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High trans temps


New Member
Sep 1, 2019
Truck Year
Hey guys, need some help isolating a problem. On a 2.5 hour trip a couple of weeks ago I noticed that the radiator fan was running while cruising down the highway at 75 mph. I switched the evic to the temperature page and noticed the trans temp was 268F. The engine oil temp was 208F and the coolant temp was 203F. I exited the highway and noticed the trans temp started dropping immediately. By the time we got through the first traffic light the temp had fallen to below 230F and I could hear the fan turn off. We got back on the highway and the temp continued to drop back down to its normal range (195F-199F). We finished the last half hour or so of the drive with the temp never exceeding 199F. On the return trip I monitored the trans temp the entire time. After about 90 min on the road it broke 200F again and started climbing. It went from 200F to 248F in about 5 min. I exited the highway, saw the temp start to drop and got back on the highway. Just like before the temp continued to drop back to the normal range and stayed there for the rest of the trip. I have repeated that same trip 3 more times over the last few weeks and same thing happens every time. I talked to my service advisor at the local dealership after the first trip and he said that there wasn’t anything that they could do about it unless it caused a CEL. After a little bit of research I found out that the CEL doesn’t come on until 275F. The highest I have seen is 272F. I ran it there for about 25 min hoping it would go up the last few degrees to cause the light to come on but it never did. I can’t imagine that it is good for for the transmission to run that hot even if it is below CEL threshold. There are so many different components in the cooling system for these trucks that I am not sure where to start. Unfortunately Chrysler doesn’t seem to want to do anything about it until it throws a code or the transmission fails. One other thing to note. I’ve already brought the truck in for a recurrent P0128 issue during the winter. They checked the operation of the 3 way coolant valve then wanted to re-flash the ECU. We didn’t do that since replacing my GDE tune is not currently an option. That problem went away when the morning temps got above 50F. I’ve read some other threads about transmissions running hot and it seems like the common culprit is the thermal management unit in the transmission heater. However, all of those threads talk about consistently high temps and not the erratic ones that I am seeing. Also not sure if the P0128 code could be a different symptom of the same problem. What do you all think? Thanks.