Originally Posted by jpeytonii

Funny you should mention this... The bigger concern with the transmission cooler thermostat is (if I am reading the hose diagrams correctly) that if it fails open it will allow coolant to flow through the radiator and back into the engine - bypassing the main thermostat.

I recently had the main thermostat replaced (after arguing with the dealer for two weeks that it was bad - "can't be bad if there are no codes...") and immediately our temperatures went from 65-70C to 89-91C. This was in a Chicago winter and I was monitoring it one day with ambient at 4F and driving on the interstate it stayed steady at 89-91C (it would go a few degrees higher during DPF regen). We are now a few months later and the temperature has dropped to ~84C while cruising on the highway. In town it will creep up to around 90C. Theses are the classic signs to me that coolant is prematurely passing through the radiator. My next step is to replace the transmission cooler thermostat.

There have been many posts in this thread asking what the proper coolant temp is. Since these engines use an old school thermostat that is designed to begin to open at 88C the proper temps to look for are slightly above that - i.e. 89-91C. Anything less than that is an indication that "cooled coolant" is being sent to the engine before it is warranted and you have a failed or failing thermostat. Then the question becomes: "which one?"

Cheers.