Step 3: Electric fan - simple troubleshooting procedure
Usually, a small control current runs through a thermal switch. When that switch reaches a set temperature, the coil of the fan relay is energized and the relay powers the fan until the contacts in the switch open again.
To test the good functioning of such a circuit, one has to find the fan thermal switch and bridge the two contacts. A relay should then be heard clicking and the fan should run. If the fan has two speed settings, it might have a three wire connection on the motor, one ground and one +12V for each speed or it will have a two wire connector feeding a variable voltage (for example, +9V for a slower speed and +12V for a higher one, plus a ground connection). These two speed systems usually work by putting a resistor in series with the fan motor to give the lower speed. On some cars, these resistances are known to crack or otherwise failing over time.
If the fan fails to run with the thermal switch contacts closed, check the wires for +12V. A fuse might be blown in the relay energizing circuit, the relay itself might have failed, etc.
It is also advisable to test the thermal switch. A simple setup involves heating the switch with warm water and checking at what temperature the contacts close. Use a good thermometer strapped right to the switch for more accurate results.