Unplug every 5v sensor and then see if you get 5volts. It's possible that a bad sensor is pulling the 5v reference down. Also, with the negative battery terminal disconnected, check for continuity between the 5v reference and ground. You could have a chafed wire grounding out and pulling down the reference voltage.
Are you sure the a/c clutch is only supposed to get 5 volts? Relays, solenoids, etc normally get 12 volts, sensors get 5 volts.