Fan viscous coupling: device, malfunctions and repair
Auto Terms,  Car device,  Engine unit

Fan viscous coupling: device, malfunctions and repair

Any internal combustion engine requires a quality cooling system. This is due to the peculiarities of his work. Inside the cylinders, a mixture of air and fuel is combusted, from which the cylinder block, head, exhaust system and other related systems heat up to critical temperatures, especially if the engine is turbocharged (about why there is a turbocharger in the car, and how it works, read here). Although these elements are made of heat-resistant materials, they still need cooling (they can deform and expand during critical heating).

For this, automakers have developed different types of cooling systems that are able to maintain the operating temperature of the engine (what this parameter should be is described in another article). One of the components of any cooling system is the fan. We will not consider the structure of this element itself - we already have about this. another review... Let's focus on one of the drive options for this mechanism - a viscous coupling.

Fan viscous coupling: device, malfunctions and repair

Consider what kind of device it has, what is the principle of its operation, what malfunctions are, as well as options for repairing the mechanism or replacing it.

The principle of operation of the viscous coupling of the cooling fan

A modern car is equipped with such a cooling system, the fan of which is electrically driven. But sometimes there are such models of machines in which a coupling is installed, which has a viscous drive mechanism. Due to the design of this system component, it is only applicable to rear-wheel drive vehicles. In this case, the engine stands longitudinally in the engine compartment. Since most modern car models are equipped with a transmission that transmits torque to the front wheels, this modification of fans on passenger cars is rare.

The mechanism works according to the following principle. The fan drive itself, in the housing of which a viscous coupling is installed, is connected to the crankshaft pulley using a belt. There are car models in which the clutch rotor is directly connected to the crankshaft. There are also options connected to the camshaft pulley.

The rotor housing of the mechanism will contain two discs, one of which is mounted on the drive shaft. The distance between them is minimal so that blocking occurs as quickly as possible in accordance with the heating temperature of the working substance or changes in its viscosity as a result of mechanical action (non-Newtonian fluid). The second disk is attached to the fan impeller located behind the cooling radiator (for more details on the various modifications and how this system component works, read in another review). The rotor body is fixedly installed so that the drive cannot constantly rotate the entire structure (these are old developments), but in the modern version the rotor is part of the fan design (the body itself rotates, to which the impeller is fixed).

Fan viscous coupling: device, malfunctions and repair

Until the mechanism is locked, torque is not transmitted from the driver to the driven element. Thanks to this, the impeller will not rotate constantly during the operation of the internal combustion engine. In winter, as well as in the process of warming up the power unit (read separately about why warm up the motor) the cooling system must not work. Until the motor needs cooling, the rotor cavity of the viscous coupling remains empty.

As the engine heats up, the bimetallic plate begins to deform. The plate gradually opens the channel through which the working fluid is supplied. It can be thick oil, silicone material, viscous gel-like substance, etc. (it all depends on how the manufacturer implements the transfer of torque from the pulley to the driven disk of the device), but more often silicone is used to create such substances. In some models of viscous coupling, a dilatant fluid is used.

Its peculiarity is that the viscosity of a given substance changes depending on the rate of deformation of the volume of the liquid. As long as the movement of the drive discs is smooth, the liquid remains fluid. But as soon as the revolutions of the driving element increase, a mechanical effect is exerted on the substance, due to which its viscosity changes. Modern viscous couplings are one-time filled with such a substance, and it does not need to be replaced throughout the entire working life of the coupling.

Viscous couplings can be used not only in this mechanism. A little later, we will look at where else such a mechanism can be installed. As for the operation of a fan with a viscous coupling, as soon as the bimetallic plate opens the inlet channel, the structure of the mechanism will begin to gradually fill with the working substance. This creates a connection between the master and driven discs. Such a mechanism does not require a high pressure in the cavity to operate. To provide an improved connection between the discs, their surface is made with small ribs (in some versions of viscous couplings, each disc element is perforated).

