HELP! 4+3 Overdrive slips out at hi engine temp

[mike53]

I have an '85 with 4+3 Nash transmission. I developed a problem where the OD will engage properly at about 145 degrees engine temp, but will kick out at about 192 deg. I have read different threads about filters, solenoids, switches, etc, but don't want to spend lots of money guessing. The car has 35k miles on it, and is still original. I did replace the OD filter and ATF about a month ago, but that didn't seem to make a difference. I have had the wiring checked, and the mechanic said it all looks fine. I have tried to find a solenoid, but don't know where to get one. I'm not even sure where to look for it. I purchased a shop manual on dvd, but it is not real friendly. I have searched for the information but couldn't find it, probably because I don't know what GM calls the parts. I guess what I need ot know is: where is the solenoid (I imagine in the pan); What is the switch, or is it the same thing; where can I get these parts; anything else I should be looking for? Can anyone shed some light on this for me? Thanks in advance.