How does AUTO 4WD work/what does it do?

[Ken226]

If you don't care about the technical, or don't want to give yourself a headache trying to keep pace with the following explanation, skip to the next post.

Anyone who chooses to follow along below, will need to read carefully, and look at each part as its called out, in order to understand.


Reference the linked PDF for part numbers, descriptions and a schematic. https://marketing.transtar1.com/acton/attachment/18758/f-147c/1/-/-/-/-/BW4444 Flyer 0917.pdf

The arrangement of the ballramp (Borg Warner calls it the mode hub, part 35, 814 and 619), is difficult to visualize and even harder to explain. But im going to try here. If I do a ****** job of explaining, please forgive.

The transfer of compressive force from the ballramp into the clutch plates flow as follows. I know its complex, but if you print the .PDF so you can look at the arrangement of parts as your reading below, it'll be easier.

When the mode hub magnet (part 910) is energized, the magnet exerts a very soft (read 12 volts, 5 amps) rearward (towards the rear of the truck, to the right hand side in the schematic) pulling force on the mode hub (part 35), which transfers it's pulling force onto the thrust bearing (128), which then transfers the pulling force onto a shim (part 221), which then transfers the force onto the circlip (407), which then transfers this force onto the shaft (part 2), The pulling force on the shaft, then transfers through the shaft, back up towards the front and onto the drive sprocket thrust washer (part 237), then through the thrust bearing (part 126), then onto another trust washer (part 237 again), then this force is transferred onto the drive sprocket (part 38), which in turn, transfers this force into the clutch basket (clutch pack kit part 625), and terminating in the clutch friction/steel plates (parts 813 and 812)

The other portion of the ballramp (part 619, cam, magnetic), applies the other half of the force, which compresses the clutch pack. It does so in the following way.

When the above mentioned mode hub is activated, and applies its rearward force through the shims, circlip, shaft, thrust washers and bearings and into the clutch pack (outlined above), the other half of the ballramp (part 619) applies this magnetically created small force, in the opposite direction, forced forward (towards the front of the truck) by the balls inside the ballramp. (part 619) applies its force against the armature (part 630), which transfers this force into the "apply plate" (part 621), which in turn transfers this force onto the clutch friction and steel plates (parts 813, 812).

If your able to follow the transfer of forces listed above, you can clearly see that the rearward pressure 0n the clutch basket, which is free to slide on its respective shaft splines, is met by forward force applied via (part 619), through (part 630 and 621), into the clutch pack (parts 813 and 812). This creates friction inside the clutch basket.

Now, in this activated state, any time the rear wheels attempt to turn without the front wheels turning along with them, the 2 halves of the ball ramp (parts 35 and 619) attempt to rotate in opposite directions, and the balls force the 2 halves apart and apply a huge compressive force onto the clutch plates (parts 813 and 812). Recall in a previous post, I calculated that for every 1 foot pound of torque the engine sends to the transfer case, there will be 24 lbs of compressive force mashing the clutch/steel plates (parts 813 and 812) together.

Also, keep in mind the shape of the ramps. If you stop and put the truck in reverse, the front half of the ballramp (part 619), reverses direction, the balls roll into the deeper part of the ramp and the front wheels are freed from their locked (clutched? "choose your own term") state, until part (619) has rotated 120 degrees, and the balls are forced up the other side of their respective ramps, after which the front/rear systems are _re-coupled_. ( I don't wanna get flogged for saying locked!)

So, you can clearly see that powering the magnet (part 910) pulls the clutch plates together with a small magnetic force, creating friction. You can clearly see that once this friction is initiated, any relative rotation differential between the front/rear drive systems will cause the ballramp halves to rotate in opposite directions. Once the ballramp halves have rotated in opposite directions 60 degrees, the balls force the clutches into compression with about a 24:1 mechanical advantage. Every 1 ft lb of rotating torque creates 24 lbs of axial force into the clutch plates.

This is how it works. This is whats going on inside the transfer case. These are facts, and they don't care about anyones preconceived notions.



if you ever decide to fix it yourself or hire a mechanic to tighten things up. Pay attention to the next post.

 

[Ken226]

Ok, we've clearly, definitively, without question, established that differential rotation of parts (35 and 619) result in the 3 internal balls riding up their respective ramps and forcing parts (619 and 35) apart, in opposite directions, mashing the friction plates and steel plates together, with a 24:1 mechanical advantage. Heres the important part:

The amount (ratio) of relative differential rotation (parts 35 and 619 respectively), needed, to fully compress the clutch plates, is determined by the amount of "slack" (for lack of a better term) (free space maybe better) in the overall system.

