"Another one" *Dj K*

"These build journal entries are part of a Special Series on the DCT major service, updated GTS filling specifications and upgraded, oversized, DCT pans

In this entry, my original DCT transmission fails, and we swap in a loaner. Would it leak? Of course it did."
- Matt

Erratic behaviours.

As Summer '23 showed up, I started noticing rather weird behaviours from my DCT: it triggered no TCU (transmission control unit) error codes yet had unusual, rapidly degrading usability. 

The transmission would only operate at full tilt: WOT, S7 and redline. The clutches had no time to slip. 

I had these symptoms: 

The first gear engagement could, at times, feel like an SMG3 dumping the clutch at at a stop and go. A normal behaviour for V10 owners, but not on the M3 V8. 

The upshift from 2nd to 3rd gear in the mid range would slip the clutch in anything above S3. The usual torque bump of the DCT would cause noticeable slippage. 

The most weird and dangerous behaviour was downshifting at high speeds from 7th to 5th to pass. It would flat out not find the gear and over rev until the TCU gave up and killed power and let it bounce off idle, at 100mph / 160kph.

This was extremely frustrating: the lack of control at these speeds isn't a situation you ever want to be in for yourself and others on the road. 

I took the car to InnovAuto and went for a diagnostic ride with @tomtom64. He noticed it right away - it was super weird. The reel below shows 



The attempted fix. 

The DCT is not finicky: there is one common problem and its leaking fluids. It wrecks havoc on the behaviour of these transmissions as the clutches are operated with fluids. 

My DCT was dry, having been fully serviced and re-sealed. Fluid level was fine. We thus tried resetting adaptations as it hadn't been done in November '21 during its initial service.

The adaptations are ran with ISTA or equivalent workshop software. It's a rapid sequence that taps the clutch discs against one another. It feels abrupt - a bit funny.

Unfortunately it didn't solve the issue. 

The temporary solution. 

My M5 V10 was down, and the E46 M3 was still in pieces. Once more, my M3 V8 had to take the helm and be my daily driver. It needed to work as I had commitments and plans that required travel. 

Unknowing yet what the problem was, I bought a spare DCT from Michael @ MPartsSource in Brooklyn, NY. 

Phil @ InnovAuto had a DCT transmission laying around the shop that he imported from Miami a few years ago. He suggested to loan it until we diagnosed and repaired my DCT, or sourced another. 

The "loaner" had previously been used to test fit the GTS banjo bolt. 

My original transmission was dropped in an hour or two. Exhaust, heat shields and the carbon driveshaft came off.

My original DCT was bone dry, showing no signs of leaks after its complete servicing in November '21.

We opened up the original DCT's side pan to inspect the mechatronics. It was a potential culprit that got eliminated later on.

The temporary DCT wasn't exactly a pristine example. It had mileage, and had obviously been leaking before being parted from its original chassis. 

The loaner's re-refit.

The TCUs of each DCT is VIN coded to our chassis. They require ISTA to be tied to your DME.

We knew this, but there's something we didn't know. 

The temporary DCT was from an '08 E9x M3. The early generation DCTs had a slightly different wiring harness than the later models. They missed a single wire that acted as a ground on the later models. 

Unfortunately we only found that out after putting it back on the car and trying to start her up. It had to come out to modify the harness. 

When it rain, it pours. Removing the transmission tore off one of the coolant hose. We replaced that too. 

With the harness adapted, and the hose changed, the loaner transmission was put back in the chassis. 

A test driver later [...] it got worse. 

I came back the next morning to pick up the car, excited about having a working DCT. The guys made me aware the 1st gear engagement was jittery, but everything else seemed fine.

I went on a quick test drive with redline shifts - things didn't go very well. The 2nd to gear shift triggered codes and fluid temperature symbols. 1st gear downshifts would whine.

The car was only drivable in D mode. I had to make do with it as I needed the car to travel. 

So, what's the issue? Take a wild guess.

Up next: we figure it out, finally - on both transmissions. 


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