What jet said in his last post exactly, more complicated then it needs to be.
no matter what the case is with the 51 is here, it sound like the amp or control unit was bad and the previous owner rigged it to work.
My control unit went out about a year ago and I had no AC control, heater side still worked. Not saying that that is your problem. You just have to go through all the test procedures and the problem will become evident.