DIY repair of viscous coupling

In detail: do-it-yourself repair of a viscous coupling from a real master for the site my.housecope.com.

Since failures occur, many owners will be interested in learning how to repair a viscous fan coupling with their own hands. Many car owners with such a device advise not to rush to throw it away, but try to repair it yourself. According to many of them, this is not difficult to do.

To begin with, it is advisable to get acquainted with its device and the principle of operation, and only then proceed with the repair work. The reasons for which they begin to pay attention to the viscous coupling are usually engine overheating during normal operation of the thermostat and other elements of the cooling system.

Do-it-yourself fan viscous coupling repair often it is necessary to perform after a long idle time of the engine, its long-term operation, as well as for some other reasons. Its installation on an internal combustion engine has one goal, it is a smooth increase in fan speed as the engine warms up. With an increase in the operating temperature of the coolant, the fan speed should increase.

A viscous coupling is a device, the principle of operation of which is the possibility of transmitting rotational motion by means of a special fluid that is pumped into it. It can be thought of as a small circular chamber filled with silicone-based grease. There are two shafts inside this chamber, and the principle of operation is as follows. The crankshaft makes a rotating motion, which is transmitted to the first shaft of the clutch. With increasing loads on the disc, its rotation speed becomes higher, this leads to an increase in the viscosity of the silicone inside the coupling.

The clutch is blocked, which leads to the rotation of the second disc, on the shaft of which the radiator cooling fan is mounted. Another principle for turning on this device is the temperature properties of the injected silicone grease. These properties are used to turn on the cooling fan when the motor temperature rises. With an increase in temperature, it is turned on, and when it is lowered, it is turned off.

Video (click to play).

The widespread use of the clutch on modern motors is due to its high reliability and safety. In case of careless handling of it, when the hand enters the rotation zone, it stops without damaging the hand.

There are very few such signs, but they exist, let's try to understand them. Most manuals for the operation and repair of cars advise you to check how the fan rotates when the engine is cold and hot. It is also stated there that on a cold engine, no special changes in the rotational speed are noticed during gas overload, but on a hot engine, on the contrary, it increases.

It is necessary to check the longitudinal play of the fan of the clutch, if any, it requires treatment. The appearance of extraneous noise during rotation may indicate a failure of the bearings. Faults also include aging of the packing glands. However, the most common type of malfunction is leakage of silicone fluid from the coupling housing.

Since fluid leaks from the clutch most often, let's talk about it first. To replenish it, you need to remove the viscous coupling from the water pump, and then proceed to disassemble it. On the removed device, under the spring plate, there is a filler hole for the working fluid. On the removed coupling, the pin is carefully pulled out and the missing amount of liquid is injected with a syringe. Most often, it is recommended to pour transmission oil into the clutch.

When refilling fluid, place the sleeve horizontally with the hole facing up.Approximately 10-15 ml is drawn into the syringe. liters of transmission oil and without a needle insert it into the filler hole. Squeeze out the liquid and wait for some time without removing the syringe from the hole. After that, wipe off excess oil from the surface of the clutch housing and put the pin in its place. The product is ready for further use.

The signal for such an operation is the noise in the area of the radiator cooling fan. To repair it, the viscous coupling must be removed from the car. In most cars, it is mounted on a water pump pulley. This ensures reliable thermal contact with the coolant. It is necessary to unscrew the three bolts of its fastening, and it easily comes up from the engine compartment.

After that, you can start replacing the bearing. This operation can be performed only after complete disassembly of the unit and draining the liquid. In the future, it will be replaced with fresh one. The bearing must be removed only with a puller, otherwise the entire assembly may be damaged. After installing a new bearing, it is collected and filled with fluid.

Whatever happens, do not rush to throw away the old node and buy a new one, if it has not scattered at all. Do-it-yourself fan viscous coupling repair is an affordable operation that does not require special knowledge and tools. A bearing puller can be found from colleagues in the garage cooperative, the rest is not in short supply today.

