Transfer case on AWD models: was it meant to be reliable ?

[oddball]

I made a video where I tried to explain and show the internals of the transfer case.
I don't know why there so little videos on youtube on this transfer, so I decided to try upload one.

So, it seems like the wave spring pushes against friction disks and provides AWD feature all the time. Not sure how many % does it send to front wheels, but definitely not more than 50%
The only way how these disks may get overheated is through uneven amount of rotation on front and read wheels.
Also, if you got an AWD problem dashlight, it's either oil pressure sensor failure or oil pump failure. Both are easy to fix.

Excellent video, thanks for recording and posting!

For the clutches, the ones that engage on the outside and are just plain steel are called "steels", and the ones that engage on the inside and have friction material are called "frictions".

Next time, spread out the clutches. You want to make sure the frictions aren't burnt or flaking apart, and make sure the steels don't have hot spots or are warped. It can be difficult to tell how much friction material has been worn from the frictions if you don't have a new one to compare against. Usually a transmission builder would assemble a clutch pack and check the clearance, then replace parts as necessary to get the desired clearance.
In this case, I expect that if you assemble the basket with the pressure plate (the thing you called a "cap" on the basket) and WITHOUT the wave, there should be a very small clearance between the basket and the lip of the pressure plate. That should be just slightly larger than the actual thickness of the wave.
Of course, I'm assuming that pressure plate doesn't have any flexibility and the bottom inside of the basket doesn't either.

That wave spring is not enough to engage the clutches. Sure, I bet installing the rear cover does compress the wave plate, but it's not going to be the entire distance. That is just to cushion the engagement so it's not harsh. Many automatic transmissions include wave springs or steels in various locations depending on the design of the trans. Many of the parts in that area are identical to an automatic transmission, and all the concepts are the same.

There's definitely an ability to change the AWD split. It's quite common to pull the "AWD Fuse" and run the car on a 2 wheel dyno. That fuse powers that oil pump. So no oil pressure = no power to front wheels.

It looks like there is a piston in the rear cover. This piston doesn't rotate, so it presses against that large torrington bearing, which presses on the basket pressure plate, which engages the clutches. It's difficult to tell how that comes apart as I'm not seeing an obvious snap ring in the video.
But, that must be a piston. The clutches need to be applied from one side or the other. The other common method is to feed oil through a shaft then through seals into the basket and have a piston inside the basket. That's obviously not the case here. So the piston must be in the case. (ha ha)

ACTUALLY! Spoke (typed) too soon. Looks like the piston might be on the back side of the mid case. I see a snap ring there. That could then push the entire basket against the rear case. Not certain about it though. Note that the piston only needs to move a very short distance. The standard rule of thumb is 0.010" per friction, and looks like there are 9 frictions, so about 0.090" or about 2mm. The fluid acts like a lubricant so the plates are designed to just float past each other unless there's enough pressure to squeeze the fluid out. Given the intent of this clutch they could easily design it to be closer, like 0.040" or 0.050".

On third thought, I'm convinced the rear case has the piston. There doesn't appear to be a provision to manage the rotation of the basket on the mid case, and it appears that when the basket is pushed forward, that large shoulder on the transfer gear engagement actually presses on the transfer gear. So the basket bottoms out there, instead of pressing on the mid case. But that doesn't answer WTF there's a snap ring on the mid case.

AWD codes could also be due to readings from the speed sensors that the ECU doesn't like.

My assumption is there's a fixed lube orifice. Run the pump at or below a certain duty cycle and it only moves enough oil to lube the clutches and gears. Higher duty cycles progressively increase the clamping of the clutches.

