To check your radiator fan switch, turn on the ignition, open the car hood, and listen and watch for the radiator fan to engage. Some vehicles engage the radiator switch when the car is started while other newer models employ a thermostatic switch. Idle the vehicle until it reaches operating temperature and starts to overheat without engaging the fan, then shut the vehicle down to further diagnose the problem.
The radiator switching system typically consists of only four parts: the switch, the fan, the relay and a fuse, so you must diagnose where the problem is. Check the fuse fist; if it is good, run a jumper wire from the positive battery post to the wire connection on the fan motor. If the fan engage,s then you have eliminated both the fan motor and the fuse as potential problems.
Check the ground wire coming from the switch to make sure that its connection point to the car frame is clean and secure. Disconnect the jumper wire, and turn the ignition to the run position without starting the vehicle. Take the wires off of the switch leads, and briefly touch them together. If the fan starts, then the problem is with the switch. If the fan doesn't start, then the problem is with the relay or the wiring between the ignition switch, the relay and the radiator switch.