Back in November 2021, I started noticing my E92 M3 DCT transmission was leaking from all over the place. This Build Journal entry details what we found out and how we fixed it. This entry is applicable to most if not all BMW DCT transmissions.
The DCT transmission is a marvel of engineering and remains to this day the best automated transmission BMW M ever fitted to a car.
G8X guys, my bad - ZF 8 speeds aren't all that, and SMG3 owners need to take a pause and be rational about what we got with the S54 and S85.
BMW didn't plan for this.
The DCT transmission are notoriously known for being designed as Lifetime Fluid by BMW M. That may very well be accurate for street use and under normal use.
Unfortunately for us, BMW didn't expect its DCT transmissions to leak as we now know today.
This is further reflected in BMW's own notes on its Technical Service Bulletin on the DCT.
They specifically mention to be very selective in DCT fluid maintenance as the Genuine BMW Pentosin DCTF-1 (83220440214) fluid supply and availability is scarce, and extremely expensive at close to 40$ per liters.
The entire DCT system is filled with up to 7.8 liters of it, some say it's 9 liters. It isn't clear.
Fortunately for us, we have permanent fixes to most of the DCT leaking issues and access to much more affordable DCT compatible fluids.
I sort of planned for this.
I was planning the TTV Racing S65 DCT Lightweight Flywheel and decided to combine with a complete DCT transmission service. It still came out to be an expensive while you're in there maintenance.
I say sort of as I was initially expected only the main pans to be a leaking problem. I had a MDCT Motorsport Main Oil Pan and Side Pan ready to go.
...but not like this:
It was leaking from everywhere: top, side & bottom.
Spot 1: The Mechatronic O-Ring
The main oil pan is often the easy target to point fingers at for leaking DCTs and it is where oil will obviously accumulate.
My experience was different.
The Mechatronic O-Ring was worn and obviously leaking all over the transmission.
You can see the accumulation of oil and dirt at the top of the transmission.
Spot 2: The Side Pan
The OEM side pan is made from aluminum, and has less tendencies to leak versus the OEM plastic main oil pan which tends to warp and leak.
Yet it still can, and it should be planned for when pulling the transmission.
The OEM aluminum side pan still use a rubber gasket which tends to dry out over time. The side pans covers the electronics of the DCT which bath in transmission fluid.
Spot 3: The Main Pan
The obvious culprit was also leaking on my transmission. This is another case of BMW using plastic where it should have used another material.
The S65 and S85 and both riddled with such cost cutting measures: throttle actuators, VANOS covers, coolant bleeder screw, power steering fluid covers, etc.
It's still just a BMW, even if it has an M badge.
The revised plan.
As we clean and analyzed the leaks to better understand where they originated, we decided to go all in and perform a total service:
- Genuine BMW Suction Cartridge Filter (SKU: 28107842828)
- Genuine BMW Side Filter (SKU: 28107842840)
- Mechatronic O-Ring (SKU: 28607849555)
- New Coolant Hose (SKU: 17222283594)
- New Coolant Hose O-Rings (2x) (SKU:17222245358)
- 9x liters of LiquiMoly 8100 Series DCT fluid
We had to change the coolant hose as it was appeared to be slightly leaking and we didn't want to take any chances.
The Installation Process
Mission Critical Torque Specs
Stop here, and go look up the torque specs of every bolt inside the DCT transmission and its pan.
You don't want to be that guy and overtorqued and snap the head of a bolt.
Don't be like Matt on this one :)
The Side Filter Swap.
The filter located on the side of the transmission filter is held with a C clip. you'll need the proper tools to get it out and to re-align it to the enclore once you replace that filter.
The Main Suction Filter Swap.
The main suction filter is behind the main oil pan. You remove to remove 2x screws to get it out, along with the pick up tube if memory services me right.
Be mindful to not thread 'em.
Swap and replace.
The Mechatronic O-Ring
The O-Ring is easily removed by sliding out the connector. No need to undo the bolt if memory serves me right.
The Main & Side Pan
This is relatively straight forward.
The Oil Pick Up Tube Extender
The main challenge is fitting the oil pick up tube extender - you want to quadruple check your fitment to the tube to ensure it won't unclip.
Here's how it should sit when it's all well and clipped in.
Be mindful that you are fitting an aluminum extender to plastic fittings. Don't be too harsh.
Bolt the pick up tube back to the transmission directly.
I'm sure you've been wondering what the hell that black circle thingy is at the bottom of your OEM pan.
It's a magnet!
Unscrew and transfer the magnet from the OEM main oil pan to your new main pan.
A reminder to check your OEM torque specs.
The Gasket Maker
You do this step last once you have quadruple checked all your torque specs.
Be mindful of the instructions regarding application and time to dry before it glues properly.
You will need to time the torquing of your bolts within that timeframe.
Fitting the GTS style Banjo Bolt
The GTS style Banjo Bolt takes place for the main fill plug on the right side of your DCT.
Look for the main fill plug by the lower left of the side pan, near the green sticker.
The banjo bolt is made to allow overfill of the main oil pan as the fill point sits higher than your OEM fill plug.
It comes with a few washers that act as spacers to clear the transmission casing design. You torque the blue bolt to the casing per OEM specs.
The Filling Process
I assume you've fitted the transmission back on, haven't filled any transmission fluid off the car, properly bolted and triple checked your torque specs of the transmission itself and properly refitted all coolant lines.
Congratulations, you're nearly there.
The official BMW process.
BMW officially recommends this to be accomplished using the dealership GT1 tools.
My understanding is this tool allows the tech to monitor the transmission temperatures as they deem it critical to precisely measure the fill rate of the transmission.
The reason is thermodynamics.
The process we used.
You will need a few simple tools to perform the fill process when doing it the DIY way.
- Fluid pump
- Fluid thermometer
- Minimum of 9x liters of DCT fluid
Before you begin the process, make sure the drain plug on your new pan is properly torqued.
Step 1: Fill her up cold.
Using the pump, fill up the pan with the engine off until it drips from the Banjo bolt fill plug.
The fluid must be below 90F.
Step 2: Heat her up and fill her up hot.
Start her up, maintain 2,000 RPM for 1 minute.
Using the pump, fill up the pan with the engine off until it drips again from the Banjo bolt fill plug.
The fluid must be below 104F.
Notes on fluid quantity.
We had 9 liters of LiquidMoly 8100 Series DCT fluid ready to go. This beyond what is usually needed as we needed to account for 2 things:
- We overfilled from OEM specs by 1.8 liters.
- We needed to account for fluid loss from the coolant hose change.
The LiquiMoly fluid is inexpensive enough and readily availably for you to overstock on it.
Better safe than sorry.
Reassemble, test drive and re-check torque specs & any leaks.
No coding or adaptations are needed.
If it all checks out, you're good to go.
That's it, enjoy!