Since the viscous coupling on my PASSAT ordered me to live a long time, I began to look for information about its repair. I found interesting information on the Internet. Added to myself. Maybe someone else will come in handy.

Yuri POLYAKOV,
truck driver, St. Petersburg

For a long time, one of the most incomprehensible parts of my truck was the cooling fan drive clutch, popularly referred to as "viscous coupling". The purpose of installing the clutch is to smoothly change the fan performance, depending on the ambient temperature and engine heating. But how exactly it works, and even more so - how it works, I must admit, for a long time had no idea.
The years passed. One truck changed another. Everything was spinning and did not create any problems. However, at certain moments I began to notice that the engine of my MAN began to warm up well in hot weather. There was no reason to sin on a thermostat or a clogged radiator. Overheating began only in the heat of the ups, when the speed of the oncoming flow of the oncoming air decreased noticeably. It became more and more clear that the problem lies in the viscous coupling.
I began to compare its rotation on a cold and on a warm engine. There seems to be no difference - in both cases it scrolls rather tightly.
I asked all my acquaintances who understood anything in these matters, about its structure and principle of operation, and contacted the technical centers. The opinions were very different, but, as it turned out, no one really knows a damn thing ...

Since there were no holes for filling the working fluid on my coupling, I decided to drill it in the body myself. Having carefully examined what was left of the old clutch, I chose a suitable place.
With the help of a syringe, he quickly filled the internal cavity and, having cut a thread in the hole, screwed in a screw as a plug, lubricated with a "thread lock" for tightness.

After analyzing the comments of motorists on forums and social networks, it became obvious that drivers have a very abstract idea of such a simple and interesting in many respects device - viscous coupling. So, read the details about the principle of operation, checking and self-repairing the viscous coupling.

Viscous coupling — it is a special device that rotates the cooling fan thanks to a special fluid. It has a round shape with a silicone base filled with grease; serves for smooth fan control. At first glance, the principle of operation will seem complicated, however, if you look at it, this is not so: the crankshaft rotates, transferring energy to the first shaft of the clutch.The device then accelerates, which makes the silicone inside it more viscous. The clutch is blocked, after which the second disc begins to rotate, on which the radiator fan is located.

The viscous coupling is used on almost all motors, since the device is reliable and safe. If a hand is inserted into a moving mechanism through carelessness or inexperience, the device will stop, thus preventing injury.

After a long period of inactivity of the car, the viscous coupling needs an oil change, as well as a check of the condition and operation in general. In addition, failure is possible due to wear or some other reason.

It is quite difficult to recognize a breakdown of a viscous coupling, but there are ways to check its performance.

Look at the speed of the device when the engine is cold and hot. In the first case, strange sounds are usually not observed, and the number of revolutions is correct. With a hot engine, the picture is different: extraneous noises are heard, and the rotational speed of the viscous coupling may not correspond to the norm.

Various noises are often caused by faulty bearings. Also, the cause of the device malfunction may be the gland seal, or the leaked special silicone fluid.

If you notice engine overheating, do not rush to do replacement viscous couplings. You may be able to repair the broken part yourself.

The most common cause of failure is silicone leakage from the base of the part. To fill in new fluid, you need:

Remove the viscous coupling from the water sediment, and then disassemble it.
On the disk of the device itself there is a plate with a spring, under which there is a hole for the silicone fluid. You need to use the utmost care to remove the pin, and then fill with a syringe with grease. Keep in mind that during such a repair, the part is laid horizontally.
It is enough to draw fifteen milliliters of oily liquid with a syringe.
Slowly pour the silicone inside.
Wait a few minutes without removing the syringe from the hole, so that the liquid has time to deeply flow into the viscous coupling.
Wipe the surface of the device from excess liquid if necessary.
Replace the pin and then install the part.

