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Relative new PSD owner here for over a year and use my F350 to mostly tow my race car.
Last week while towing back from MD to NJ we got stuck on the pike in stop n go, mostly stop traffic for over an hour and trans temps worked their way up to 190. The weather out was about 90-95 degrees and humid.
The reason I ask is I had a 93 F250 gasser and a couple years ago while towing the same car and coming over the bridge into NJ the trans got so hot (no gauge on truck, my fault) that it blew thru the pump and got me stranded on the NJ Turnpike. So because of that I am always watching the trans temp and don't want a repeat of that.
190 is not too bad. Especially considering it wa 95 degree outside. Normal temps not towing are @ 70 degrees above OAT. Towing as you know is a different story. You were almost 100 degrees above outside air temps but that was because you were in stop & go traffic and the TC was unlocked creating alot of heat. 225 is the pull over and let her cool down point.
edit: on the newer models with the trans temp guage in the dash, 238 is when the needle gets set to the H (Hot) position.
I don't think that's too bad, it's getting pretty close but still under 200. Above that and you should start to get nervous, I hear people say 210 or so is getting bad.
Ive never hit more than 180 while towing, I have a bigger cooler though and my trans pan has fins on it. For stop and go traffic thats not bad I wouldnt worry about it till about 210.
Ive got a stock tranny with no additional cooler, 275K miles, tow a 11.5K# 5er, and have never had the tranny temp go over 200. I think you are doing well, I see 190 when I tow the 5er in stop and go traffic, but it always comes down to 160 once the TC locks and some wind gets under the truck.
As others have noted stop and go traffic is killer on the trans for temps since the TC isn't locked and you aren't getting much air through the cooler.
90-100 over OAT is pretty normal and you are just fine at 190-200. I would check your fluid occasionally to make sure it doesn't start getting burnt smell (just good idea no matter what anyway if you tow at all)
Towing in stock form up and down hills my Ex's trans ran about 180ºF-185ºF, but climbed to 205 after a long slow uphill tow.
I changed the fluid, installed the 31-row cooler from a 6.0, and installed a BTS valvebody (probably no effect on temps), and on my last tow, it ran around 145ºF on the highway (fewer hills, but same speed), and 155ºF in town.
Next time you flush an fill the trany go with synthetic ATF. It handles the heat better.
That's a good start if you don't have the time, or the resources to get a bigger OTA tranny cooler. Although, your temps are right in line considering the conditions.
I have the larger 6.0 cooler also. I tow a 9500 fifth wheel and on my last trip back from Kentucky Speedway, in the mountains of east Tennessee and western North Carolina, I saw 101* on the outside temp gauge and the tranny maxed out at 160. It normally runs outside temp PLUS 75 up to the 75* point then it usually hangs up around 150 to 155. This time it spiked up to 160 plus or minus 1 or 2 but that was it and only briefly. I am a firm believer in the 6.0 cooler. My V10 gasser used the Mercon V synthetic fluid and there was some contraversy on here about wheather or not you could use the Mercon V but I never had any trouble with it. When I got this 2002 diesel I have now, I was going to change to the Mercon V but everyone talked me out of it, then all of a sudden you were allowed to use Mercon V, so I might change it out next fluid change.....Just my pennies worth...Dave