This site has given me quite a lot of pointers so I figure its good to pay back too, I did an atf flush on a '13 GLK350 (Blue ATF) and as we all know the 7 spd 722.9 (of any era) is an overflow type filler system which requires the fluid to be at a specific temperature.

I did a small test on my phone app torque, an IR gun was showing close to 8 degrees celcius short on temperature, the multimeter and temperature probe is too fiddly and was also showing 4 degrees short when taped to the pan (its messy to dip in the oil) - I was comparing it with an autel maxicheck, I started fiddling around on my old Torque Pro app on my old andriod phone and it was showing perfect ATF temperature with the following,

You need to add a custom PID to it as under.

PID: 2130
Min: 0.0
Max: 200.0
Scaling x 1
Device type: Celcius
Equation: L-50
OBD header: 7E1

I found it with help of another freind of mine who is a bit more into electronics.

The maximum allowed check temperature is 45C, so best start the check at 39C and pump half a litre of ATF (usual garden sprayer, 3/8 clear hose and fill adapter in pan), remove the fill hose and observe it draining out, if it is then unscrew the adapter and install the drain bolt, if its not then pump some atf in and recheck, ideally pack it up before it goes beyond 45C.

tips:
use a spirit level on the pan to ensure it is straight.
before installing new bolts, run a thread chasing tap to make sure the holes are squeaky clean
anneal the copper washer on the pan drain bolt for best seal.
The torque converter drain is tiny and will take ages.
the exhausts are hot when testing the fluid level.

notes:
if you would like to flush the cooler, its very easy as Mercedes Benz use General Motors cooler line fittings on these later cars, you can buy them at an autoparts store for cheap or just cut them from some GM car in a scrapyard, then use the cooler cleaning product with a hose and flush the cooler backwards (bottom to top). You can also use this same area to flush the transmission if you are doing a multiple flush job (3 fills etc) - the max you can get out per starting cycle is 3 litres as its the pan capacity where transmission filter is, the pan is deeper but that is designed such to keep debris at the lower parts of the pan (thats what the big dimples in the pan are for, they support the filter in raised position)

If you have an android phone, the torque app is 5 dollars, the bluetooth dongle is about 20 from amazon - and the PID entry is free, so for 25 dollars you get a pretty decent instrument to test the ATF temperature and it sits right next to you when you are under the car, The filler adapter can be had on ebay or amazon for 10 or 15 dollars, while a garden sprayer with gallon capacity is less than 10 dollars - 2 gallon one is not more than 15 dollars. 134FE ATF is about 85 to 95 dollars for a case of 12 litres (depending on where you live and what supplier you contact)

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)