I have my trans temp guage sender on the hot line coming out of the transmission. Towing 6500lbs. at 65-70mph in overdrive on flat highway my guage reads 210 degrees. Is that O.K.? If I kick out the overdrive the temp drops to around 195 but my max speed is 65mph at 2500rpm. I thought about doing a couple things: Change my tire size from 33" to 35" and only tow in drive. That might let me reach 70mph and keep my rpms at 2200-2500. Replace the P.O.D. injectors with bosch 190's. That would bring the egt's down too. Bypass the trans cooler line into the heat exchanger. This truck has a DTT built trans with a 91% converter and a electric fan trans cooler under the bed. Bypassing the heat exchanger might keep the engine heat out of the trans fluid or vice versa. Change the axle ratios from 3.54 to 4.10. More rpms in overdrive might keep it cooler. Anyway sorry for the long post Just looking for advice

 

IMO The simplest answer is to slow it down a little. But as long as the fluid cools down readily I would not be real worried. I agree 210 is getting hot, but nothing to panic over especially since it cools down. I would not bypass the heat exchanger, it is helping not hurting. Even with the electric fan your under bed trans cooler is not as efficient as one mounted in the grille at highway speeds, so you might consider moving it if that is possible as well as going to a larger cooler. Also you might consider changing to synthetic fluid as it tolerates the heat better. Your injectors and EGT's have nothing to do with trans temp. Changing the tires to a bigger tire will lower your RPM's but worsen the stress on the trans.

 

210 isn't that bad for what you were doing. I wouldn't bypass the engine trans cooler. If you don't want the expense of synthetic fluid, just change the regular atf more often. The overdrive is supposed to cut out automatically at around 280. I have never seen that and in the summer I can be hard on the tranny. I just replaced the TC with a DTT but I haven't had an oppurtunity to pull the boat in hot weather yet.
As far as the 210 is concerned, I have seen 190 at a stop light. Once I start moving it drops back to the peg at 160.

 

Install the 3200rpm governor spring.

Then you can run 70 at 2700rpm w/o OD all day. It's a little loud, but the temps stay under control.

Cheapest mod you can do & one of the best.

Regards, DBF