DIY: PDK clutch fluid and gear oil change without using PIWIS fill mode

This is a DIY on how to replace the fluids in the 981 Boxster/Cayman PDK transmission without using the PIWIS fill mode. Difficulty level: Similar to an engine oil change.

Before I start, I can confirm that the procedure I’ve put below works. I’ve compared the fill level I’ve achieved using this method to using PIWIS fill mode. They are identical as long as you are only draining and filling. I’m confident it would be the same if you are replacing the pan and filter. But if you are doing any further work, maybe dropping the valve body or replacing the clutch pack, you will need to use PIWIS fill mode.


Video of the fluid change process



Video explaining some of the procedure and comparing fluid level using PIWIS fill mode.



I’ve put some general information prior to getting to the DIY below. Sorry for all the words. I’m just trying to get all that is in my head down on so paper that people will be comfortable with the process. TL;DR, go to the bottom for the DIY.

I understand that people will be reluctant to touch something that seems on the surface to be complicated, and if performed incorrectly, extremely expensive. Hopefully what I’ve written below will clear up some of the mystique and give you the confidence to do the DIY. Standard caveat: you are doing this at your own risk.

The first bunch of information is about the clutch fluid change. Further below is some info on the gear oil change.

This procedure would be the same for a 911, but the transmission is a mirror image of the Boxster/Cayman due to engine/transmission orientation being reversed.

The information below has come from a number of sources. I’ve spoken to a number of Porsche techs and attempted to suck their brains of all the information I can. I personally have worked reasonably extensively with ZF automatics for a number of years. I’ve stripped and rebuilt transmissions, rebuilt valve bodies, etc, and feel very comfortable with what is going on inside. I’ve done the same, but to a lesser extent the same with manual transmissions.

A double clutch transmission is a combination of both an automatic and manual gearbox. Whilst the gears/diff are close to identical to a manual, and use a standard hypoid gear fluid, the clutch and gear change control is pretty much identical to an automatic. This part of the transmission uses a very low viscosity, highly specialized fluid that is similar to ATF.

Gear oil is any fully synthetic GL5 75W-90 gear oil.
Clutch fluid is a double clutch transmission automatic transmission fluid. Do an internet search for 'DCT ATF' for examples. Many fluid manufacturers now are making these.

Clutch fluid replacement information

Clutch fluid replacement is close to identical to that for an automatic, and much of the discussion below is for this.

See the image below. It’s a simple diagram of what is going on in the control part of the transmission. It’s not exactly how things are laid out in the transmission, but it shows what flow is happening.




Simply:
- There is a big bath of oil at the bottom of the transmission in the pan. This is full of fluid. The filter sits in the fluid with the input close to the bottom of the pan. There are some super strong magnets that sit in the pan that grab any ferrous particles circulating in the fluid.

- The filter is not like an engine oil filter. They are made from a strong fibrous material that never needs replacing. It’s not designed to catch small particles, only big stuff. Small ferrous particles are caught by the magnets. Other particles, like from the clutch frictions disks, continue to circulate in the fluid. This is all the crap you see when you drain the fluid.

- Fluid is sucked from the pan by the pump and sent to the valve body. The pump is always pumping if the engine is turning. The valve body is the brain. It controls everything. It’s full of small aluminium pistons, springs, valves, etc. At the end is has the solenoids that control where the fluid flows. These solenoids are controlled by the transmission control module.

- The valve body controls fluid flow to the clutches, shift actuators, etc. If you ever have the opportunity to look at a valve body when opened, do so. It’s a wonder of modern engineering. What is clear when you look at one is how finely machined they are.


Pan and filter layout



Keeping good oil in this system is critical. Poor oil will degrade valve body operation, wear the valve body bores and seals, and degrade clutch engagement. If the valve body isn’t operating correctly, everything downstream will deteriorate. Also the fluid is full of additives such as anti-shudder compounds that are essential to smooth operation.

