Viscous Fan

Another cooling component that is most often overlooked is the viscous fan coupling or clutch fan coupling. It is important to understand how it works. This component is probably responsible for more engine failures than any other cooling part on a vehicle simply because its failure is so gradual and unnoticeable. I have seen guys build up a new engine and not bother to replace the viscous coupling. Manufacturers fit them to most model cars and trucks as they are fuel savers and they allow the engine to produce more power. A fixed fan will also drain horse power from an engine powerplant.

The viscous fan is free to turn on it’s axis as the motor revs. In other words the engine RPM and the viscous RPM differ in their speed of rotation. It is designed to start locking-up at round about the time the engine reaches its desired temperature range which is controlled by the thermostat. The thermostat actually controls the viscous fan in principal because it regulates the engines expected temperature range. As the radiator cools the hot water from the engine, the thermostat allows only enough water in at a time, keeping the water at a constant temp within the head and block. The viscous coupling will then only engage again once the temperature rises again.

The problem arises however when the viscous comes to the end of it’s life span which is difficult for most people to understand because it always appears to be spinning in time with the engine revolutions. There is only one way to check it and that is when it is cold, on start up in the morning. You will hear the distinct rush of air as you rev up the engine. This noise will fade away and quieten down completely after a 20 seconds or so. Watch the temperature climb on the temp guage, listening all the time. Once the temp reaches the half way mark after a few more minutes, the viscous fan coupling should reengage itself and you will again hear the wooosh sound of air rushing through the fan blades as you rev the motor.

This indicates that the viscous coupling is working. If the temp guage reaches the half way mark and I mean, not a hair over the half way mark and the guage keeps climbing over the half way mark then you know it’s time to replace the coupling. They don’t come cheap so what a lot of guys do is remove them and fit solid fans adaptors. The problem with these are that when the vehicle attains a speed of let’s say 110 km/h the fan reaches a static condition whereby the fan actually slows down the air flow through the radiator.

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