I'm sure this issue has been addressed in this forum before, but I figured I would ask again. I've got a 08 f150 fx4 with the 5.4 and my transmission has been running around 235 on the highway lately. Now i attribute some of that to the new 35s I have on it, but I can't think that is the only reason. I checked the cooler and lines, no kinks, bent fins, anything of the sort, even removed the cooler and washed the fins out to make sure it wasn't plugged with dust and dirt. Changed the fluid 5k miles ago, just a swap with filter, didn't flush it or anything of the like. The overheat light on my edge tuner flashes constantly whenever I drive on the highway, even with cruise at 65, the check engine light is on due to a bad O2 sensor, but it doesnt throw any codes regarding transmission temperature. What gives? Faulty sensor? Engine Temp seems to be in normal range, fluid is circulating through the trans cooler so it doesn't seem to be plugged. Engine stayed cool enough at idle in 100 degree heat during the summer, so i don't think it it is the fan clutch. I'm flat out of ideas. I need a solution rather quick, since I have a couple 1500 mile trips in the next 3 months. Would throwing a couple electric fans behind the cooler help since it is in a crappy spot thanks to the ford design and doesn't get enough air flow? How about throwing a heat sink in on a frame rail to drop extra heat? Larger trans pan? I'm open to any and all ideas.

 

If you have a thermostatic bypass on the cooler, that may be stuck open?

 

To read transmission codes, you need a scanner capable of reading them. Most aren't.

Have you checked the fluid level? Did you put the correct trans fluid back in it when you did the service?

As glc said, the thermal bypass might be suck open. If it is stuck open, I'd imagine the two line from the trans to the bypass valve would be hotter than the two lines going to and from the coolers.

If the bypass valve is good, then you'll need to check line pressure to make sure it's good. Then a trans fluid flow test.