It sounds like you have a pressure switch on the ac that is not turning on the fan. It sounds like the fan is turning on because the vehicle has hit operating temperature and the fan comes on to keep the vehicle cool. The vehicle might have a couple of fans and one specific for the ac. You need to get a wiring schematic and find out why it's not turning on the fan. It could be a sensor, a relay or the fan. It could also be a short in the wire or a bad connection. I would recommend taking it to a professional repair shop. Wiring can be difficult if you are experienced but impossible if you don't know what you are doing.