[dawnnewsome3]

It may be low of freon check and see if it has pressure easy way is to remove the cap from the suction line the larger of the two lines . If it does get a can of freon with attachment hose and refill , it may be just low enough to not activate the pressure switch . As for it not working on defrost it doesn't hurt anything , the reason it cycles on and off during defrost has to do with changing the airs humidity to help defrost the windshield easier.