My A/C works well at all fan speeds except for the highest fan speed. When I turn the fan switch all the way up the compressor disengages. The fan runs as it should at all speeds. What is causing the compressor to drop out at max speed? I have a used A/C control that I substituted and the results are the same. I pulled the fan speed resistor and it is as stout and primitive a piece as you will ever see, it is fine. Even though the fan relay was OK I replaced that too. Your thoughts?