I have a problem in which I can put the tranny in overdrive when the car is cold. As it warms the car will slip out of overdrive. When fully warm, OD will not engage at all. I've replaced the relay. Reading the threads, I'm thinking the solenoid is bad. Can this part be replaced, and if so what are the steps?

 

Go here..half way down download publication TP30058-3
All there is to know about the M46 transmission.
http://www.k-jet.org/documents/greenbooks/200-series/