So, the rotational force from the engine to the fan blades is transmitted through a viscous material entering the rotor cavity and falling on the perforated coating of the disks. The viscous coupling housing is completely filled with this substance, due to which centrifugal force is additionally formed, as in the engine pump (for details on how the water pump of the cooling system works, it is described in another article).

Fan viscous coupling: device, malfunctions and repair
1 - the valve is ajar (warmed up engine);
2 - a slight bend of the bimetallic plate (warmed motor);
3 - fully curved bimetallic plate (hot engine);
4 – the valve is completely open (the motor is hot);
5 - drive from the internal combustion engine;
6 – viscous coupling drive;
7 - oil in the mechanism.

When the antifreeze in the radiator is cooled to the required degree, the bimetallic plate takes on its original shape, and the drain channel opens in the clutch. The working fluid under the action of centrifugal force moves into the reservoir, from where, if necessary, it begins to be pumped into the coupling cavity again.

The operation of the viscous coupling, if the working fluid is based on silicone, has two features:

  1. The connection between the discs is not only ensured by centrifugal force. The faster the driving element rotates, the more the silicone is mixed. From intensity it becomes thicker, which enhances the engagement of the disc group;
  2. As the liquid heats up, it expands, which increases the pressure inside the structure.

In the process of uniform movement of the machine, the motor runs at a relatively stable speed. Due to this, the fluid in the coupling does not mix intensively. But when the driver starts to accelerate the vehicle, there is a difference between the rotation of the driving and driven discs, due to which the working environment is intensively mixed. The viscosity of the liquid increases, and the rotational movement begins to be transmitted with greater efficiency to the group of driven discs (in some models, not one disc is used, but two sets, each of the elements of which alternates with each other).

If the difference in rotation of the disc packs is very different, the substance becomes almost solid, which leads to blocking of the clutch. A similar principle of operation has a viscous clutch, which is installed in the transmission of the machine instead of the center differential. In this arrangement, the car defaults to front-wheel drive, but when each drive wheel starts to slip, the spike in torque difference activates the clutch lock and engages the rear axle. A similar mechanism can also be used as a cross-axle differential (for more information on why the car needs a differential, read in another article).

Unlike the mechanisms used in the transmission, the modification for the cooling fan is equipped with a special reservoir in which the volume of the working substance is stored. When the motor is at the warming up stage, the thermostat in the OS line is closed (for details on the operation of the thermostat, see here), and the antifreeze circulates in a small circle. In cars that are operated in cold regions with frosty winters, for this purpose, you can use the ICE preheating system (read about it in detail separately).

While the system is cold, the drain valve located in the clutch housing is open and the rotating drive disc throws the fluid from the reservoir back into the reservoir. As a result, the viscous coupling does not work due to the lack of clutch between the discs. The fan blades do not rotate and the radiator is not blown. As the air-fuel mixture continues to burn in the engine, it heats up.

Fan viscous coupling: device, malfunctions and repair

At the moment when the thermostat opens, the coolant (antifreeze or antifreeze) begins to flow into the circuit to which the radiator heat exchanger is connected. The heating of the bimetallic plate (it is attached to the viscous coupling housing in the front, as close as possible to the radiator) is due to the heat coming from the radiator. Due to its deformation, the outlet is blocked. The working substance is not ejected from the cavity, and it begins to fill with liquid. The liquid gradually expands and becomes thicker. This ensures a smooth connection of the driven disc, which is attached to the driven shaft with the impeller.

As a result of the rotation of the fan impeller, the air flow through the heat exchanger increases. Further, the cooling system works in the same way as when installing a fan with an electric motor. When the coolant is cooled to the desired parameter, the bimetallic plate begins to take its original shape, opening the drain channel. The substance is removed by inertia into the tank. The clutch between the discs gradually decreases and the fan stops smoothly.

Device and main components

Consider what components the viscous coupling consists of. The device consists of the following key elements:

  • A hermetically sealed body (since it is constantly filled with liquid, this part of the mechanism must be sealed to avoid leaks);
  • Two packs of perforated or ribbed discs. One packet is the master and the other is the slave. Regardless of the number of disc elements in each package, they all alternate with each other, due to which the liquid is mixed more efficiently;
  • A dilatant fluid that transmits torque in a closed housing from one disc package to another.

