1990 240DL AW70 4-speed automatic transmission w/OverDrive(OD). 189K. I am 2nd owner since 1995 at 41K. This is now my daughter's daily driver. I had opportunity to drive it on 12/27 (first time I have driven it in 6-8 weeks and noticed the car does not shift from 3rd to 4th gear. My daughter drives locally so really has no occasion to drive over 45-50 mph, or for extended periods of time/miles (trips less than 10 mi one way. She said it's been "doing that" for several weeks, probably for the last few hundred miles. So, yesterday I removed the transmission pan and replaced the screen/filter and the transmission fluid, and added about 8oz of Sea Foam "Trans Tune" and took it for a 12-15 mile test drive to let it warm up, not exceeding 50 mph. It still does not shift from 3rd to 4th gear. All other gear selections work fine when I shift manually (P, R, 1, 2, D, N). All gears shift automatically smoothly: 2nd at about 15 mph, 3rd at about 25-30 mph. The car would usually shift to 4th just past 40 mph, but no longer does so. It just hangs in 3rd gear even to speeds up to 50 mph. Letting up on the accelerator after achieving 40+mph does not cause it to shift. "Kick-down" cable is fine and the transmission downshifts when I sharply depress the acccelerator. OD button switches on-off w/orange arrow as usual, however it does not "down-shift". I assume this is because the car needs to be in high (4th)gear in order for the OD switch to command the down-shift. Would appreciate any advice on what is causing this and what I can do to remedy and get 4th gear operational again.
(BTW: My daughter reminded me that she "told" me the car "sounded loud" back when this all started (but she never made comment to me about shifting). She said "You know,Dad, if the car had a tachometer, I would have known right away it was related to the transmission." (That's my girl.)
Thanks again. Marc