Torque split is due to two factors: a) slippage of the clutches, and b) design of the gearbox. Wet clutches don't enjoy partial engagement, so I doubt they do that a whole lot. Especially under power. I mean, it's possible, and it is done, but the design life will be pretty short. There's quite a few frictions in there, which helps, but they're small. For comparison, an overrun clutch is usually 2 frictions, a forward clutch is usually 4 frictions, and a direct clutch can be anywhere from 3 to 6 frictions.
The design of the box includes the gears. That intermediate gear is freaking huge. Some of that is to get enough distance so the forward shaft has clearance, but I bet that also factors into the torque split.

I find it very interesting that they use taper bearings on the intermediate gear.

The holes in the hub (at 7:00) and the slots and the basket are just for lubrication. ATF is also a coolant - you want ATF flow between disengaged plates to cool them.

You said initially the wave was broken when you opened the unit. I expect that was the first failure and broke due to manufacturing fault (hairline crack, bad heat treatment, whatever). The chunks then clogged up the filter which reduced oil flow which caused things to get hot and start melting.

There's a couple of other interesting little design features that I'm not entirely sure how they factor in:
1) The teeth on the basket that engage the transfer gear are much wider than the teeth on the gear. At first glance I thought the engagement teeth on the gear were a synchro!! That implies they expect the basket to move backwards and forwards quite a bit.
2) The engagement teeth on the transfer gear aren't fixed to the gear. When you turned the gear train back and forth I could see there's a little bit of play between the two. That's a bit odd as I would have assumed that would be a single forging. Or, well, powdered metal casting these days. I'm curious if there's more going on in that gear. Just seems like a minor bit of unnecessary complexity. Actually I take that back - with a replaceable interior, that same gear part number can be used for the transfer gear and the output gear, with the output gear having a different insert that engages the output flange. Still, now you have at least 3 part numbers where otherwise there would've been 2. Hm.

 

[oddball]

I think if you run the pump faster it would not help to send more torque to front wheels.

Look at this from another angle: if you split 50/50 torque between front and rear wheels then you need, well, equal or similar axles (the shaft which goes from differential to left or right wheel) on front and rear to hold the torque. But on AWD models the front axle is much more thinner than the rear one. I'd say front one is twice thinner, meaning it's designed to hold 3-4 times less torque (area = p * radius ^ 2).

There are just no signs to believe front wheels can get more torque at any moment.

Length has more to do with the size of the shafts in this case. The front transfer shaft is very short, so doesn't need much. And even then, driveshaft design is more about critical speed and weight. Diameter is then just calculated based on the handling capacity of the desired material. It takes *very little* solid steel to handle hundreds, if not thousands, of foot pounds of torque. Most driveshafts are hollow because the length required would make a solid shaft wildly heavy. But once you go to a tube, the torque capacity goes way down.

The diameter and length of the shaft determine its critical speed. Above that speed the shaft gets unstable and will vibrate regardless of how well its balanced. The fun bit is the critical speed of a traditional driveshaft (trans to rear - 50" to 65") using common material (2.5" steel) is right about 5,000 RPM! A thicker shaft has a higher critical speed. So they'll design to the minimum size to handle the load, then possibly increase size to get acceptable critical speed.

For comparison, look at the half shafts on any car with independent rear suspension. The wimpy Honda with almost no power to the rear will have something like 1/2" things that look like spaghetti noodles, but that's enough to handle ~100HP. Move up to something more exciting like a camaro with 500+ HP and you're still only talking 1.5" or so.

Anyway, I expect the rear driveshaft is a tube while the front transfer shaft is a solid shaft.

 

[MerlintheMad]

^^^Are you guys writing the "Gearhead's Bible"?

 

[oddball]

^^^Are you guys writing the "Gearhead's Bible"?

Barely even scratches the surface..........

 

[FarmerMagnum]

You were right the first time. 47370b is the piston unit. It's trapped in the first case he pulls off. That applies pressure to the basket when enough oil pressure is delivered.