If you are not well versed in cars and do not know how certain parts work, it is better not to start repairing yourself. The point here is not the possible breakdown of parts of the vehicle, but the difficulty of putting everything back together.

Also, bearings are a common cause of viscous coupling failure. There is only one symptom of such a malfunction: various kinds of noises in the area of the cooling radiator.

To repair the device, the first step is to remove it. To do this, unscrew the three bolts that secure the part. After that, the viscous coupling can be easily removed from the engine compartment.
After removing the device, you can start replacing the bearing. Replace them only when you have disassembled the unit and drained the oil fluid. Use a special tool to remove the bearing - a puller. If you use the tools at hand, you can completely damage the node.
Having installed a new bearing, you can start installing the device. Do not forget to fill in new silicone fluid, which was drained before repairing the viscous coupling.

When you notice a "misbehaving" of a coupling, you do not need to change the entire part right away, as it can often be repaired. Special skills and abilities for this business will not be required.

The only difficulty that can arise is finding a puller to remove the old bearing. The tool is not sold in every car store, which makes it difficult to repair the viscous coupling on your own. If you have visited all the car dealerships you know and did not find a puller, ask your friends-drivers. The rest of the details are easy to find.

Not all such devices have a hole for filling oily liquid. If you are a "beginner", do not try to repair the device yourself. Experienced craftsmen make the holes themselves. You can, of course, also try drilling the hole at your own risk.
Do not use brute force when handling the disc. If the aluminum on the currency is bent, the viscous coupling cannot be repaired - only a complete replacement of the device.

Open the hood of the car and flip out some of the fan housing latches.
Unscrew the bolts with a 6-point hex wrench.
Remove the cooling fan.
Rotate the cover 180 ° to the right. There is no other way to remove the part. Therefore, it will not work to get to the viscous coupling.
Unscrew the viscous coupling with a 36 wrench. The tool jaws should be no thicker than 10 millimeters.
After taking out the device, clean it from dirt and dust.
Next, you need to rivet the bimetallic plate of the viscous coupling on one side.
Pull out the part disk and fill with a syringe with PMS-100 lubricating fluid.
Assemble the viscous coupling structure back; install the device in the car.

There are no fundamental differences in removing the device. You need to unscrew three screws securing the impeller and then three more to remove the cover.
Then, taking out the viscous coupling, carefully remove the upper disc, which serves to block the fan.
The entire device is lubricated with special oil. Repair the viscous coupling to preserve it. If you do not know the exact name of the liquid, it will be extremely difficult to find it.
Simply replacing the bearing will not work, since you first need to grind off the flare on the shaft.
Place the device in a vise for greater ease of repair.
The bearing also needs to be ground off. If this is not done, the base of the viscous coupling can be severely damaged.
Buy a sealed type bearing with no visible balls at any automotive store. Its cost is not more than 100 rubles.
Before seating the bearing, coat the seat with sealant.
Assemble the viscous coupling and then install it in the car.

The name of such a detail, and even more so how it looks, is not known to many. Basically, car mechanics and mechanics are engaged in repair and replacement. Experienced craftsmen make repairs with their own hands, for example, for a rigid connection with a fan shaft, holes are drilled in a viscous coupling, a thread is cut with a tap and bolts are screwed in. Let us examine in more detail what a viscous coupling is, signs of malfunctions, and methods of self-repair.

Viscous coupling, it is also a viscous muff (from the Latin viscosus - viscous) is a mechanical device that transfers torque using a viscous fluid.

In this video, see what is the clutch in the cooling fan device

The principle of operation of the viscous clutch is carried out thanks to the belt drive, which connects the clutch rotor and the crankshaft pulley of the internal combustion engine. Also, there are car designs in which the clutch is installed directly on the crankshaft or camshaft. Image - DIY repair of viscous coupling

Spring.
Bimetal plate.
Inlet B.
Intake duct A.
Camera.
Check valve.
Check valve.
Rear springs.
Front reservoir.
Rotor.
The body of the device.
Rotor shaft.
Bearing shell.
Rear reservoir.
Back plate.
Rotor.
Front plate.
Lid.

