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Why would you turn it off with the AC on? You need the fan to keep the high side pressure in range. You could install a pressure switch on the high side but it would hardly be worth it.
Modern systems use a high side pressure transducer where the PCM closely monitors high side pressure. If it determines system pressure is in optimal range for conditions it can energize and de-energize the fan relay as needed. Unless you can closely monitor this you run too high a risk of damaging the system. Relying on ram air isn't good enough as high side pressure can climb too rapidly.