At the risk of boring people, I’ll give an example of how regular changing of fluid is good. I recently sold a 2004 4.2L Audi Allroad. We purchased the car at 117,000 km at only 4 years old. Soon after purchase, the torque converter lockup clutch showed signs of imminent failure. Bad shuddering on acceleration (torque converter lockup clutch slipping and shuddering), and the transmission was shifting poorly. I immediately changed the transmission fluid, and all the symptoms disappeared. The only thing was the lockup clutch activation was slightly jerky when cold, but nothing that the average Joe would notice. From then I changed the fluid every 10,000 km, at the same time I did an engine oil change.

At 225,000 km the transmission was performing identically to how it did after the initial fluid change. I needed to remove the engine a few years ago to fix some known manufacturing issues. The transmission was out, so I took the opportunity to rebuild it, and I wanted to install a rebuilt torque converter. When I pulled it apart, it looked like new inside. The friction disks in the clutch packs still had the writing on the contact face that is printed during manufacture. I was utterly amazed. The disks had thousands upon thousands of actuations, and the writing on the friction disks where they contact the adjacent steel disk was easily visible.

I’m completely convinced of the value of fluid changes. Good fluid makes valve bodies work correctly so the correct pressures are sent to the components. Also, good fluid is essential to get the correct interaction between disks in the clutch packs.

Back to replacing the fluid

Refer to the following images of where things are on the transmission.








The pump I use


I’ve tried lots of different pumps over the years, and the one pictured above is what I use now. They work really well, are cheap, and they are very easy to clean so they are spotless for the next job. To clean you don’t have to disassemble. Just pump solvent through it (I use fuel), then get some compressed air and blow into the intake hose. The two one-way valves inside are orientated so that the air flows straight through, removing and evaporating any remaining solvent. The only problem with these pumps is the hoses tend to pull out of the housing easily. I’ve secured my hoses so they can’t be removed.

When filling with fluid, you need the engine running (this means the pump is pumping), and the rest of the components in the transmission full of fluid (clutch pistons, actuators, etc). Then you fill so it’s just dripping out of the fill hole. If you turn the engine off, and hence the pump, some oil that is circulating around the transmission falls to the pan, which then causes it to overflow out of the fill hole. This is why the engine needs to be running when doing the final filling.

Also, the oil needs to be at the correct temperature, about 40 deg C. How important is this you ask? Some will say that it’s critical. It’s actually not. The difference in density between 40 and 60 deg C is tiny. Having talked to the manufacturer about this, and done experiments to measure the difference, it's negligible. Certainly not enough to make any difference to the operation of the transmission if not perfect when doing the fill. Having a diagnostic readout of fluid temp is not required. If you grab the clutch fluid cooler supply pipe (the one closest to you on the left side of the transmission) and it feels gently warm, that’s perfect. 30 deg feels cold. 50 deg feels hot. 60 deg is nearly too hot to hold onto. It’s amazing how sensitive your hand is to temperature change. The problem with this transmission is that the pan is plastic. Metal pans are much better like this. I used to always change transmission fluid using the diagnostic readout, but after a while I realised it was a waste of time. I could easily get it bang on just by feeling the pan.

How important is the fluid level? Well it depends how you drive it. The transmission can’t risk oil starvation at the bottom of the pan at the filter intake. Sucking air means immediate loss of pressure, and immediate clutch slip or possible gear disengagement. Super bad. This type of car might be doing a hill climb at 30 deg nose up and max acceleration, soon followed by max cornering force, then 30 nose down and max engine braking, or any crazy combination of these. The fluid is sloshing all over the place. This needs to be catered for. If you are driving like this, yes, the fluid level needs to be dead on. If you are driving like a human, 100 ml here or there will make no difference at all. Like a standard automatic, slippage only occurs when the fluid level gets very low due to the input of the filter being near the base of the pan.

Do I replace the filter? No. I used to do this all the time on automatics, but after pulling them apart and seeing they were like new inside I didn’t bother anymore. Especially a transmission like this with a pan that has aluminium torque to yield screws etc, and you need to remove the brace below the pan (wheel re-alignment afterwards). The juice to squeeze ratio is way too low for my liking. Also, the workshop manual doesn’t mention this when doing a fluid change. Yes, a bit of oil will remain in the pan if you simply drain. But regular changing of fluid will keep what’s inside perfectly fresh. By comparison, when you do an engine oil change, you only replace 7.5L of the 10L that is in there.

