I spent probably way too long trying to understand how it all works. The gist of it is that you're definitely guaranteed to end up with more "fresh only" oil if you use the "hole in the pan" method over the cooler lines. But is that 1% better utilization, or 50%? No idea.

But I dug enough into it to find how the oil flows through the ZF 6HP transmissions. The below are some pictures I took, some I dub up online, some are even ripoff AliExpress parts (in some cases the best view I could find), but for the purposes of this explanation it doesn't matter.


First, all the oil that goes from the sump uptake into the oil pump goes back out into the mechatronic unit for further distribution. The holes at the bottom are from left to right: oil discharge (into mechatronic), suction (or "re-discharge", mechatronic doesn't want more oil at this time), and intake (where the plastic uptake from the cover plugs in):

In some of ZF's documentation it's ambiguous whether oil that's unwanted at the moment is dumped into the pan for re-uptake, or whether it's "spun" in place within the oil pump. The correct answer is the latter. That's good news for the "hole in a pan" method of flushing.

Now, the body of the transmission with the mechatronic removed looks like this:

You can see the intake port there in the bottom-left, then "suction" and discharge. What's even more interesting are the various other ports. From ZF's docs:



So in the picture above this one the "to cooler" is the elongated hole to the top-right of the oil ejection port (the large one with the rubber sleeve). So if all you want to do is evacuate the cooler with the mechatronic off you should be able to pump fresh fluid (or perhaps blow compressed air) into that port.

Interestingly the "lock up clutch release" port may allow for draining the fluid out of the torque converter. ZF's own documentation has this:

But it isn't elaborated on. I found some forum posts, e.g. this one speculating on this point, but nobody seems to have tried it and reported back. That's a compressed air gun above.

Anyway, once oil gets into the mechatronic it's distributed around the unit by various valves and ports, and oil going to the cooler is only one of those paths. I found the best illustration of the overall oil flow to be this picture:



That shows the mechatronic opened and turned upside-down. Critically because the oil discharge port (remember, discharging from the pump, this is oil going into the mechatronic) is now at the bottom that oil's drained out, so you can clearly see the path the oil's taking as it enters the mechatronic. It's the large passage going from top-left to mid-right in this photo.

The right side of this photo is where the solenoids are, so as they actuate the various gears get put into motion. And on the left side of this photo is where the fluid management (among other things) happens.

These two diagrams from ZF's documentation show that in detail. Now the unit's been turned around again, so the solenoids are on the left, fluid management on the right. The "WD-V", "WK-V" etc. is in reference to the schematic I posted earlier in this thread.



Anyway, I haven't been working up to some grand point all this time, except to say that it's clear that oil going in and out of the cooler is only one of many side-passages. There will be oil that goes directly into the mechatronic that just goes to solenoids and drips into the pan, up into the various gears and then drops down and splatters into the pan area etc.

I hope some of that rambling is useful to someone.

 

Anyone have an idea of which length the pan screws are? I'm looking to replace the fluid and wanted to have replacement bolts ready in case the existing ones are stripped. I've read on some BMW forums that have the same tranny that they're 30mm but also saw a merchant listing them at 20mm.

 

Buy a Zf kit the screws will be included

 

 

 

7 liter of zf lifeguard 6 oil , pan with filter and magnets included, new screws for the pan. That would be enough fir a simple oil change.
appreox 3 liters would stay in the torque converter.
you would need apprx 12 liters for a flush so sll the old oil is out.
The little blue box has different seals in it that are worth changing as well. Esp. On early models.
bridge seal, mecatronic sleeve, 4 ltlle tubes ... to prevent pressure loss.
I di oil change the simplest version 7 liters.
Made a huge difference when changed first time after 20 on my years xj6 2003 with 90 t miles tranny was behaving like a stuborn mule. Never had a change before
on my xj8 2003 228t miles it was still worth it . Had 2 known oilchanges . One at 80t miles. Second at 180t miles

 

Well, screws are steel; case is aluminum. if something going to get stripped it will be aluminum. No if's, but's or maybe's!
The replacement screws are longer, so they will grab more fresh thread.
It is really easy to strip threads in the transmission flange, and there are a lot of screws there.
I suggest to use torque limiting screwdriver and no power driven tools.
As a back up when good old Murphy rises for occasion. Have a this handy:

 

As an update to this thread see my post performing a transfusion on a 6HP28 X351 here 6hp28 transfusion

 

What usually strips is not the threads on the screw or in the transmission, but rather the small size Torx socket in the screw head. But with galvanic corrosion, the threads certainly could strip.

 

@Don B In my case the torx socket head was fine. I installed helicoils in two stripped holes. I have to mention, mechanic was there before me: the filling plug did not require a breaker bar to loosen it up. I was installing new screws with hand screwdriver to finger tight and then used small clicking torque wrench. Two screws got stripped way before click. It could be they were striped before I got there.
Not any more!

 

After following this fluid exchange method I thought I would clear transmission adaptions. When I hooked up SDD TCM adaption clear is a road test so some inherent risks of running laptop SDD on battery and vehicle without auxiliary power supply. Is there another way to clear adaptions? JTB00040 implies its a stationary method followed by a road test?

 

My recollection is that clearing the transmission adaptations can be done with the vehicle stationary. The adaptations should eventually reset on their own, but the process can be accelerated by driving the vehicle while monitoring the shift data onscreen in SDD and following its instructions. I think it has always taken me at least one hour to complete all of the adaptations, and usually longer.

 

No help on your question but when I need to drive and use SDD I got a 12 VDC adapter to plug the laptop into the car. As long as the engine is running you will have enough power. It was somewhat hard to get the right adapter as the factory one (Dell) was expensive so I got a cheap off brand one on EBay.
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It's difficult replacing the filler level plug whilst avoiding the hot exhaust pipe. Any heat proof gloves I own are also too thick and cumbersome.

I don't think it's been mentioned before but wrapping the pipe with a Plumber's Soldering mat tied in place with wire will protect your hand when getting the plug back in.

Plumber's Soldering mat: heat protection when inserting the filler plug. (hex wrench shows where the plug goes)

Hope that's helpful.