Each manufacturer uses its own base for the working fluid, but often it is silicone. When an organic liquid is vigorously stirred, its viscosity rises to an almost solid state. Also, modern viscous couplings are presented in the form of a drum, the body of which is attached to the impeller with bolts. In the center of the body there is a freely rotating shaft with a nut to which the drive pulley or the motor shaft itself is screwed.

A little about the use of viscous couplings

In addition to the cooling system of some car models, the viscous coupling can be used in one more system of the car. This is a plug-in all-wheel drive (what it is and how such a car works is described in a separate article).

More often, modifications of such a transmission with a viscous coupling are installed in some crossovers. They replace the center differential, due to which, when the driving wheels slip, the group of discs starts spinning faster, which makes the fluid more viscous. Due to this effect, the drive disc begins to transmit torque to the driven analog. Such properties of the viscous coupling allow, if necessary, to connect the free axle with the vehicle transmission.

This automatic mode of operation does not require the use of sophisticated electronics. Of the other varieties, with the help of which the secondary axle can be associated with the leading one, this is the 4Matic all-wheel drive system (it is described here) or xDrive (this modification is also available separate review).

The use of viscous couplings in four-wheel drive makes sense due to their simple design and reliability. Since they operate without electronics and accessories, viscous couplings are cheaper than electromechanical counterparts. Also, the design of the mechanism is quite strong - it can withstand a pressure of up to 20 atm. There are cases when a car equipped with a viscous coupling in the transmission has worked for more than five years after being sold in the secondary market, and before that it also worked properly for several years.

Fan viscous coupling: device, malfunctions and repair

The main disadvantage of such a transmission is the late actuation of the secondary axle - the drive wheels must skid a lot in order for the clutch to lock. Also, the driver will not be able to forcibly connect the second axle if the road situation requires the activation of all-wheel drive. Plus, the viscous coupling may conflict with the ABS system (for details on how it works, read here).

Depending on the car model, the driver may encounter other disadvantages of such a mechanism. Due to these shortcomings, many automakers are abandoning the use of viscous couplings in all-wheel drive transmissions in favor of their electromechanical counterparts. An example of such mechanisms is the Haldex coupling. The features of this type of couplings are described in another article.

Functional Testing

Checking the viscous fan clutch is easy. According to the vehicle operating instructions, this must be done first on an unheated internal combustion engine, and then after it reaches operating temperature. This is how the mechanism works in these modes:

  • Cold system... The engine runs, the driver raises the engine speed several times for a short time. A working device will not transmit torque to the impeller, as the outlet must remain open and there is no coupling between the discs.
  • Hot system... In this case, depending on the temperature of the antifreeze, the overlap of the drain circuit will depend, and the fan rotates slightly. The revs should increase when the driver presses the accelerator pedal. At this moment, the engine temperature rises, the pump drives hot antifreeze along the line to the radiator, and the bimetallic plate is deformed, blocking the outlet of the working fluid.

The mechanism can be checked independently without diagnostics at the service station in the following ways:

  1. The motor is not working. Try to crank the fan blades. In doing so, some resistance should be felt. The fan must not coast by inertia;
  2. The engine starts. A small noise should be heard inside the mechanism for the first few seconds, which gradually dies down due to some filling of the cavity with the working fluid.
  3. After the engine has run a little, but has not yet reached operating temperature (the thermostat is not open), the blades will rotate slightly. We fold a sheet of paper into a tube and insert it into the impeller. The fan should block, but there should be some resistance.
  4. The next step involves dismantling the coupling. The device is immersed in boiling water so that its internal parts are heated. An attempt to turn the blades must be accompanied by resistance from the mechanism. If this does not happen, this means that there is not enough viscous substance in the clutch. In the process of this work, you can additionally dismantle the heat exchanger of the cooling system and flush it.
  5. Check for longitudinal play. In a working mechanism, this effect should not be, since a constant gap must be maintained between the discs. Otherwise, the mechanism needs repair or replacement.