The amount of (slack) or (free space) in the system is adjustable. Hence the amount of differential rotation needed to fully compress the clutch pack, is adjustable.

This adjustment is accomplished by using a shim (part 221) of a different thickness that the one currently installed. Notice that on page 2 of the schematic, there are a total of 9 different part numbers for part (221). Each of these shims have a different listed thickness.

The thicker the shim you use, the smaller the amount of differential rotation needed to fully compress the clutch plates. They range from 3.95mm to 5.3mm. A difference of only 1.35mm.

It should also be noted, that using a shim too thick could result in clutch pack friction while the magnet is not energized. Resulting in having the ball ramp assembly engaged all the time, even when 2wd is selected.

If anyone decides to experiment with these clearances, we would love to hear back as to what your results were.

 

[chrisbh17]

I for one appreciate all of the detailed analyses you have provided. Im not a mechanical engineer but have gained a much better understanding of how its supposed to work, which is more than I can say about any documentation from BW or FCA.

I find it interesting there are so many different shim thicknesses available. Since FCA doesnt assemble the xfer case, is it just up to BW to determine which one is necessary and actually use the correct one? Or is it just "whatever comes out of the bin"?

I know you explained how it all should work (i.e. what wheels should do what), but Ive also seen you mention that there probably are some t-cases out there that arent working correctly (even brand new). My concern has been if mine is, or rather WILL, work the way it should. Still havent had the correct weather really to try it out, so Im just assuming I got a good one (but you know what happens when you assume!)

 

[VA10]

And to be quite honest, you'd have got the same results if you had tested 4 auto and 4 lock in the opposite order.

4 lock first would have slipped a bit, then 4 auto would have engaged quick.

Likely, your first test rotated the ballramp into it's engaged position. It was still in that position when you switched to 4lock.

If u crawl under your truck and look at the transfer case, you'll see 1 single wire separate out of the main harness and enter the transfer case through a hole near the rear drive shaft.

The main harness plugs into a stepper motor, which operates a shift fork that shifts between low range and high range.

But that 1 wimpy little 14ga wire, rated for 12volts and a few amps, is all that operates the computers on/off controller of the 4wd modes.

That single wire powers the electromagnet that commands the ballramp on. Through that wire, Power on and the 4wd engages, power off and the next time u make a turn or stop, it takes the frontpart of the ball ramp rotates backwards a little, (relative to the back portion of the ball ramp), taking the pressure off the balls and a wimpy little wave-washer spring pushes the ballramps apart.

hmmm I did do that.....the truck did not behave like you are explaining. there could have been many factors in my very basic test that went overlooked. I will retest tonight.

Would coming to a complete stop prevent what you are pointing out? My 4auto is no where near as instantaneous as 4hi. No matter what order I shift in or out of 4hi or 4lo.

Are some cases or trucks programmed differently? Maybe build batches had/have slight differences?.........idk I after using this t-case for a while I understand there some concerns. And they are valid concerns. But I am starting to think there is high level internet hype regarding this t-case.

 

[Ken226]

No, I think your right.

4 auto would have the additional lag time in waiting for the computer to sense the spin, and send power to the magnet.

4 auto has to have wheelspin, then computer activation, then ballramp differential rotation.

4 lock only has to have ballramp differential rotation.

Your results seem to me, to be exactly what should have happened.

 

[skupko9680]

My recommendations based on it's function and my experience:

Things to avoid.

1. Avoid getting into situations where you need to switch into 4wd while your sitting still 'Stopped', AND will need all 4 wheels to pull at the same time. Such as stopping halfway up an ice covered hill to shift into 4wd.

In this situation, the rear wheels will brake loose first, and likely spin half a turn or so before the front wheels engage. And the front wheels will engage quickly, probably spinning a little too.
Activate the 4wd at the bottom, and the application of power combided with the flexure modulus of the tires will have all 4 engaged and pulling before you even get to the hill.

2. Avoid areas where you'll need to go back and forth between forward and reverse without wheelslip. When you switch from forward to reverse you'll need double the wheelslip to engage the system. Then, when you switch back to forward, you'll need double the wheelslip again. Sucks if you need to rock back and forth.

3. Avoid sudden high speed engagement of the 4wd . This isn't likely to be a problem, unless you get your driving advice from the internet. The traction control will prevent damage, unless you've turned it off.

