Transmission Behavior

I have a '98 Grand Caravan with AWD and a 3.8 liter engine. It has the 4-speed automatic transmission. The last 2 or 3 summers the transmission has begun to act up, especially when during the hottest part of the day. It’s also worse if I use the A/C. The last 3-5 miles of my trip home after work, it’ll stop shifting into high gear. The manual says there’s an electronic sensor that causes this behavior when it senses conditions that could be damaging to the transmission. What conditions are these? I bought some ATF and a filter (also did this last summer). Will this help? Anything else I can do?

hmm, the problem seems to be heat related. Does your grand caravan have a transmission fluid cooler in the radiator? Wait and see what the pros have to say on this one, I am not going to venture a guess this time, despite my tendancy to post the first thing that comes into my head. But I will ask some questions that might help those other guys. How many miles on this beast? What kind of driving, city or road? Have you followed the service manual like religion? When was the last time you had it serviced, in miles?

Almost 240,000 miles. Yes, I think the fluid goes thru the radiator, which could be helpful. For many, many miles I have had to drain the coolant with a backflow approach–the draincock doesn’t work (is there anything I could use to break up whatever is gumming up the draincock?). Mostly road miles. Like I said, I replaced about 4 1/2 qts. of ATF last summer and changed the filter. Had done the same about twice before. Have never had the transmission serviced otherwise.