It is not necessary to carry out further checks if at some stage a malfunction of the fan is detected. Regardless of whether the clutch needs to be repaired or not, there is always a need to service the cooling system at the end of the summer season. For this, the heat exchanger is removed and any contamination in the form of fluff, foliage, etc. is removed from its surface.

Symptoms of a malfunction

Since the fan in the engine compartment is designed for forced cooling of the motor during its operation, overheating of the power unit is one of the main signs of a clutch malfunction. It should be noted that this is also a symptom of failure of other elements of the cooling system, for example, a thermostat.

The motor will overheat due to the fact that a leak has formed in the clutch, and the fluid either poorly transfers rotational forces between the discs, or does not provide this connection at all. Also, a similar malfunction can manifest itself as a result of untimely operation of the bimetallic plate.

Fan viscous coupling: device, malfunctions and repair

When the clutch does not engage properly, the impeller stops rotating or performs its function with minimal efficiency, no additional flow of cool air is supplied to the heat exchanger, and the motor temperature quickly rises to a critical value. If the car is in motion, the radiator is blown efficiently, and forced airflow is not required, but when the car stops, the engine compartment is poorly ventilated and all mechanisms and assemblies are heated.

Another sign of a viscous clutch problem can be identified by starting a cold engine and seeing how the fan behaves. On an unheated unit, this mechanism should not rotate. the opposite effect is observed when the working substance loses its properties, for example, it solidifies. Due to the longitudinal play, the discs can be in constant engagement with each other, which also leads to the constant rotation of the blades.

The main causes of the fault

The key reason for malfunctions with the operation of the viscous coupling is the natural wear of the mechanism parts. Therefore, each manufacturer establishes a certain schedule for the scheduled maintenance of vehicle mechanisms. The minimum working resource is from 200 thousand kilometers of car mileage. In the aftermarket, a car with a viscous fan will always have decent mileage (you can read about how to determine if the mileage on a used car is twisted) in another article), so there is a high probability that attention will need to be paid to the mechanism under consideration.

Here are some other reasons for the failure of the viscous coupling:

  • Deformation of the bimetallic plate due to frequent heating / cooling;
  • Bearing breakage due to natural wear;
  • Broken impeller blade. Because of this, runout is formed, which accelerates bearing wear;
  • Depressurization of the case, due to which a leak of the working substance occurs;
  • Loss of fluid properties;
  • Other mechanical failures.

If the driver does not monitor the cleanliness of the mechanism or heat exchanger, then this is another reason for the failure of the device.

Fan viscous coupling: device, malfunctions and repair

The control of the moment of activation of the mechanism must be carried out at least once a month, especially in summer, since the motor especially needs cooling during the hot period. Even if the new viscous coupling does not do its job well, perhaps there is a reason to install an electric more powerful analogue. By the way, some motorists, for greater effect, install an electric fan as an auxiliary element.

How is the repair done

So, when the driver notices that the car's engine has begun to overheat more often, and other parts of the cooling system are in good working order, a viscous coupling should be diagnosed (the procedure is described a little higher). As we have seen, one of the device breakdowns is a silicone leak. Although the user manual indicates that this fluid is poured into the mechanism once at the factory, and it cannot be replaced, the motorist can independently replenish the volume lost as a result of depressurization or replace the fluid with a fresh one. The procedure itself is simple. It is much more difficult to find the right working substance.

In stores, these products are sold under the following names:

  • Fluid for repairing a viscous coupling;
  • Oil in the viscous clutch;
  • Silicone substance for viscous couplings.
Fan viscous coupling: device, malfunctions and repair

Special attention should be paid to the repair of the viscous clutch, which is used in the connected all-wheel drive system. In this case, it is recommended to select a new fluid in accordance with the type of substance used before. Otherwise, after the repair, the transmission will not connect the second axle or will work incorrectly.

To repair the viscous coupling, which is used in the cooling fan drive, a universal analog can be used. The reason is that the torque transmitted through the discs of the mechanism is not as great as in the transmission (more precisely, such a large power take-off is not required in this case). The viscosity of this material is often sufficient for the operation of the mechanism.

