Viscous fan clutches

How do they work and what is the correct way to diagnose them?

The fan clutch or visco clutch as it is sometimes referred to is typically one of the least well understood components in an engines’ cooling system and consequently it is also one of the most commonly misdiagnosed parts in the vehicle as a whole. However, with the correct understanding of it’s workings and with the correct equipment it is not all that difficult. In this article we will therefore try to demystify the subject and hopefully help you to understand whether the part is indeed faulty or not.

So, first off, what is the full scope of functions that this part needs to perform and how does it actually work? The purpose of the fan clutch is to control the engine fan, not only to allow it to supply cooling air to the heat exchangers at the front of the vehicle, but to make it do so only when it is necessary, and to supply only the necessary amount of air when it does. The latter two points are important to note as they are often overlooked or not understood to be part of the primary function of the fan clutch. The reason why this is important is that the engine fan will draw power from the engine in direct relation to the speed at which it is turning and if the cooling load on the various systems collectively is low then only a small amount of air is required. The fan speed therefore should be correspondingly low so as not to draw in excess air thereby robbing the engine of power unnecessarily and consequently wasting fuel and money. The point then is that the fan clutch is not only there to drive the engine fan but to do so efficiently.

So then, how does it actually do this?
In a nutshell the viscous fan clutch is made up of two halves mated to each other and capable of rotating semi-independently of each other. The input side is coupled to the engine either directly or by means of a belt and therefore rotates in a fixed relation to the engine. The output side is mounted on a bearing which is in turn mounted on the input shaft so as to allow it to rotate at a speed different to that of the input side. There is a seal on the input shaft which keeps the assembly sealed. The input and output sides have a system of mating concentric grooves and the sealed space between them is filled by an amount of silicone oil which varies in relation to the amount of drive required from the fan clutch. The amount of oil in the grooved space between the two halves is controlled by a valve connected to a thermal element located at the front of the fan clutch. The oil is released from a storage reservoir incorporated into the assembly when more oil is required and it is pumped back into this reservoir when less oil is required in the working space. As the thermal element heats up in response to hotter air resulting from an increase in thermal load the valve will allow more oil to flow into the grooved space between the two halves. This increases the viscous shear forces (friction) between the input and output sides and thereby causes an increase in the speed of the output side. The fan therefore draws more cooling air through the heat exchangers which balances the cooling capacity of the air with the thermal load of the system.

Now that we have a basic outline of how the system works it is necessary to understand the system characteristics of a properly functioning viscous fan clutch in order that we may be able to interpret system characteristics which are out of line and thereby differentiate a functional fan clutch from a failed one.

In a correctly functioning system we would see that when an engine is started the fan clutch will be engaged to exactly the same extent as it was when the engine was last shut down. This means that even if the engine is ice cold when started, the fan would be fully engaged if the engine was running at full thermal load when it was last shut down. This is because the oil volume in the grooved space between the input and output sides is still the same as when the engine was shut down and the clutch stopped rotating. When the engine is first started again (when it is cold) the oil in the working chamber (the grooved space between the input and output halves) will start to circulate back to the storage reservoir thereby decreasing the drive force transmitted by the fan clutch back to the point where the fan is completely disengaged. At this point the fan will rotate with only a very small drive torque resulting from the frictional forces in the bearing. As the engine heats up, the air coming through the heat exchangers will heat up also, in response to which the thermal element on the front of the clutch will start to progressively open the valve controlling the oil flow. This will cause the drive torque of the system to progressively increase in balance with the thermal load on the system up until the point where it is fully engaged again.

A small deviation from the above pattern will be seen when a cold start up is performed due to the effect of oil viscosity which is higher at low temperatures than at operating temperature. The effect of this is that when the oil is cold the amount of fan engagement will be higher for a given amount of oil in the working chamber than when the oil is warm and a certain amount of fan engagement will therefore always be present on a cold start up but it will fall away relatively quickly as the oil heats up (due to internal friction) and the fan clutch reverts to the correct (low) level of engagement in response to the low thermal load under that condition.

With the above in mind it is now possible to determine whether a fan clutch is faulty or not. Essentially there are two ways in which a fan clutch can fail. The first and more obvious one is if the part is no longer transmitting the required drive torque necessary to allow proper cooling. The other is if the part becomes ‘jammed’ up and no longer varies the drive torque supplied in response to changes in thermal load. The first failure mode becomes evident in elevated cooling system temperatures and the second mode becomes evident in excessive noise from the fan due to the fact that it does not disengage at any time whatsoever. In addition the second failure mode should also become evident in increased fuel consumption.

This brings us to the next question, ‘How do we test for the above?’ The correct way to test a viscous fan clutch is to determine the amount of slippage allowed by the system at the various operating points that the system goes through while warming up. The pattern that should be evident is that on a cold engine the clutch will be engaged at a level in line with the engine temperature at the time of shut down and should disengage again within three to five minutes after start up. This setting should then hold until the thermostat opens and the engine coolant begins circulating. Only at this point would the air being sucked through the heat exchangers in front of the fan start to warm up and only from this point on should the fan clutch start re-engaging. By the time that the coolant temperature entering the radiator reaches full operating temperature the fan clutch should be fully engaged. At this point the measured slippage in the fan clutch should be no more than 5%.

The correct way to determine slippage is to measure fan speed relative to engine speed and divide the former by the latter. Anything less than 95% would become progressively more and more problematic as a healthy system should be able to maintain this level of performance. This measurement is best made using an infrared tachometer with small pieces of reflective tape on the input shaft to the fan clutch as well as on a suitable place on the fan hub between the base of two adjacent blades. At the same time the engine coolant temperature should be measured using an infrared thermometer so as to ensure that the temperature is indeed at operating temperature when measuring slippage. It should be mentioned at this point that it is not possible (not to mention downright dangerous) to determine how much slippage is present by trying to hold the fan blade with ones hand and then starting the engine, simply because it is not possible to differentiate between 5% slip and 10% slip or even 15% slip for that matter. To put this into context, a system with 10% or 15 % slip in the fan clutch could already have significant cooling problems.

Since the above tools are of a specialised nature, it is recommended that where it is suspected that a vehicles viscous fan clutch is failing or has failed, the vehicle be taken to the closest Silverton Radiators outlet where this testing can be undertaken and a diagnosis can be made by someone who has experience in cooling system troubleshooting. In addition Silverton Radiators stocks a wide range of OE manufactured viscous fan clutches and quality replacement parts to cover most requirements. In particular Silverton Radiators stocks the original Behr range of visco fan clutches being a partner to Behr via the Behr Hella Service joint venture. In summary then, your most reliable partner when it comes to engine cooling problems remains as always, Silverton Radiators.