Transmission Temp ?

I have a '00 F250 with the 7.3 powerstroke and the 4 speed auto tranny in it. I use it to pull a 35' fifthwheel toyhauler weighing in at about 13K loaded up. I'm kinda bothered at the auto being realiable for the long haul though. It hasn't given me any reason to doubt that it can handle the weight and in fact does better than I had expected. I bought and installed a transmission temp gauge just to be able to watch the thing. My biggest question is how much heat is too much? I know that the cooler the better, but whats a cutoff of safe for this particular vehicle? I've heard say that around 200 is as hot as it needs to get before damage, but it this a good rule of thumb or will it handle more heat? Thanks for all responses.

Generally anything above 250 is seriously bad news.

200 is a reasonable upper limit.

Some years ago, I did see a bar graph from a transmission parts supplier that displayed an inverse relationship between trans life and temp;

that is, higher temps do not result in immediate failure, but will reduce trans life.

A constant 200 deg or less resulted in 100k+ miles of life.
A constant 250 deg = 11k miles
300 deg= 500 miles before failure.

I also recall this depended somewhat on how the transmission was made.
For example, some used bronze Torrington bearings, which are much better at coping with heat than the nylon bushings found on older, light-duty automatics.

I used to have a F350 with a non-turbo 7.3 and I cooked the auto tranny really good within one season of snow plowing until I put a cooler on it.

Thanks for the reply. I have now have my trans temp guage installed and its working. I have yet to hitch up the hauler and pull it since installing, but just under normal driving conditions I've noticed that the tranny usually stays between 125 and 150 degrees, and thats with temps in the upper 70's and lower 80's but without the added weight of the trailer. I just want to be sure that I'm not gonna get it too hot because I'd rather have to slow down and pull out of overdrive than replace the tranny. Also I have heard alot of talk about the BTS (Brian's Truck Stop) trannies anyone here ever use one or know of any other super high performance tranny builder that they recomend. I saw where you could have him rebuild your tranny for somewhere around 3K and it's suppose to withstand a ton more punishment, whereas a stock rebuild is gonna run somewhere around 2k and still you get stock parts. Kinda seems worth the extra bucks if all the fuss is true.