Example situation that could cause this: While stopped, you turn off traction control, engage 4 auto, then hit the gas. You'll immediately get high velocity wheelspin in the rear wheels, and the computer will power the ballramp mechanism, which will hammer the front wheels into engagement with a violent shock load.

If the traction control is ON, it will detect the high velocity wheelspin and reduce power before engaging the ballramp.

Things to do:

Turn on 4wd early. Once the rear wheels have turned a few times and the ball ramp has engaged, it'll stay engaged as long as your applying some power.

Don't be afraid to trust the stability control and traction control. Contrary to internet lore, it makes things better, not worse. The only time it's a net negative is when high speed eheelspin is needed to centrifugally spin mud out of the m/t tires lugs.

Don't try to outsmart the computer. It's programmed to start/stop actions based on sensory inputs. If your doing something the programmers couldn't forsee, the computer and traction control will behave in unpredictable ways.

Example: you put your truck on ice, and shift into 4 low to show everyone how bad your 44-44 sucks. You get out to film your rear wheels turning while the front wheels are stationary.

In this situation, the following occurs, cyclically, in fractions of a second. The rear wheels start to rotate, the ballramp electromagnet is energized, and the rear wheels start to rotate, depending on how slick the ice is under the front wheels, they may start to spin too, The stability control interprets this and wheelspin and begins selectively braking both the front the rear wheels (which causes the ballramp to begin clocking back and forth, and stop compressing the clutches, just making a nasty ass grinding noise), the wheels stop and the abs stops braking, then the wheels start rotating again, then the ballramp engages, then the abs kicks in again, then the ballramp releases, then the abs releases, then the wheels rotate, then the ballramp engages. The cycle repeats, multiple times per second, over and over, generating heat, wear and possibly damage. The exact timing, force and results of this cycle will depend on relative friction under each wheel, condition of the clutch friction plates, and the idle power transferred by your torque converter.

Don't: Test the 4wd by holding the brake and throttle at the same time. It creates a lot of stress on driveline systems, AND if you do happen to accomplish your goal of getting the rear wheels to spin, your ABS and stability control will interpret this as wheelspin and begin doing strange things, as well as reducing power. Also, while the ABS is modulating the pressure in the brake lines, the ballramp will be rapidly engaging and disengaging (picture the ballramp plate driving the front sitting motionless as the other plate is hammering back and forth), as the ABS is selectively modulating the right and left, front and rear brakes independently, reducing/adding power, and trying to figure out WTF is going on.




If you really want to test this system, here is how to do it:

leave the truck in 2wd, leave the traction control alone. back into some deep mud, snow or ice and get the rear wheels really dug in and stuck good. Stuck good enough that in 2wd, you cant get out but not so deep that the front wheels cant get you out. Leave the front wheels on some nice, dry, high traction surface. Or, put the rear wheels on a set of roller dollys, while leaving the front wheels on pavement.

Now, put it in 4 lock or 4 auto, and give it some gas. The instant the rear wheels slip, the traction control will slow things down to prevent it from engaging too violently, the ballramp will transfer all (100%) (the rear wheels will still be turning, but this doesn't count as torque since they aren't pushing against anything), of the engines torque to the front wheels and all questions and mysteries will be answered. The Hemi, and that 44-44 will yank your ass out of that mud so fast you won't believe what happened. All 410 ft lbs of torque will be applied to that contact patch of rubber between the front tires and the road surface.

When I started seeing all the internet complaints about the 44-44, this is the first test I did. I went over to Canyon Creek Road, just east of Glacier, WA near Mt Baker and stuck the back wheels in a couple feet of snow, with the front wheels on some pavement. I left my first -front wheel only- black mark. When all 4 wheels are spinning, the ECS will allow you to apply all the power you want.

Don't believe me? Before you respond and tell me how wrong I am, go find a muddy ditch.

I drive a 2012 Chevy Tahoe for 8 hours a day, 5 days a week, and my Ram Sport the other 2 days, in the Cascade mountains. The Tahoe has the 44-05, which is smaller but otherwise identical to our 44-44. It behaves the same way.

I get that Tahoe stuck plenty, since someone else owns it, i'm a lot braver in it. Above the snowline I have plenty of opportunities to stick the rear wheels in 2wd, then pull out with the front in 4 auto.