Here's a link to the part - I'm too new to post working links
www.hyundaipartsdeal.com\genuine\hyundai-piston-assy~47370-4j000.html

 

[FarmerMagnum]

Also, given how simply this transfer case seems to work I don't think it would be difficult to build a circuit to control it and fake out the ecu. Either that or this would be right up the JB4's alley. We'd just need readings from the oil pump and sensor at various instances like launch, under accel and traveling at highway speeds to understand it. The box would just send sensor values to the ECU in relation to what it expects to see while delivering values we want to the pump. A rwd or drift mode box would be cool.

 

[dgorbenko]

I made couple videos, the first one regarding problem in my transfer case, and the second where I tried to speculate on how the oil interacts with friction plates. The second is boring, 17 minutes, sorry in case you are going to watch it

Also, I must admit that during filming these videos my understanding how everything works was changing. So, don't blame me if you see something weird at the beginning.

I hope some of you will find answers to your questions or assumptions. Once i get new friction plates I'll be able to assemble the transfer, fill it with oil, put 12V to the pump and see how it works: can we regulate the torque split by voltage applied or not ? Will it send any torque to the front wheels without pump working ?

 

[GennyGuy]

Barely even scratches the surface..........

Keep it coming.

 

[oddball]

As for why the middle clutches were bare: heat builds up from the middle of a clutch pack. Once the oil movement slowed due to the blocked filter then heat built up quickly in the clutch pack. That's why transmission builders put thicker steels in the middle of clutch packs whenever possible. The ones in the middle get hot fastest, which causes the friction material to fail and flake off. Once enough comes off then there's enough clearance to let the clutches start slipping. But it's still partially engaged. So the slipping causes MORE heat and MORE friction material to fall off until finally enough has failed that there's enough clearance for the clutches to spin freely. In your case, the wave spring broke and fell out, so material kept wearing away until you have the current stackup where the pressure plate just touches the top steel.
Once you get the new frictions (and steels, I hope!!) you'll find that there's clearance between the pressure plate and the basket when things are squeezed together, and that clearance is just a bit more than the thickness of the wave.
All of your clearance/distance/thickness/rattle tests are pointless because you're using heavily damaged clutches. Results will be completely different with new clutches. Even the frictions that look OK probably aren't. Several of them are probably worn much thinner than they are supposed to be.

Food for thought: if there was no control on engagement then there would be absolutely no need for a clutch pack and no need for a pump. Splash lubrication is plenty for a gear train, just like used in countless truck transfer cases for decades. So something's going on, just have to figure out what! It doesn't have to be obvious!

The wave plate is also the return spring for the apply piston. Like I said earlier, the case partially compresses that spring, but it won't be all the way. There will be just a very little bit - maybe 1mm - that the pressure plate can be moved - by the piston in the rear case - to apply pressure.

Moving that piston might be pretty hard. Pistons usually use lip seals that make a good seal. This one doesn't need to move very far and they never rotate. Usually I have to apply air after removing the retention features to pop a piston out of its pocket. You can clearly see where the seals are: there's one around the outside of the piston at that machined surface, and then on the far inside. Those grooves are where the rear housing was machined out and the piston slips in.

To keep messing with this, you'll need to map out all the galleys in the back and what they're doing with them. I very much expect that there is a small bleed orifice that feeds that one big hole you can see which spills fluid into the oil guide and into the filter. The rest will route oil to the back of the piston.
One thing you can do is put the pump back on, the pressurize the system via the sensor hole. It might take A LOT of air to flood the bypass and get pressure to the piston. Might have to block that lube hole with your finger. This also assumes all the galleys in the back are just tied together, which I would expect. The galleys are also much more complicated than is really required, so the engineers must have been planning for other things, too. Just note that the piston will likely fly out and it could be very - very - difficult to reinstall. Lip seals are very annoying. Heck, it might use o-rings which are even worse. Lip seals are self-sealing, so it doesn't *have* to be a tight seal. O-rings are self-leaking, so it *has* to be a tight fit.




Anyway, you don't have to believe me, but this is how that unit works.


