Use the following engine running test to test your fan clutch.
- Block off the radiator and run the engine with the air conditioner operating to help warm the coolant.
- When the engine is cool, the fan will not pull air.
- As the engine warms, there should be a noticeable increase in the noise level from the fan.
- If the fan clutch doesn't engage before the temperature gauge shows hot, it must be replaced.
- When a warm engine is shut off, the fan can turn a small amount, but it should not continue to freewheel. If it turns more than four or five turns, it is probably defective.
- When the engine cools down after the radiator is unblocked, the fan should disengage.
- The operation of the clutch can be checked with a timing light. Manufacturers' service manuals give the procedure and engine speed specifications.
Timing light with inductive pick-up. Courtesy of Actron Manufacturing Company.
A handy way to check fan clutch engagement is to write a number on the engine side of each fan blade with a marking crayon, and then point a timing light at the fan. If the drive pulleys are of equal size, the numbers will be stationary if the fan is locked up. If the clutch is slippping, the numbers will run backwards.