There is a pressure switch on the drier that cuts off the compressor when it is to low on freon..Have you chaged it over to R134 yet or is it still R12? You need also to put gauges on it to see what kind of pressure you have to figure out if it is low freon or bad switch or relay or something else. You need to trouble shoot a little deeper, with test light you can check if voltage is at switch and going through to comp..........