BMW F30 Transfer Case Replacement

Two weeks ago, I determined that my F31 had a failing transfer case. The F30 generation of the BMW 3-series was available with model years from 2012 to 2019. All non-M3 models of this era were available with BMW’s all-wheel drive system: X-Drive. X-Drive is a rear-biased system that uses a transfer case to get power to the front wheels. Most X-Drive BMWs use a transfer case called the ATC 35L, and there have been a few mid-mileage cars that had transfer case issues. 

Below are the symptoms and diagnosis of the issue, along with the remedy and results.

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Symptoms:

At about 55,000 miles, I noticed a slight hesitation while accelerating. I have felt something similar to this on previous cars (BMW E30, VW Golf R) so I started to try to hone in the symptoms to figure out what was wrong. A couple of key symptom observations:

  • The hesitation happened at varying places across the rev range. 
  • There were no codes, and nothing showed up in a scan.
  • It was more prevalent when going up a hill, or passing. Driving this car, 95% of the time the hesitation wasn't felt.
  • It was not as strong as a spark plug or ignition coil fault, the cutouts were more gentle and the car clearly powered through it.
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Diagnosis:

With past cars, a hesitation was the result of something like cracked/faulty ignition coils, plugs that needed replacing, or even an AFM that was unresponsive. I looked at the AFR using a bluetooth OBDII tool and the Torque app, but everything looked good. With 50k+ miles on the car at the time, I first replaced the plugs. The plugs that came out of the car all looked really good - no fuel fouling, no white charring, just looked like lightly used plugs that didn’t need to be replaced. The coils also had no visible signs of fatigue.

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After it was determined that the spark plugs were not the source of the issue, I changed ignition coils. Unfortunately, the symptoms persisted.

Finally, I had to enlist the help of my new friend John Tovaas at Black Forest Motors in Bozeman, MT to check for a transfer case fault. John drove with me to verify what I was experiencing. He inspected the plug/coil work that I had done and said it looked good. Next, the power for the transfer case control module was pulled, which effectively put the car into rear wheel drive mode. And…. the car drove perfectly smooth. No hesitation. A faulty transfer case had been confirmed. This is such a quick easy diagnosis, I recommend starting here if you have similar symptoms.

Remedy:

The transfer case is not considered serviceable. There is no separate drain plug for the fluid, and parts to rebuild the ATC 35L are scarce. A replacement transfer case is required. Fortunately, units are available. Black Forest Motors did the transfer case swap (pictures below).

A couple of interesting tidbits about the install:

  1. Black Forest Motors is a one man shop. The exhaust needs to come off as part of the procedure, and John from BFM has a great procedure to do it by himself. He attached ratchet straps to the front and rear, unbolted the exhaust, and then he lowered it down using the ratchet straps until the exhaust was out of the way.
  2. The book time for the transfer case swap is just under 5 hours.
  3. The primary transfer case bushing was replaced at the time. Removing the old bushing is hard to do without damaging it, so using a new one is good practice.
  4. The original control module was reused as it was in great shape, but an adaptation procedure must be initiated so that the control module knows to re-learn the behavior of the new hardware. 
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This was the first time I have had the car on a lift. It is remarkably clean underneath (aside from road grime). It was also nice to see behind the heat shields and braces.

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Results:

The first thing I noticed during my validation drive was the….. steering. Odd, right? The car is a model year 2014, and in order to run the transfer case adaptation, all of the control modules needed to be updated. In my research, I discovered that around the time of the mid-cycle refresh of the F30 generation (BMW calls this the Life Cycle Impulse, or LCI), BMW addressed complaints that the electric steering lacked feel and feedback by updating the steering module parameters. So, with a 2019 steering update loaded on the car, things felt very different and much improved. 

The next thing I noticed - smooth, direct power. To evaluate whether or not the transfer case really was the issue, I first had to acknowledge my subjective nature; I’m the kind of guy who can wash a car and think it drives better. I had to push myself to be objective. To try to tease the former hesitation, I did several mid-throttle and wide-open throttle runs through the gears. Without a doubt, the F31 feels smoother and more connected. The hesitation is completely gone.

Additional Notes:

  • The transfer case is a remanufactured unit. New units are generally not available. It has a two year, 24,000 mile warranty.
  • A new transmission bearing set / bushing was installed.
  • The transfer case control module was in great shape and was reused.
  • Running the control module adaptation reset was missed the first go around. You could tell immediately that the existing control module was accustomed to commanding a failing transfer case. Once the existing control module was connected to a fresh transfer case, it was being too aggressive. In a full lock turn out of the garage, you could almost feel driveline binding. Once the control module was asked to “relearn” the transfer case it was connected to it went back to feeling like a brand new car.
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