Where the 44-44 and the 44-05 both really work well is climbing really steep, loose packed rocky mountain roads. There are tons and tons of time when all 4 wheels are spinning at the same time, and the ballramp stays engaged, all 4 wheels keep powering on. The hardpack level switchbacks are easy, because you can make steep, 180+ degree turns without dragging or binding, and when you start climbing again, all 4 wheels are pulling again. In this application, the traction control works like anti-spin diff, when a wheel spins over a rock or spins out climbing loose gravel, the TC brakes it and moves power to the other wheel pretty seamlessly.

Thanks just what I was looking for1

 

[ClarkandAddison]

Maybe a dumb question, but I just bought my first 4wd truck. I have the 2017 with Express trim which doesn't have the "4wd Auto" option. Does this mean that I have a different tc than the 44-44? Does "4wd Lock" actually lock my truck or does it work like those with the "4wd Auto"?

 

[mohemipar]

Maybe a dumb question, but I just bought my first 4wd truck. I have the 2017 with Express trim which doesn't have the "4wd Auto" option. Does this mean that I have a different tc than the 44-44? Does "4wd Lock" actually lock my truck or does it work like those with the "4wd Auto"?

Express has the 44-45. So yes, different transfer case. Yours will actually lock. No auto mode.

 

[ClarkandAddison]

Express has the 44-45. So yes, different transfer case. Yours will actually lock. No auto mode.

Thanks. I knew I didn't have the auto mode but wasn't sure if the 4wd lock actually locked or not.

 

[mohemipar]

Thanks. I knew I didn't have the auto mode but wasn't sure if the 4wd lock actually locked or not.

Yep, same case as the one in the HD trucks. Also is in the Rebel and I believe Tradesman 1500s.

 

[Hemi395]

The HD trucks have the BW 44-46 or BW 44-47 tcase. One of those is a manual shift tcase but I don't remember which...

 

[mohemipar]

The HD trucks have the BW 44-46 or BW 44-47 tcase. One of those is a manual shift tcase but I don't remember which...

Yes this is correct. I had a brain fart. Though the electric ones operate the same. I think the 46 is the electric.

 

[Podcast]

And to be quite honest, you'd have got the same results if you had tested 4 auto and 4 lock in the opposite order.

4 lock first would have slipped a bit, then 4 auto would have engaged quick.

Likely, your first test rotated the ballramp into it's engaged position. It was still in that position when you switched to 4lock.

If u crawl under your truck and look at the transfer case, you'll see 1 single wire separate out of the main harness and enter the transfer case through a hole near the rear drive shaft.

The main harness plugs into a stepper motor, which operates a shift fork that shifts between low range and high range.

But that 1 wimpy little 14ga wire, rated for 12volts and a few amps, is all that operates the computers on/off controller of the 4wd modes.

That single wire powers the electromagnet that commands the ballramp on. Through that wire, Power on and the 4wd engages, power off and the next time u make a turn or stop, it takes the frontpart of the ball ramp rotates backwards a little, (relative to the back portion of the ball ramp), taking the pressure off the balls and a wimpy little wave-washer spring pushes the ballramps apart.

I've had a question floating in my head throughout this thread that I did not specifically see answered (however it likely was with all the replies and I simply missed it). What is the difference between 4auto and 4lock? From what I've read they are the same thing.

 

[Ken226]

They aren't the same thing.

Others will likely chime in and indicate otherwise, but as someone's who's engineering background specializes in gear drives and mechanical power transmission, I'm saying they are different.

The difference is very small in the way it's handled by the computer, but significant in the way it's manifested inside the transfer case.

To put it simply, in 4 auto, when the ABS wheelspeed sensors detect the rear wheels rotating at a higher speed than the front wheels, a current is sent down that single wire entering the transfer case electromagnet. When the front wheels speed up and match the rear wheels speed, the computer shuts off that current.

In 4 lock, that same wire powering the electromagnet, is continuously powered.

The resulting difference regarding actual power output to the wheels is as follows:

In 4 auto--your in 2 wheel drive untill the computer detects loss of rear wheel traction, then the computer powers the mode hub. A ballramp clutch engages and sends power to the front wheels. When all 4 wheels are turning the same speed, the computer shuts off power to the mode hub. The ballramp clutch remains engaged as long as torque is passing through it, regardless of the powered/unpowered state of the mode hub. As soon as all 4 wheels have equal traction, the torque passed through the ballramp clutch disappears and the system reverts back to 2wd. (Kinda the way the old steering wheel lock, locked up your ignition switch. U have to turn the steering wheel a little, take the torque of the ignition switch in order to turn the key).