Before proceeding with the repair of the coupling, it is necessary to check how much silicone fluid is in the device. For each fan model, a different volume of substance can be used, therefore information on the required level should be found in the user manual.

To add or replace fluid in the clutch, you must:

  1. Dismantle the mechanism from the car, and remove the impeller from the clutch;
  2. Next, you need to put the product horizontally;
  3. The pin behind the spring-loaded plate is removed;
  4. There must be a drain hole in the coupling housing. If it is not there, then you will need to drill it yourself, but it is better to entrust this procedure to a specialist so that the discs are not damaged;
  5. After these procedures, about 15 ml of liquid is pumped through the drain hole with a syringe. The entire volume must be divided into several portions. In the process of pouring, you need to wait about a minute and a half for the viscous substance to be distributed in the gaps of the discs;
  6. The mechanism is reassembled. To keep the device clean, it must be wiped off, removing the remaining silicone substance from the surface, which will contribute to accelerated contamination of the case.

When the driver hears a fan noise as it rotates, this indicates bearing wear. The replacement of this part is carried out in the same way as filling the liquid, with the exception of a few additional manipulations. In this case, the liquid itself must be replaced with fresh one.

To remove the bearing from the housing, you must use a bearing puller. Before doing this, it is necessary to remove the flaring along the edge of the mechanism housing (it prevents the bearing from falling out of the seat). It is not recommended to dismantle the bearing using any improvised means, since in this case damage to the contact surfaces and discs cannot be avoided. Next, a new bearing is pressed in (for this, you must use the option with a closed socket with the appropriate dimensions).

The repair process should in no case be accompanied by large efforts on one of the shafts of the device. The reason is that even a slight deformation of one of the discs is enough, and the clutch will be unsuitable for further operation. During the repair process, you may notice that there is a thin film of lubricant on the device. It should not be deleted.

As practice shows, most motorists who decided to independently repair the fan viscous coupling have difficulties associated with assembling the mechanism. In order not to confuse what to connect where, it is better to capture each stage of disassembly on a camera. This will provide step-by-step instructions for reassembling the device.

As mentioned a little earlier, instead of a fan with a viscous coupling, you can install an electrical analogue. This will require:

  • Buy a fan of suitable size with an electric motor (often these components of the cooling system are already sold with a mount on the radiator);
  • Electrical cable (minimum conductor cross-section must be 6 square millimeters). The length of the wiring depends on the size of the engine compartment. It is not recommended to run wiring directly or near vibrating or sharp elements;
  • 40 amp fuse;
  • Relay for turning on / off the fan (the minimum current with which the device is capable of operating must be 30A);
  • Thermal relay that operates at 87 degrees.

The thermal relay is installed on the radiator inlet pipe or you need to glue it to the metal part of the pipeline, as close as possible to the thermostat. The electrical circuit is assembled by analogy with the VAZ models (the diagram can be downloaded from the Internet).

Selecting a new device

Like the selection of any other part for a car, the search for a new viscous fan coupling is not difficult. To do this, you can use the services of online stores. Even if the device offered by this or that store is too expensive, you can at least find out the catalog number of the mechanism. This will make it easier to find the product on other platforms. By the way, many online car dealerships offer both original parts and their counterparts.

It is best to look for original products by VIN-code (about what information about the car it contains, as well as where to find it in the car, read in another article). Also, in the local auto shop, the selection can be carried out according to the car data (release date, model, brand, as well as the characteristics of the motor).

Fan viscous coupling: device, malfunctions and repair

An important factor when choosing any device, including the viscous coupling of the cooling fan, is the manufacturer. When buying many auto parts, you should not trust the packing companies, but this does not apply to viscous couplings. The reason is that not so many companies are engaged in the manufacture of these products, therefore, in most cases, the product will be of the required quality, and the cost will differ from the original. Such firms usually supply couplings to factories that assemble vehicles.

Noteworthy are the products of the following manufacturers:

  • German firms Behr-Hella, Meyle, Febi and Beru;
  • Danish manufacturer Nissens;
  • South Korean company Mobis.

