1. Autos

Your suggestion is on its way!

An email with a link to:


was emailed to:

Thanks for sharing About.com with others!

Questions and Answers

Fan Clutches

Q. Hi Vincent, how you doing? I was working on my kids car, the other day. He has a clutch fan on his radiator. I don't know anything about a clutch fan. I know when you start the car, the fan runs.

I did see an article that says if there is a fluid leak around the fan, you should replace it. Also with the engine off, if you can spin the fan easily with your finger then it needs replacing. But every clutch fan that I have seen, is easy to spin. So I don't guess I understand.


A. Hello J.D. Good to see you again. To answer your question I'll give you some info on what a clutch fan is and then how to check it.

The fan drive clutch is a fluid coupling containing silicone oil. Fan speed is regulated by the torque-carrying capacity of the silicone oil. The more silicone oil in the coupling the greater the fan speed, and the less silicone oil the slower the fan speed.

Two types of fan drive clutches are in use. On one a bi-metallic strip and control piston on the front of the fluid coupling regulates the amount of silicone oil entering the coupling. The bi-metallic strip bows outward with an increase in surrounding temperature and allows a piston to move outward. The piston opens a valve regulating the flow of silicone oil into the coupling from a reserve chamber. The silicone oil is returned to the reserve chamber through a bleed hole when the valve is closed.

On the other type of fan drive clutch a heat-sensitive, bi-metal spring connected to an opening plate brings about a similar result. Both units cause the fan speed to increase with a rise in temperature and to decrease as the temperature goes down.

In some cases a Flex-Fan is used instead of a Fan Drive Clutch. Flexible blades vary the volume of air being drawn through the radiator, automatically increasing the pitch at low engine speeds.

Do not operate the engine until the fan has been first checked for possible cracks and separations. If there are signs of leakage, then it needs to be replaced.

Run the engine at a fast idle speed (1000 RPM) until normal operating temperature is reached. This process can be speeded up by blocking off the front of the radiator with cardboard. Regardless of temperatures, the unit must be operated for at least five minutes immediately before being tested.

Stop the engine and, using a glove or a cloth to protect the hand, immediately check the effort required to turn the fan. If considerable effort is required, it can be assumed that the coupling is operating satisfactorily. If very little effort is required to turn the fan, it is an indication that the coupling is not operating properly and should be replaced.

If the clutch fan is the coiled bi-metal spring type, it may be tested while the vehicle is being driven. To check, disconnect the bi-metal spring, and rotate it 90 degrees counterclockwise. This disables the temperature-controlled free-wheeling feature and the clutch performs like a conventional fan. If this cures the overheating condition, replace the clutch fan.

Another way to check it is to listen to it. Let the engine idle and when the clutch fan heats up enough to engage, you'll hear a change in sound. It will sound like a roar which indicates that it is actually pulling air.

Additional Information provided courtesy of ALLDATA

Back to Index

©2017 About.com. All rights reserved.