Regarding filling procedures that are in workshop manuals, they have to create a procedure that will work in any situation. Hot, cold, new transmission, transmission previously full of fluid. This is why PIWIS fill mode exists.

From the research I’ve done, fill mode does the following. It turns off the coolant flow to the clutch fluid heat exchanger (to stop it from heating up to quickly), and it does something to the solenoids so the clutch pistons and shift actuators get filled with fluid. Having used fill mode, you hear it clunking about as it’s doing this.

But here’s the thing, if you are simply draining the fluid and filling, all the clutches and actuators are already full of fluid. Even if you stop the engine, the valve body holds the fluid in. The only way to get the fluid out of here is to unbolt the valve body. Even if you drop the pan and filter, the valve body still holds all the fluid inside everything downstream of it.

Filling a transmission that has just had a drain is completely different to one that was filled from empty. The procedure described in the workshop manual is the same for both. Filling a transmission that was previously empty has the thing burping and farting all the air out. Also, if there is air where their needs to be fluid, the transmission can do some weird things until everything is full.

So why is this important? Because if you are simply doing a drain and fill, fill mode isn’t required to fill the stuff downstream of the valve body, as it’s already full of fluid.

Also, turning off the cooling (actually warming) of the clutch fluid isn’t that necessary to have the temperature correct. If you start the engine from cold and get under the car, you can feel the clutch fluid temperature by grabbing the output from the transmission to the clutch fluid heat exchanger. I’ve also used an infrared thermometer on the top of the pan to check this is a good indication of the clutch fluid temperature. Grabbing the pipe works perfectly. I would suggest to people getting under the car and doing this so they are comfortable with what is going on prior to doing a fluid change.

When you start the engine from cold, it is clear the rapidly warming coolant is directed to the clutch fluid heat exchanger to warm the fluid. It doesn’t happen so fast that you can’t do the final filling prior to it getting too warm. It probably takes about 5 mins to get to about 50 deg if the start temperature is 25 deg. Loads of time to pump in the last bit of fluid so it’s full well before it gets to 40 deg.

When I did the fluid change on my transmission, I was pretty clinical about measuring what came out and what I put back in. 3L came out and 3L went back in.

The process I used was

- Prior to drain, I removed the fill plug with the engine going and the transmission at 40C to see what the level from the factory was. It was just at the bottom of the fill hole.

- Drain. I left it overnight on the lift to drain as much as possible, and also to cool for the change the next morning. Installed drain plug the next morning. Outside temp 15C. Pan temp 20C.

- Engine off, filled until dripping out (2.5L fluid went in). Pump remains in fill hole.

- Start engine, go back to fill hole and pump until fluid dribbling out. Another 0.5 L went in prior to dribbling out.

- Select R, D then P, a few sec in each. UPDATE: I don’t do this anymore. If you are draining and filling only as per this DIY, selecting the gears makes no difference to level.

- Go back to fill hole and check. It was still dribbling out.

- Removed pump hose from fill hole. Due to the size of my pump hose being only slightly smaller than the fill hole, when I removed the hose, fluid was coming out at a good rate. I estimated the fluid level to be about 5mm higher than the bottom of the fill hole. I installed and torqued the fill plug at this time. Looking inside the chamber where the fluid goes in, it’s quite small, and the difference in level I estimated to be a relatively small volume of over fill.

- Fluid temp still about 25C. It was about 2 minutes after starting the engine where I had the fill plug re-installed.

- I fitted the wheel, lowered the car, and took for a very short drive. Gently drove up to about 70 kph to ensure gears all the way to 7th were engaged. R was used getting out of shed. All shifts and clutch uptake perfect. I then returned home. After car lifted, I started engine again, removed fill plug and checked level. It was now just dribbling out. Temp of oil in pan was 35 C using an IR thermometer. Fill plug re-installed.

