Is coolant in transmission fluid extremely rare?
Yes.
Unless the transmission fluid cooler in the radiator springs a leak.
Tester
No. All it takes is a failed transmission cooler that is integrated into the radiator.
you might have tranny fluid in your radiator coolant too.
This was a common problem with certain Acura and Honda models a while ago.
Why do you ask?
Don’t drive the car in that condition. Antifreeze will trash the tranny in a few seconds.
New radiator, flush and fill antifreeze, drain tranny completely and refill with new tranny fluid (proper type for the vehicle), say a prayer with rosary beads.
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A while back I mentioned my 2018 low mileage RAV 4 doing a little transmission slipping for a few minutes in very cold weather. Then I saw a spot of trans fluid under it for first time ever. Took it to dealer, they didn’t find anything wrong with it, and acted like coolant in trans was almost unheard of.
I still have 14 months of powertrain warranty, they said bring it back if it gets worse.
Vehicle owners describe any transmission abnormality as “slipping”; Delayed engagement, upshift flare, delayed upshift. If the technician does not experience the condition, it is up to the customer to return with more detailed information.
I have not seen a failed transmission cooler in at least ten years. Every shift complaint should not lead to a suspicion of a transmission cooler failure.