Interesting. I always thought it was the engine temperature sensor which measured the engine coolant temperature and passed the information back to the ECU which in turn commanded the radiator fan. Nothing to do with the A/C.
In old cars A/C system and engine managemente were completely separate, and there were two external fans: one for engine cooling, another for A/C cooling.
In recent cars (last 20 years, I mean), often with A/C standard fitted, they use a single fan operated by the engine ECU: this because this ECU follows the fan and idle management, both influenced by the A/C compressor. So now the external fan can run both for engine cooling (cooling fluid too hot and/or vehicle speed too low), or for A/C (a/c fluid too hot and/or vehicle speed too low). Heat exchangers and circuits are separate, but there is a single ECU.