Slow shifting from first to second

Which transmission do you have, the three-speed or the four-speed with overdrive?

This happened to my 1988 Grand Caravan after many years and 140,000 miles of never hitting wide-open-throttle. While trying to find the elusive cause of intermittent cruise control operation, I had test lights hooked to the servo's wires so I could watch what was going on. When I started the engine, one of those lights grounded the appropriate solenoid and the servo pulled the throttle wide open and held it there until I ran around and turned the ignition switch off. A little while later, when I drove onto the highway, it had a real high shift point, and it had a delayed down-shift. I had to manually put the shift lever in "1" to take off from some stop signs. Since it was my own van, I just kept driving it like that, and found the shift points gradually returned to normal over about a six-month period.

The cause of this problem was varnish build-up in the bore for the throttle valve. To add to the misery, the transmission fluid had only been changed once, at 84,000 miles, because the side cover rusted out and started leaking. I changed the filter at the same time. Years after that, I regularly used that van to drag around a tandem axle enclosed trailer that is bigger and heavier than the van, but I never changed the transmission fluid again. I just retired that van two years ago due to it being so rusty, the carpet was the only thing holding the front and rear together, but the transmission was perfect and had never had a single thing done to it other than the filter, fluid, and side cover.

The first thing to look at is if the return spring is missing for the kick down linkage. That linkage is right up on top in front of the brake master cylinder. Push it down and it must spring back up. If that is working properly, this is one of the few times you might consider adding a chemical to the fluid to dissolve any varnish. I am not sold on transmission flushes because people tend to do those well after a problem has developed, and those are usually mechanical problems, meaning broken parts or dry-rotted/cracked lip seals. No mechanic-in-a-can is going to fix a mechanical problem. Consider quizzing the customer on their driving habits and see if they recently hit wide-open-throttle and that is out of character for them.