BMW ATC 35L Transfer Case Repair and a Prevention Method
Hello everyone, I believe I have figured out why our transfer case output flanges fail.
Background story:
I noticed that I had a mild-moderate oil leak around the transfer case area. I put it on a lift and I could tell it was coming from the rear output flange seal. I ordered a new seal (just in case) and then days later went to try to install the seal.
2014 328Xd wagon, 89,000 miles, maximum power tune for the last 18,000 miles. This is the factory original transfer case, with a date of late 2013. No symptoms or weird drivability issues.
I put the car on the lift the second time, took off exhaust, all the panels, and the driveshaft partially. I got to the output flange, and immediately noticed a problem. A wobble in the output flange!
That explains the leak...
I pulled off the circlip and pulled out the flange by hand. I noticed that the splines were quite worn down:
From inspection of the splines, this is NOT instant shear force that wore them down. This looks to be from constant
vibration.
For proof, look at the symmetry of the wear on each side of the spline. It's even on both sides with sharp peaks. This means they were wearing clockwise and counter clockwise equally, even though the flange spins mostly in one direction for the life of the car.
Splines/output shaft was still holding down the torque, and I was still driving around.
This is not a metallurgy problem or too much torque.
More pictures here
https://f30.bimmerpost.com/forums/sh...&postcount=133
I changed my mind about changing the seal and went and picked up my new transfer case from home, which I have been saving for an 'event'.
I transferred the transfer case mount from the original case to the new one, because it was in great shape.
I couldn't just install this new case in, and call it a day... but rather, I pulled off the output flange.
I started with the circlip:
And the flange pulled out...
BY HAND. I didn't use a gear puller.
That was
huge alarm for me, because it's NOT pressed in nor held in place by a huge nut with high torque.
Essentially, the circlip is the only thing holding it in place.
I believe this is where ATC (designer and manufacturer of these ATC 35L cases) made the mistake regarding the output flange.
I discussed this with a seasoned transmission and drivetrain specialist.
This design makes it very susceptible to vibration, with eventual wear down of the teeth. It wouldn't matter how much power the car is making; more importantly, how much vibration the engine and drivetrain is putting on those spline teeth.
It makes complete sense why the 328xd is the most common of the xDrive vehicles to have it fail, followed by the 328xi. These engines simply generate more vibration.
Sitting at a light, idling with brakes pressed, the engine vibration is always hitting against the splines of the transfer case, the output flange not really secured to the shaft.
Prevention:
A common successful solution for many manufacturers is to use a compound on the splines to prevent vibrations and movement. Ferrari, Lotus, and BMW themselves use this method on splined shafts of axles to keep them from having play.
There is a spline shaft specific Loctite 660 that is probably the best for this:
https://f30.bimmerpost.com/forums/sh...8&postcount=37
I first cleaned the output flange and shaft with brake cleaner. I sprayed it on a rag and cleaned the shaft carefully without spraying inside the transfer case.
At around 5pm it was extremely hard to find Loctite 648/680/660 locally, so I used high temp Loctite 272 instead. This is rated at 3000psi of shear force. The full cure time is 24hr + depending on temperature.
Install the flange:
Clean excess, put o-ring back in, tap in the spacer, and lift the shaft up with flat screwdrivers, to fit the circlip:
Install to car:
Additional Prevention Step:
If your original flex disk (giubo) is aluminum with inserts (BMW 26117605629 , 105mm bolt circle to driveshaft, for cars before 07/2015)
You should update it to the superseding part number BMW 26117610061 (all rubber giubo, 105mm bolt circle, for cars before 07/2015)
4 hour install time, with my wife helping (best helper ever), on a lift, indoors away from the chilly weather. Started swapping it around 4:30-5pm, and we were packing up the tools at 9pm.
I drove it home gently as the Loctite cured some more, no issues.
I will clean up the old transfer case and take pictures of the not-fully worn splines, and of the signs of vibration on the spacer and circlip. Having caught it pre-failure and comparing to the brand new transfer case allows for quite a bit of insight to be gained.
** UPDATE 6/26/20 **
I've received some more insider info regarding this and the Service Bulletin SI B27 01 20 from BMW.
BMW does indeed blame the aluminum flex disc, the counter vibration onto the output flange, and the abrasive action of the worn flange rust speeding up the process of wear.
Interestingly, they didn't propose this SIB for the gas 320xi/328xi which would benefit as well.
The proposed fix to go to a rubber giubo and stopping the wear on the flange is still very relevant and valid.
There are no updates to the transfer case nor the output flange.
Loctite and rubber giubo should fix this one right up for its lifetime.
**