AC Problem
 

I have a problem with the A/C on 89 Maxima. Blower fan works fine on all 4 speeds, but the compressor kicks out on #4. It works fine on 1, 2, & 3. Freon level is good. It can't quite cut in in Arizona summers without high fan. Seems as though it is a sensor or relay, but I don't know where to start.

Thanks