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)