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Discussion Starter #1 (Edited)
I think this DIY requires relatively little skill and tools, but it is time consuming. It took me about 6 hours over 2 days, but I took my time and tried to enjoy the process. Total cost was $201.97, plus another $112.98 in tools. Considering a dealer charges over $1,000 for this work and often does a poor job, this is a great value DIY. If you’re going to follow this guide, check your data card to make sure you have the 7G-TRONIC PLUS transmission, also known as the 722.9, or the NAG 2. My apologies to @Mountainaire for this similar DIY post. Procedure is essentially the same but with a little more detail added.

List of Materials

Mercedes 722.9 Transmission Service Kit - $125.68

Automatic Transmission Fluid Pentosin 134 FE (5 Liter) – $50.96

½” ID Vinyl Tubing - $9.99

Mityvac MVA588 ATF Refill Adapter $15.34

Etekcity Lasergrip 800 Digital Infrared Thermometer Laser Temperature Gun $22.99

Motive Products- PowerfillPro and Power Extractor- 2 Gallon (1745) $89.99


I did extensive research for the job before I started and read the WIS manual procedure over and over. I found a really good YouTube video that shows the full procedure for the 722.9 transmission, although not for an SLK:
This video can pretty much be used for a step-by-step guide, but I will post a few clarifications with pics of my experience below.

1. This project can be done with a jack and jack stands, but do not attempt it unless you know how to jack up all 4 corners of your car safely. Here is a pic of my setup.

2.When installing the gasket on the transmission pan, the YouTuber lubricates it with a silicone lube. The WIS manual says specifically not to do this, so don’t.

3.Since there’s no dipstick, the fluid level is set by running the car until the trans fluid reaches 45 degrees Celsius. The WIS manual stresses the importance of this because more fluid will drain out of the overflow pipe as the temperature rises. You could end up a liter short if you bring it to full operating temperature, or a liter too much if the fluid is cold. My OBD scan tool with the Torque app does not show trans temp, so instead I used a $20 infrared temperature gun (link above) after seeing this technique used by many others. I found the best place to get an accurate reading is right where the refill adapter screws into the pan. See pic here.

4. Only 4 liters of fluid drained out through my transmission pan plug, and since the total capacity of the system is 9 liters, a simple drain and fill doesn’t do much good. I’ve read that some Mercedes dealers will only go this far during a transmission service. The YouTube video was short on details for hooking up the tubing to the fluid return line, so here is a few pics to help out:

Before removing the return line, two heat shields also need to be removed, held on by E12 bolts.

It also helps to remove the first two small support clips that hold the line close to the transmission. Also held by E12 bolts.

The return line is connected to the transmission with a single E12 bolt. It can be removed with the same E12 socket that is used to remove the transmission pan.

I was originally intimidated by the part of procedure that involves starting the car to pump out the old fluid, but I found it to be really easy (with the help of my lovely non-greasy wife). My original plan was to do a drain and fill, but I couldn’t access the drain plug in the torque converter even after I hand cranked the engine (another thing I initially found scary) to get it to line up with the access hole.

Just for another point of reference, here is another decent video on doing a simple drain and fill on a 722.9 transmission. He does a good job of explaining how to torque the bolts on the fluid pan. The torque on these bolts is 4 Nm PLUS another 180 degree turn, and remember these bolts cannot be reused. The torque on the drain plug to the oil pan is 22 Nm.


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