I've read lots of people over the years mentioning how difficult it is to get an ATF level reading at 80C...that as soon as you pull over to measure the level the temp starts to drop. So, after I changed the fluid on the CDI today, I was playing around with my scanner which reads the ATF temp. I drove around town and was able to get it up to 65C or so....then it would fluctuate in the 60-65 range. But when I pulled into my driveway and let the car idle, I was surprised to see the temp actually rise to well above 80.

Anyway, glad to get precise readings on the CDI....for the 98 and the 91 I will continue to do ambient temp measurements since I never know for sure when it's at 80.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

this phenomenon happens because your ATF cooler is built into the radiator, when you are driving along your coolant temperature is about 82C (well - my car is)

when Im stuck in traffic or idling the temperature rises to 98C - the ATF temperature will surely rise with it.

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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)

I have a temp gauge on my motorhome and when it is hot, I can see it go up with extended idling. As stated, you're plugged into the radiator's cooling system and you have no airflow.

Dkr.

Idling puts a large strain on the automatic transmission due to the torque converter having to slip to keep the engine from stalling.

So shifting into neutral is good idea for extended periods of idle. Even gets the rpms and the oil pressure up. That fan is moving air through the rad at idle. More rpms, lower temps.

Why would you expect a 'steady' 80C? The ATF heat exchanger lives on the cool side of the radiator. if the temperature at the top of the system is 180F, a delta T of 20F across the core is what you would expect. It would only get hotter if the engine was under heavy load or the fan was off. So for sure it's going to be tough to have the transmission at a steady 80C. If the fan comes on, it will drop right down to about 65C, and that's where I would expect it to stay under most operating conditions.

I guess it drops from 80 and then creeps back up. As a practical matter, I need to find a warm temperature that is stable for at least a few minutes to allow me time to measure the level.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

you are worrying too much about it, Your concern for such exact measurement is the reason there is a cool scale on the dipstick. Use that for the cold fill. Once you drive around and measure it between 70-85C - the dipstick would show correct on the hot scale.

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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)

I usually use the cold measurement...but, up here in New England, haven't a a 25C day yet.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife