83 300SD

I can't get the AC to blow cold air only hot. I've replaced the monovlave and every once in a while everything will work great and the AC works fine. But these occasions are getting further and further apart. I can punch buttons and get it kick on the AC, but after a while its back to hot air. The climate control unit will constantly click, click, click click by itself at about 3 times a second. I recently bought a used climate control unit off of Ebay and took it apart and inspected all of the solder joints (whick looked good) and it does the exact same thing. Did I buy another broken climate control or is there something else causing the thing not to work properly?