So I got a thm400 in my rv, it shifts great, it's quiet, had no idea I had a problem until I installed a temp gauge. I drive for about 45 minutes on a nice flat road and it'll creep up as high as 280, which is right about where i pull over for 10 minutes, and then it's good for another half hour. It gets hotter going up hills, and only seems to keep a safe temp going downhill. I've tried a mechanical gauge and an electronic, both with the same results. Gauge sensor is on the oil cooler line going to the cooler. The cooler is a massive 2 foot x 4 foot cooler with 2 electric fans on it. There's lots of room up front, so it sits by itself, no other radiators around, with the 2 fans pulling on the back. Fluid smells and looks fine, when I disconnect a cooler line, fluid gushes out, so it seems to be recirculating. Using high quality fluid, tried standard and synthetic with the same results. The previous owner never had a temp gauge on it, so who knows how hot he ran it without pulling over. The engine always runs nice and cool even when pushing it hard up a hill. So I don't know if overheating the hell out of it in the past would cause a problem like this now? Like I said, it works great, except for what the gauge is telling me. Was gonna try a deep aluminum pan next, but I feel like I may have a bigger problem. Any ideas?

 

You cannot allow an automatic transmission to overheat. 280 F is not correct. You might well need to install an aftermarket fluid cooler.

 

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