OK, so I replace the solenoid with a new one--still not working. Yesterday, I went through all of the wiring and connections and double, triple, and quadruple-checked the operation of the solenoid, and it seems to be functioning as it should: there is an audible clicking sound from both the relay and the solenoid when I press the button on the shifter I(it's an automatic). I put it all back together, listened again for noise, and then took it out for the moment of truth. It won't go into fourth gear. There are only 136K miles on the tranny--seems a little young to be having major failures.

The only thing I can think of is that perhaps the solenoid itself is getting contaminated (I noticed that the tranny fluid dipstick seems to have some of the yellow ridges on the pull handle chipped away--I have no way of knowing whether they fell down the tube). Can I decontaminate the solenoid itself? I am also wondering whether too much or too little fluid would affect function (I think I have too much in, but it's hard to read--I pulled the dipstick once after following the fluid checking instructions in the manual and it appeared dry, and I pulled it again and it read over the hot high mark). Very frustrated. How might I fix this problem? As always, thanks.