So after dealing with an RPM fluctuation for a few thousand miles I decided to try a transmission fluid change. The original fluid had been in since new as I recently got the vehicle at 123k.

I followed the procedure for a normal drain and fill. I got out approx 3L of foul smelling dark fluid and the 3L of fresh didn't make a difference on the RPM fluctuation.

I wasn't happy with the dilution and wanted a full exchange. Also wanted a new filter. Some have gone with a metal pan with replacement filter but I chose to replace the original ZF pan with another with built in filter. I then proceeded to completely exchange my fluid and I'm happy to report that the RPM fluctuation is gone!

I'll share my set up for the exchange below.

 

Materials:
1) 10ft 1/2 inch inside diameter hose cut in half
2) two hose clamps
3) container with markings by the litter
4) large funnel
5) m18 1.5 fitting with hose barbs
6) 11-12L of clean fluid

 

1. Get the vehicle level
2. Drain what you can from the pan and replace drain plug. (Not needed if pan just changed as in my case)
3. Disconnect the passenger side transmission cooler outlet infront of the front axle and slide on one hose and clamp tight. Put the other end in the container and tape down so the pressure doesn't throw it around. (May need to drop skid plate, mine was already off)
4. Remove fill plug and thread in the fitting then attach the other hose and clamp tight. Attach funnel to the other end and fasten up high on something.
5. Refill with same amount that you drained out earlier from the pan.
6. Have a helper start engine and run through the gears while you refill with the amount that is coming out from the transmission cooler. (May need to stop engine if you can't keep up with the fill)

 

7. Once coming out clean or you run out of fresh fluid (hold 1-2L back, to top off later) you can reconnect the cooler line up front.
8. Now fill till fluid runs back out the fill tube. This is easy and clean with the fitting threaded in and hose connected since all you need to do is lower the hose into a clean container and if full the fluid will come back out the hose.
9. Lift hose/funnel back up and run engine. Repeat number 8 now while engine running up to temp (look elsewhere for the temperature, I just estimated).
10. Leave engine running and thread out the hose and fitting. Wear gloves, that exhaust is hot!
11. Replace fill plug
12. Shut off engine
13. Feel good knowing you didn't just dilute the garbage that was in there

 

There's a good discussion on this process here: https://www.disco3.co.uk/forum/robbi...ge-141425.html

It's on my list of jobs over Christmas.