We turned our a/c on yesterday for the first time. We didn't notice anything out of the ordinary until several hours later when we adjusted the thermostat to force it to stop running. The outside unit continued to run even after I set the thermostat to off. The copper tubing leading into the blower unit in our basement was extremely cold with frost on it. After several more hours of it running for no reason, I had to shut the circuit breaker off in order to stop it. My outside unit is a Jantrol?? presumably the original unit when the house was built in 1988. Any ideas??

Thanks,

I would check for power to coil on outside contactor for 24 volts, if none is present and power is on load side of contactor, it may be welded closed. Replace/clean?
If 24 volts is present at contactor coil, then it is thermostat or control wiring problem.

I agree you need a new contactor.