It basically has an electro-magnetic clutch attached to the input of the rear differential. The prop shaft to the E-clutch is live anytime the vehicle is rolling. The vehicle is FWD or full 4x4. There's nothing other than a 50/50 split that kicks in if wheel slip is detected.

People generally experience a grinding noise or vibrations when they go bad and of course the fix is to replace the coupling which is about a $3000 dealer job. You can find mail-order replacement couplings for around $500.
No, the older Tucsons like Eric O changed a bearing on are different and generally solid. The newer ones burn the clutch packs or grenade the housing, or grenade the transfer case. There's a bang and then oil on the ground and no more drive to the rear wheels. The clutch packs go quietly, exit stage left... Part of the problem is the Hyundais don't allow ANY difference in wheel speeds, so wear on even 1 tyre can be devastating as you can't drive 2 axles at the same speed if there's 1 tyre of differentr size, let alone 2 on the same axle...
 
No, the older Tucsons like Eric O changed a bearing on are different and generally solid.

Different how? The Eric O one had an an electro-mechanical clutch in it like my 2019 Pathfinder and many SUV/CUVs still do today.
 
Thanks for the replies. Have not narrowed down the vehicle choices, just mostly poking around. Could be anything from Subaru, Toyota, Ford, Hyundai, or Nissan (with no CVT).

Are there quick general checks that one would usually do on these AWD systems to make sure they're functioning?
 
No, the older Tucsons like Eric O changed a bearing on are different and generally solid. The newer ones burn the clutch packs or grenade the housing, or grenade the transfer case. There's a bang and then oil on the ground and no more drive to the rear wheels. The clutch packs go quietly, exit stage left... Part of the problem is the Hyundais don't allow ANY difference in wheel speeds, so wear on even 1 tyre can be devastating as you can't drive 2 axles at the same speed if there's 1 tyre of differentr size, let alone 2 on the same axle...

Don‘t hear much of that happening and there are many who probably don’t rotate as often as they should. I don’t mean to question your knowledge, but it doesn’t make sense to me.
 
Thanks for the replies. Have not narrowed down the vehicle choices, just mostly poking around. Could be anything from Subaru, Toyota, Ford, Hyundai, or Nissan (with no CVT).

Are there quick general checks that one would usually do on these AWD systems to make sure they're functioning?

Drive it, pay attention to NVH, check for codes and check for fluid leakage are some thoughts off the top of my head.

People love 4x4 / AWD, but there really is a lot of extra parts and complexity added to the vehicle to have it. Not only do you have the coupling, most have some type of transfer case or power transfer unit that is mounted between the transaxle and front right CV shaft. Then you have the rear prop shaft and it's carriers/bushings/bearings, the rear CV axles, rear diff which is usually hung on bushings and the list goes on. It's great when you need it. Not so great when parts need replacing.
 
Don‘t hear much of that happening and there are many who probably don’t rotate as often as they should. I don’t mean to question your knowledge, but it doesn’t make sense to me.
I'm guessing half of the santa fe's (DM) with AWD I get in contact with have had AWD parts replaced. In fact, I can't think of any that didn't right now... Will have to go overe the list to find any that didn't have work done. Part of the problem is Rotations are not practiced here and people only go to a workshop once a year. If the tyres don't get rotated then, they likely don't make it another year. A LOT of people didn't know the AWD didn't work until we told them, or the transfer case was leaking. Holed transfer cases or clutch pack assemblies are the least common.

Tucson is better but still quite a few. Less tucson with AWD though. No kona as of yet, but very few of those have AWD
 
This is an old thread but the closest thing to this subject I could find in search...
I have a related question about old Subarus (a 2004 Forester): do they use Torsen differentials?
A Torsen would provide limited slip without clutches, purely mechanical without electronics either. Torsens are great differentials if you can live with their limited torque bias ratio (something like 3:1, not full lock).
PS: from what I read, since it's a 5-speed stick it has open diffs front & rear and a viscous coupled center diff.
 
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This is an old thread but the closest thing to this subject I could find in search...
I have a related question about old Subarus (a 2004 Forester): do they use Torsen differentials?
A Torsen would provide limited slip without clutches, purely mechanical without electronics either. Torsens are great differentials if you can live with their limited torque bias ratio (something like 3:1, not full lock).
PS: from what I read, since it's a 5-speed stick it has open diffs front & rear and a viscous coupled center diff.

The rear diffs in the STi are (were?) Torsen, but I'm not sure if they still are as I haven't kept up with them in a while. Most Subarus around that era were a viscous center diff with sometimes different rear diffs combinations but I'm not sure if that's still true today.
 
I may be scoping around for an AWD vehicle for a family member soon and will be looking at used vehicles in the $5000 range. I've always been hesitant on used AWD vehicles since it just seems like more parts that could break.

I'm noticing a lot of these modern AWD systems use clutch packs in their transfer cases (and sometimes in the differentials too) to shift power. My understanding is that these are wet clutches of which in theory the wear is minimal since it relies on hydrodynamic lubrication meaning there's no physical contact of the clutch plates. Is this typically true of all AWD systems, and how is the long term reliability on this? Is the fluid the only part that breaks down and needs servicing?

Would like to hear your thoughts and experiences on these AWD systems. Which manufacturers are known to be more reliable and which to stay away from?
Sorry for 5K you are not getting much, probably an old 2.5 Subaru and with that the AWD system would be the least of you worries, chances are it is one start away from needing an engine and/or replacing a rotted out rear sub frame. These can set your wallet on fire in short order.
IF you can find an old one with the EJ22 closed deck engine and no rust that would be your best bet.
 
The weak point on several AWD systems in fords and Volvos, is the 90 degree PTO shaft off the transmission. I think these are probably one or two related families of aisin. The bevel gear and shaft sits in its own fluid and often isn’t serviced.

haldex units also are prone to be overlooked and neglected. They can sometimes be resurrected with hands-on care. As mentioned above, honda’s AWD using dual pump systems is hard on fluid. They work fine if you even half reasonably maintain them, but in a $5k car, I doubt the 2nd or 3rd owner cared.
 
Subaru uses a few different systems - the most common is a viscous coupling system that’s trouble-free as long you’re not being stupid - and care is taken to make sure all tires are within a difference of 2/32nds of a inch. No special fluids - ATF/CVTF for automatic model, and gear oil for the front diff and manual models.

Honda’s Real-Time 4WD and VTM-4(now called SH-AWD by Acura) use clutches but differently. Regular(every 30K) fluid changes with OE Honda DPSF or VTM-4 fluid will keep the rear diff happy and the front PTO is serviced with GL-6 gear oil.

The Haldex system used by VW/Audi, Volvo and GM does need regular fluid and filter changes with the proper stuff.
Front transfer with Acura SH-AWD uses 75/90 GL-5 with hypoid. Correct Honda pump fluid rear diff or second gen 07-2013 use ATF in rear diff
 
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