So I was driving on the freeway in my 1990 240 cruising at 65 or 70 and all of a sudden it downshifted for no reason. the overdrive arrow went on so i was like ok thats wierd and pushed the overdrive button. it wouldnt do any thing, the arrow stayed on and it didnt switch to the top gear like it should have. It could be the switch but I didnt push it in the first place so I'm not sure. Where is the problem and how do I fix it?

 

The same thing hapened to me. You can try the button but will probaply be od. selinoid at the trany., it is an expencive part and if you dont use it not a good investment. I pulled mine and cut grove in it like the bypass plate the sell on IPD for 40$. The whole thing took me appr. 45 min and only down side is light still comes on on acation, but does not down shift.

 

Have you checked fuse #11?