Also, just a warning, clutch pack clearance is a very important specification. Checking it can be difficult, and correcting can be even more difficult. Finding the specification and verifying it might be difficult. NO clutch pack can be blindly assembled and run, even when using all OEM components. Too much clearance and there's not enough engagement, causing heat and failure. Too little clearance and there's not enough room for slip, causing heat and failure.

 

[oddball]

Bench testing torque split will be very difficult because it'll have breakaway torque in the dozens to hundreds of foot pounds range.


Also, I'm thoroughly convinced that the fuse trick works. If it didn't, there would be A LOT of AWD stingers with massive front end damage from flying off of two wheel dynos. And probably some dead guys, too.

 

[dgorbenko]

I believe you, because this is my first opened transfer case, and I have no knowledge how it should/must/can work, so I have to believe )
I have some knowledge over mechanical stuff, but I see you have much more.
And very thank you for spending your time trying to explain me probably some basis stuff. I appreciate it.

> NO clutch pack can be blindly assembled and run
how to assemble clutch pack in such case ? I'll try to find some materials to read on.

Btw, since it will take me some time to find plates, I'm going to assemble the case without friction and steel plates, put the transfer back to stinger, and use this car if i need. I don't expect some issue to happen, unless AWD module may tell me it "doesn't feel the front wheels get some torque". Do you think it may be OK as a temporary solution ?

Since you don't mind to explain some basic things to me, then what if... imagine some stinger has front and rear wheels and they take exactly the same amount of rotations per mile. Then, imagine this car running at some speed on normal road. Good stable friction on all wheels. Also, assume the friction and steel plates are separated between each other with such distance they don't experience friction at all. Question: will they rotate at the same rate ? Because they should, right ?
And I'm sorry if I asked this question before and you answered one.

Cheers.

 

[oddball]

No worries, and I'm very glad you're interested to learn. Taking the time to video and post is also incredibly helpful! I'll likely never see the inside of one of these units otherwise (my car is RWD).

To check clearance on this pack, assemble all the clutches into the basket, then put the pressure plate on top without the wave. Press them tightly together and measure the gap between the lip of the pressure plate and the top of the basket with feeler gauges. Since you measured the wave at about 1.2mm, I bet the gap will be around 2.2mm to 3mm. Now subtract the thickness of the wave (1.2mm) and you have the actual clutch clearance (1mm to 1.8mm is my expectation). Do this with the clutches dry and everything clean.


Note, before final assembly, soak the frictions in ATF for 15 to 30 minutes. They'll soak up a lot of fluid. This is necessary to make sure they don't burn before they soak up fluid naturally. You can't check clearance at this point because they'll actually swell just a tiny bit, and the specification is determined with the frictions dry.

Yes, I think you can reassemble with no clutches and run the car. Keep everything else in there (pressure plate, wave spring, etc) to keep all the guts in the right place. The front wheels just won't receive any power. I would want the oil guide - even the melted one - in place to make sure the gears are lubed correctly. That wouldn't be strictly necessary, especially if it won't be driven a whole lot and won't be driven hard. You will absolutely get a check engine light and error codes, but they can be ignored.


For your last question, yes, in that case all the components will spin together. There will be slight gaps between the frictions and steels but they should be rotating together.



Monitoring the duty cycle of the pump would be fascinating. Even though the AWD has open differentials, I wouldn't be surprised if the ECU reduces the pump duty cycle whenever the car is in a turn. That would let some slip in the clutches to prevent crab-walking or lurching.

 

[vipeboy2000]

No worries, and I'm very glad you're interested to learn. Taking the time to video and post is also incredibly helpful! I'll likely never see the inside of one of these units otherwise (my car is RWD).

To check clearance on this pack, assemble all the clutches into the basket, then put the pressure plate on top without the wave. Press them tightly together and measure the gap between the lip of the pressure plate and the top of the basket with feeler gauges. Since you measured the wave at about 1.2mm, I bet the gap will be around 2.2mm to 3mm. Now subtract the thickness of the wave (1.2mm) and you have the actual clutch clearance (1mm to 1.8mm is my expectation). Do this with the clutches dry and everything clean.


