Maytag LAT8606AAE won't spin unless I push down on control knob

Have a Maytag washer LAT8606AAE, which I bought new . . 20 years ago. Problem I have is that it won’t spin unless I push down (not in) on the large control knob. It’d be great to replace the part that’s causing the problem, just don’t know where to start . . Problem has got to be something behind the large control knob. Any ideas?