Any one have any suggestions on what needs to be checked for radiator fan. It's not turning on and not sure what connections need to be checked. Any suggestions would be appreciated.
It would help to know the make, model, year and engine type of the vehicle. There are essentially two main types of radiator fans; mechanically driven and electrically driven. Depending on the type of fan you have fitted, fault finding will be a slightly different procedure.
With an electrically operated fan, the fan (or fans) are driven by virtue of a small electric motor. It typically is as simple as voltage is applied across it's pins, the fan turns. Some systems utilise a multiple speed motor which may have a number of pins. These are usually in the form of a single ground pin and several live pins to allow multiple speed operation.
In order to start the fan turning, a thermostatic switch is screwed into the cooling system, usually into the back of the radiator, which is calibrated to allow electricity to flow at a specific temperature. The output of this switch is wired to the input of the fan motor.
The simplest diagnostic is to bridge the pins on the loom to the switch and observe the fan. As soon as the pins are bridged, the fan should start to spin. If it does, this could point to a faulty switch (or airlocked radiator / low coolant / jammed thermostat). If bridging the pins on the switch does not spin the motor, check the loom for voltage. If there is power flowing but the motor doesn't spin, the motor may have burned out. If there is no power in the loom, it could be a connector, fuse or break in the cable somewhere.
With the mechanically operated fan type, the fan is usually physically mounted on the end of the engine and spun mechanically by the rotation of the engine. In this type of system there is usually no electrical system to test. To prevent the fan from spinning all of the time, it isn't physically bolted directly to whatever pulley is driving it but it mounted on a viscous (or bimetallic) fan clutch. This clutch slips freely at low temperatures but as the temperature rises, the clutch locks up so the fan starts to spin with the rotation of the pulley wheel.
When these clutches get old, they can fail in a way that means they slip all of the time so the fan never spins at full strength. Be mindful when testing them that a spinning fan can cause injury. I'd typlically "feel" the resistance of the fan with the engine cold and switched OFF. Then run the car up to temperature and feel the resistance (again, with the engine OFF). This isn't a particularly technical fault finding method but I have successfully identified and rectified a fault on my old BMW 3 series in this way.