Note, before final assembly, soak the frictions in ATF for 15 to 30 minutes. They'll soak up a lot of fluid. This is necessary to make sure they don't burn before they soak up fluid naturally. You can't check clearance at this point because they'll actually swell just a tiny bit, and the specification is determined with the frictions dry.

Yes, I think you can reassemble with no clutches and run the car. Keep everything else in there (pressure plate, wave spring, etc) to keep all the guts in the right place. The front wheels just won't receive any power. I would want the oil guide - even the melted one - in place to make sure the gears are lubed correctly. That wouldn't be strictly necessary, especially if it won't be driven a whole lot and won't be driven hard. You will absolutely get a check engine light and error codes, but they can be ignored.


For your last question, yes, in that case all the components will spin together. There will be slight gaps between the frictions and steels but they should be rotating together.



Monitoring the duty cycle of the pump would be fascinating. Even though the AWD has open differentials, I wouldn't be surprised if the ECU reduces the pump duty cycle whenever the car is in a turn. That would let some slip in the clutches to prevent crab-walking or lurching.

this thread has been an awesome read for me. I love learning about the mechanicals.. Looking at those videos, the internals are very similar to the bmw transfer cases which makes sense since i believe magna produces them for both and i have looked up many videos on those as an awd bmw was going to be my next car until i got the stinger.
on your last point though, i'm guessing it would especially on the gts or indigo models in the states and the top trim in canada as they do have awd with the LSD.

 

[oddball]

this thread has been an awesome read for me. I love learning about the mechanicals.. Looking at those videos, the internals are very similar to the bmw transfer cases which makes sense since i believe magna produces them for both and i have looked up many videos on those as an awd bmw was going to be my next car until i got the stinger.
on your last point though, i'm guessing it would especially on the gts or indigo models in the states and the top trim in canada as they do have awd with the LSD (Limited Slip Differential).

Yup, a handful of special AWDs came with limited slip. Especially the ones that advertised AWD with "drift mode" as a limited slip is pretty useful for that.
Maybe Canadians just hoon around more than US?

 

[D.J.]

Yup, a handful of special AWDs came with limited slip. Especially the ones that advertised AWD with "drift mode" as a limited slip is pretty useful for that.
Maybe Canadians just hoon around more than US?

lol

Or it's for enhanced snow traction ! - plus hooning around

 

[vipeboy2000]

Yup, a handful of special AWDs came with limited slip. Especially the ones that advertised AWD with "drift mode" as a limited slip is pretty useful for that.
Maybe Canadians just hoon around more than US?

Haha it definitely does. And drift mode does work.

 

[oddball]

lol

Or it's for enhanced snow traction ! - plus hooning around

Who would be crazy enough to drive this car in the snow?
I vote 100% hooning.

 

[dgorbenko]

You will absolutely get a check engine light and error codes, but they can be ignored.


For your last question, yes, in that case all the components will spin together. There will be slight gaps between the frictions and steels but they should be rotating together.

You may be surprised as I was because i didn't get any error codes. As I was saying, I assembled transfer case with everything except friction and steel disks, drove 10 miles on regular roads + highways, so far no error codes. And I did not pull the AWD fuse.

Btw, after I assembled TC I send 12V to oil pump, and for the first 2 seconds I heard steady sounds, then it changed to lower tone, I guess because the pump achieved some pressure level. I stopped the pump, waited a minute, did the same test and I heard the same sequence. So, I guess you were right, and next time I disassemble it I'll try to see how the piston is moving.