This drive and re-check was done after a discussion with a Porsche tech who explained to me what the fill mode was doing, and without using fill mode, what could be done to ensure all the actuators were full when checking the level. The difference in level between before and after the drive I estimated to be about 50ml. Clearly there was some small volume in the transmission somewhere that needed to be filled via gear engagement. Maybe it was just air bubbles settling out. Who knows.

For most people, lifting the car again is going to be a PITA. I am fortunate to have a lift in my shed, so lifting again was a no brainer. Knowing what I do now, I don’t plan to do a re-check when I do this in the future. I’ll just fill so it’s flowing out with a little bit of overfill and just leave it like that.

Results after the change. Shifts are the same (beautiful), but clutch uptake in R and 1st seems to be smoother, especially at very slow speeds where there is deliberate slip. I actually didn’t expect any change. My car has only 50,000 km and has never lived in the city.

UPDATE: I have now completed the clutch fluid change twice. Each time you only get about half of the fluid out so I did it twice, with some driving between, to get about 75% new fluid in there. The first time I checked the fluid later by driving a bit then lifting again and checking. The second time I didn’t bother because I didn’t need to top up after doing this when doing the first fluid replacement.
Today I checked using PIWIS oil fill mode. It’s been about 500 km since the last fill, and using PIWIS fill mode the level was perfect from last time. Also, selecting different gears when filling made no difference to the level, so I wouldn’t bother doing this.

Gear oil replacement information

With regard to the gear oil change, the only thing that is different to a normal manual gearbox is the oil cooler. This oil cooler has a small pump that is attached to the rear of the internal shaft that runs forward to the diff. Oil only gets circulated when the wheels are turning.

When draining, you need to release the main plug at the back as well as the small plug under the diff. The oil is in the same chamber, but clearly there are two low points from where you need to drain. Filling is via a single fill hole.

When you drain the gear oil from the rear drain hole, initially it will drain quickly, but then over the next ten minutes, more and more dribbles out. This is the oil cooler slowly emptying. When you initially fill, the oil cooler is still empty, and needs to be filled. You do this by putting the transmission into 1st at idle (when up on the lift), and let the wheels turn slowly for two minutes to turn the pump and ensure the oil cooler is full. Then you stop the engine and top up the oil.

If you are going to replace both fluids, do the clutch fluid replacement first, then the gear oil. This is because you don’t want the clutch fluid cavity to get too warm prior to replacement.

Clutch fluid replacement DIY

1. Lift car to get access underneath. It needs to be level. Remove rear left wheel. Engine needs to be cold.

2. Use masking tape to cover the gap between the transmission housing and pan on left (image above). Clean with degreaser prior to taping so it sticks. This covers the gaps between the pan and casing so they don’t fill with oil.

3. Loosen fill plug on left side. When loose, screw it back in finger tight. You want the check you can remove this before draining the pan.

4. Remove drain plug and drain fluid from pan. Once drained install plug. 15Nm. Be careful. It’s an aluminium plug and the T30 torx fitting is easily stripped if you use a poor tool.

5. Remove fill plug. Use pump to fill until it dribbles out of fill hole. Leave pump connected so you can do the final fill easily.

6. Start engine.

7. Fill with pump until dribbling out. Should need just a bit more.

8. Hand on heat exchanger supply pipe. When temperature is gently warm (approx. 40C). Remove pump hose. Ensure that fluid level is a good solid dribble.

9. Install fill plug. 27 Nm.

10. Turn off engine.

11. Clean pan and strut brace under transmission.



Gear oil replacement DIY

1. Lift car like above.

2. Remove main gear oil drain plug. Allow to drain for 10 mins so the heat exchanger empties. Install 27 Nm.

3. Remove diff drain plug and drain oil. Install 10 Nm.

4. Remove fill plug. Use pump to fill until dribbling out. Install plug finger tight.

5. Get in car. Close door and start engine. Put into manual gear change 1st. Brake and park brake off. Allow wheels to slowly turn at idle in 1st for 2 minutes. Keep door closed. You will get warnings on the MFD if not.

6. Brakes on. Select P. Engine off.

7. Fill with pump until dribbling out. It will take a few more pumps due to the heat exchanger now being full of fluid.

8. Install fill plug. 27 Nm.

9. Clean pan and strut brace under transmission.