Special attention should be paid to the products of recently entered the market of Turkish and Polish manufacturers. If there is an opportunity to choose another manufacturer, then it is better not to be tempted by the budget price. To determine the reputation of a company, it is enough to pay attention to its assortment.

Usually, worthy viscous couplings are sold by firms that produce radiators and other elements of cooling systems for transport. If you have experience in purchasing a high-quality radiator, then you should first look for a suitable viscous coupling in the catalog of this manufacturer.

Advantages and disadvantages

Failure of the engine cooling system is always fraught with serious damage to the internal combustion engine. For this reason, in no case should one ignore the slightest symptom indicating a breakdown or imminent failure of one of the system elements. So that the motorist does not need to frequently go to the service station to overhaul the motor due to its overheating, which in itself is one of the most expensive procedures in servicing the car, manufacturers developing cooling systems have tried to make its components as reliable as possible. It is the reliability of the viscous coupling that is its main advantage.

Other benefits of this mechanism include:

  • A simple device, due to which there are few nodes in the mechanism that are subject to rapid wear or breakdown;
  • After a winter inactivity of the car, this mechanism does not need maintenance, like electronics, if the car was stored in a cold and damp room;
  • The mechanism works independently of the car's electrical circuit;
  • The fan shaft can rotate with great power (this depends on the speed of the motor and the size of the drive pulleys). Not every electric fan is capable of delivering the same power as the power unit itself. Due to this property, the mechanism is still used in heavy, construction and military equipment.

Despite the efficiency and reliability of the viscous coupling for the cooling fan, this mechanism has several significant drawbacks, due to which many automakers refuse to install a viscous coupling on the radiator fan drive. These disadvantages include:

  • Not every service station provides services for the maintenance and repair of these mechanisms, since now there are few specialists who understand the intricacies of the device;
  • Often the repair of the mechanism does not lead to the desired results, therefore, in the event of a breakdown, you have to change the device completely;
  • Since the fan drive is connected to the crankshaft, the weight of the device affects this part of the motor;
  • The mechanism is triggered not by electrical signals, like an electric fan, but due to the thermal effect on the bimetallic plate. Many motorists know that mechanical devices are not as accurate as electrical counterparts. For this reason, the viscous coupling is not activated with such accuracy and speed;
  • Some COs allow the motor to cool down for some time after it has stopped. Since the viscous coupling works exclusively by rotating the crankshaft, this option is not available for this device;
  • When the engine speed approaches its maximum, there is a decent amount of noise from the fan;
  • Some models of viscous couplings need to be refilled with working fluid, even if the manufacturer indicates that such a procedure is not needed by the mechanism. The difficulty in this case is to choose the right substance, since not all operating instructions indicate which material is used in a particular case (they differ in initial viscosity and the moment when the liquid changes its properties);
  • Some of the power in the power unit is used for fan operation.

So, the viscous coupling is one of the original solutions that provide forced cooling of the radiator. This mechanism allows you to save a little battery power or reduce the load on the car's generator, since it does not use electricity for its operation.

Often, the viscous coupling serves for a long time and does not require any special maintenance. You can diagnose problems yourself, and repairs, although they are not recommended by manufacturers, can be performed even by a beginner - the main thing is to choose the right replacement components and be careful.

In conclusion, we offer a short video on how the viscous coupling of the radiator fan works, as well as on the properties of the non-Newtonian fluid that is used in the device:

Cooling fan viscous coupling - the principle of operation, how to check, repair

Questions and answers:

How does a viscous coupling work in a car? During a constant speed of rotation of the shafts, the disks in the viscous coupling rotate in the same way, and the liquid in them does not mix. The greater the difference in the rotation of the disks, the thicker the substance becomes.

What is a viscous coupling on a car? This is a block with two shafts (input and output), on which the disks are fixed. The entire mechanism is filled with viscous material. When mixed intensively, the substance becomes practically solid.

What happens if the viscous coupling does not work? A viscous coupling is needed to connect a four-wheel drive. If it stops working, the machine will be rear-wheel or front-wheel drive (whichever is the default drive).

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