Now, sorry for long boring paragraph, but I have to train my imagination
Regarding rotation speed of frictions and steels: so, I'm in a car, it's parked, I'm changing to D mode, firmly pressing on gas pedal. What happens next ? The torque is sending to rear wheels and steels are rotating because they are sitting on the rear shaft. Do steels rotate frictions ? Maybe, but I think no, because the oil pump has not yet put enough pressure on the piston (this is an assumption). It could be putting the pressure to help the car to get move traction with the ground (by using front wheels), right, but not yet.
So, frictions are shifting against steels.
After some short period of time rear wheels will push car body forward, and front wheels will start spinning as well, which will rotate frictions, as they are sitting on the same front shaft. And if there is no slip between rear wheels and the ground (again, assumption), front wheels will be rotation with the same speed as rear wheels.
Which means, frictions are rotating with the same speed as steels, which means, frictions are not shifting any more against steels.

After some time I depress the gas pedal, and maybe the frictions will get back to the same position regarding steels as they were before when car was not moving.

So, I don't see the reason to have AWD unless rear wheels are losing traction with the road. Which is obvious
Which comes to the question, if I drive carefully and don't slam on the gas pedal (expect that kick down, but there were no slippage of rear wheels), how come what my transfer case broke ?

 

[oddball]

Now, sorry for long boring paragraph, but I have to train my imagination
Regarding rotation speed of frictions and steels: so, I'm in a car, it's parked, I'm changing to D mode, firmly pressing on gas pedal. What happens next ? The torque is sending to rear wheels and steels are rotating because they are sitting on the rear shaft. Do steels rotate frictions ? Maybe, but I think no, because the oil pump has not yet put enough pressure on the piston (this is an assumption). It could be putting the pressure to help the car to get move traction with the ground (by using front wheels), right, but not yet.
So, frictions are shifting against steels.
After some short period of time rear wheels will push car body forward, and front wheels will start spinning as well, which will rotate frictions, as they are sitting on the same front shaft. And if there is no slip between rear wheels and the ground (again, assumption), front wheels will be rotation with the same speed as rear wheels.
Which means, frictions are rotating with the same speed as steels, which means, frictions are not shifting any more against steels.

You are correct. If the piston is not compressing the steels and frictions together, then they are free to spin. However, I expect that most of the time the TCU or ECU is commanding the pump to pressurize the oil system, which will push the piston, which will press the frictions and steels together. In an ideal situation where the road is level and you're going in a straight line then there's not much stress on any of the components.

Main thing to remember: if the piston is engaged, then the steels and frictions are squeezed together and the basket and shaft will turn together. Any traction difference or stress will just be transferred straight through the clutches - the holding power of that clutch pack is far higher than the tire traction. If the piston is not engaged, then the frictions and steels can rotate separately, or together, or opposite directions, or anything they feel like.

In a typical RWD car, especially with an open differential, if one wheel looses traction then all engine power goes straight to that one wheel and it spins like crazy. If that same situation happens here, the front wheels are also tied to the driveshaft via the transfer case if the clutch is engaged. Now the engine power will also be turning the front wheels which, hopefully, have more traction.

Which comes to the question, if I drive carefully and don't slam on the gas pedal (expect that kick down, but there were no slippage of rear wheels), how come what my transfer case broke ?

This was definitely a manufacturing defect in the spring/wave plate. That spring gets cycled at least every time you start the car - it's relaxed when car is off, and compressed whenever AWD is engaged which should be almost all the time while driving. You can't use plain steel for something like this - it'll loose it's "spring" after just a few cycles. So it needs to be a spring steel, and/or have a special heat treatment. I bet it broke in the same place in all the waves - the lowest or highest point (depending on how you look at it) of each bend. That's where all the stress happens in the material when the spring is formed and every time it moves.
Also, "shit happens".

 

[Morridin19]

So if you pulled the AWD Fuse is there a risk of damaging the transfer case as there isnt any active lubrication due to the pump not running?

Or (excluding the input shaft and output shaft to the rear) because the piston isnt apply pressure almost nothing is moving in the transfer case to the risk is negligible?