I suspect this may be due to varnish buildup in the valve body related to sitting for a few years. I had a similar failure to down-shift and delayed up-shift on my '88 Grand Caravan about eight years ago. I drove that van for many years without ever hitting wide-open-throttle, so varnish was able to build up in the parts of the valve bores where the valves never went. When I finally did one night, that's when the problems started, and the shifting always improved when the transmission warmed up. The problem lasted for about two months and I didn't do anything to it except drive it. Every day it was less noticeable. I even drag around a tandem axle enclosed trailer that's bigger than the van, and the shifting has been fine. Even with that I rarely exceed three quarter throttle.
There's two valves in particular that are involved with shifting at the wrong times. Those are the throttle valve and the governor. The governor valve is spring-loaded to come back to rest when vehicle speed is 0 mph. If that valve sticks just a little, the shift valves will think the car is still moving and they should be in second gear. They won't go into first gear unless you manually put the shift lever there.
I don't know if your throttle valve is spring-loaded. Mine is. That night when I finally hit wide-open-throttle, that valve got pushed to where it hadn't been for years and got stuck in that varnish. That makes the shift valves think you're accelerating harder than you really are, so it moves the shift points to higher speeds.
There are chemicals, ("mechanic-in-a-can"), that will dissolve that varnish. In my case, the transmission fluid and filter has only been replaced once, at 85,000 miles, since the van was new. It has 248,000 miles now. That fluid still looks good but it's surely turning to varnish. By the way, that isn't neglect, ... It's abuse, and I'm not recommending ignoring proper maintenance for anyone. I don't care to tell you when I changed oil last, but I still try to replace the filter once every two or three years!
I didn't need any chemical additives to solve my problem, and I don't use them for other people. You'll have to rely on the people selling parts to recommend what other mechanics have had good luck with. I don't think a simple fluid and filter change will help. That won't address the varnish buildup. A lot of people will recommend a complete flush that includes adding a can of very aggressive detergent first, but some car owners have been unhappy with the results. They blame all their future transmission problems on that flush. I have to wonder how many of those people already had an existing problem and they had unrealistic expectations the flush would magically solve a mechanical problem.
Wednesday, October 9th, 2013 AT 12:14 AM