In 4 lock--the mode hub is continuously powered. When the rear wheels start to slip, the ballramp clutch causes the front wheels to turn as well. (This mode makes your transfer case behave like a freewheeling clutch, or ratchet).

The front wheels are allowed to turn faster than the rear wheels, but, the rear wheels are not allowed to turn faster than the front wheels.

When you start acceleration, theres some slack in the system. The rear wheels gotta rotate about 1/6 to 1/3 of a turn, ish, to start the front wheels pulling. (the actual amount depends on things like your differential ratio, clearances in the clutch pack, wear and tear, ballramp module condition, the shimmed clearances from the factory, etc).


So, in short:

4 auto: wheelspin detected, computer powers the electromagnetic clutch, front wheels start pulling too. Speeds match, computer deactivates mode hub. Wheelspin detected again, repeat the above process.

4 lock: mode hub continuously powered. If the rear wheels start to slip, the front wheels are mechanically driven. (Yes, this torque is transmitted through the clutch plates).

In 4 lock, if the front wheels start rotating faster than the rear wheels (this occurs when you are driving around a curve, turning, etc), the ballramp design, -mechanically- (not computer commanded), relaxes the pressure on the clutch plates and allows you to turn without much binding.

All that being said, how it was intended to work and the results some are getting, seem to vary pretty wildly!

 

[ColdCase]

I've had a question floating in my head throughout this thread that I did not specifically see answered (however it likely was with all the replies and I simply missed it). What is the difference between 4auto and 4lock? From what I've read they are the same thing.

In other words, 4Lock activates the ball ramp and preloads the clutch so that there is just a bit of power driving the front wheels and anti spin is completely mechanical. It does not have to wait for the computer to detect wheel spin.

4Auto waits until the computer detects wheel spin before the ball ramp is activated. There is no preload.

 

[Ken226]

Exactly.

But, the theory and the actual results vary depending on condition.

There appear to me, to be several factors that can severely effect how fast (rear wheelspin before front wheels engage) the mode hub/ballramp start clamping the clutch plates together.

1 is the condition of the mode hub (part 35) friction face and the stator (part 630) friction face, and apply plate (part 621) friction face. These parts friction faces should be rough machined so theres plenty of friction. If they're worn smooth, things will be slower to engage.

2 is the amount of free clearance in the clutch pack while uncompressed. If the installed shim size creates allot of unnecessary free clearance, things will be slower to engage.

Combine the 2, and I'd expect exactly the behavior some have experienced.

 

[chrisbh17]

Just an FYI I finally got around to making a magnetic mount for my sons GoPro.

Already got one video and it confirms the front driveshaft spinning in 4WD Auto *and* 2WD.

To me it looks like the front driveshaft spins at the same speed no matter what (2WD or 4WD). Does that mean one "side" of the front differential is always "in play" in regards to what is spinning while the truck is going forward? That seems silly to me (wasnt the whole point of the axle disconnect to NOT spin anything until necessary, so as to save 0.0000000002 mpg?)

EDIT: As I take videos I will edit this post and add links.

Vid 1 -

 

[MT Hillbilly]

Both half's of the ballramp look like this:

https://cobratransmission.com/bw441...3_2122&zenid=e0bef0093352e1344418273d463603dd

You can clearly see the helical ramps that the balls ride in.

Picture 2 of those, facing each other, with a ball trapped in each groove and an electromagnet pulling hem together.
One of those plates is connected to the rear drive shaft. The other on a splined shaft to the front wheels.

Thanks Ken! this post helped me understand what is going on in there a LOT!

 

[Joes1500]

The front diconnect only disconnects one side. So you will have (power) coming from the other side , coming through the ring and pinion gear to the driveshaft.

All the dodge 4x4 front ends act the same way from 94 to present , regardless if they are solid or independent.

 

[MT Hillbilly]

This week I had no option but to drive in deep snow parking lots had ten or so inches of snow, and every thing but the main street had deep snow due to how fast it was falling and the wind. Even in 4 lock when pulling out onto the road after waiting for a break in traffic the rear would spin then the front engage, it just seems violent, is there no way to get around that? Is that not causing damage? The deep snow was made worse by having to drive through.. over the snow berm in order to get onto the main road. So, yes, I needed to spin the tires to clear my tread, and if I forgot and left traction control on, it would kill power , and take way longer to pull out onto road in traffic... if I made it through the berm at all. I do live up here so, yes, have studded winter tires, and have weight in back for extra traction.