Due to the complex design of the device, the price of a viscous fan coupling is relatively high.

In cars where rare models of the clutch are installed, if it is impossible to repair it and find a new such mechanical unit, the owners have to slightly re-equip and install an electric cooling fan.

This video simply and clearly shows how to determine whether a viscous coupling is working properly or not. We do not miss the opportunity to glean useful information, look:

The resource of this device in the design of the machine is designed to cover 200 thousand kilometers. Image - DIY repair of viscous coupling

To check if the fan and clutch are working well, you need to know the symptoms of faults in these parts:

If the engine heats up quickly, then this is the first sign of a viscous coupling malfunction. This appears due to the release of the gel or the inopportune actuation of the bimetallic plates. In this situation, when the motor is very hot, the fan or rotates at a low speed, or does not turn on at all.
The cooling fan starts spinning when the engine is still cold. This is due to a change in the properties of the gel, solidification of the lubricant and the failure of other elements of the viscous coupling.

A resource has been developed.
Mechanical damage.
The bearing has worn out or broke prematurely.
End runout of the fan impeller. Part of the propeller blade is broken.
Lack of lubrication of the coupling shaft.
Dirty coupling joint.

More reliable fan with viscous coupling than electric fan. It often happens that after winter, the fan electric drives fail and the electronic control circuit of the propeller is disrupted.

The advantage of the viscous coupling is that there is no electronics or electricity in it.

The viscous coupling provides more power, the fan rotates at a higher speed. If the clutch sits rigidly on the engine crankshaft, then the rotation speed is very high.

The power of the clutch connected to the crankshaft is several kilowatts (kilowatts). And the power of electric fans has a capacity of several tens of watts. Due to this, it is the viscous coupling fan that is used in agricultural machinery, military equipment, and special equipment.

Complex structure, more difficult to make repairs. It is cheaper to buy a new assembly.

Has a good mass. The weight on the ICE crankshaft is significant.

The temperature control of switching on is not entirely correct. There are errors in turning the cooling fan on and off.

There is an increased noise level at high revs.

According to the schedule, you have to refuel with gel or oil with the desired viscosity.

The viscous coupling takes up part of the engine power.

This list of viscous coupling shortcomings influenced the decision of the owners of auto concerns. They abandoned the use of such devices in passenger cars.If this device stops working, then first you need to fill it with gel or special oil.There are not many specialists who can fix a viscous coupling. Sometimes it’s cheaper and faster to buy a new device or switch to an electric fan.

Replacing the old one and installing a new viscous coupling is an easy job, even a beginner can do it. After dismantling the device, it is necessary to remove the fan impeller. Install a new clutch.When installing a new coupling, the bolts must be tightened firmly, but without stripping the threads. Then you need to install the fan in place.

Greetings to all owners of foreign cars who, instead of an electric fan, have a radiator cooling fan with a viscous fluid under the hood - this article is for you. When this part stops working properly over time and the engine starts to overheat, many drivers have no idea what to do with it. Many not rich drivers, going to the store and finding out its price (from $ 150 and above), try to repair their viscous fluid, but having disassembled it, they simply understand that they cannot restore it, and like most drivers, they begin to claim that it is not maintainable. But in this article we will nevertheless prove the opposite, and also prove that with a skillful approach and the necessary knowledge, EVERYTHING can be repaired.

The name of the viscous coupling.

Let's start with the simplest thing - with the name of the viscous coupling. Most of the drivers who tried to repair this part probably never thought about the name of the part itself, but in vain. The name itself speaks for itself - viscous clutch, the name confirms that such a clutch is a device that transfers rotation to the fan due to the frictional force (viscosity) of a special viscous liquid, placed between two disks - master and slave. The driving disc is attached to the engine shaft (more precisely, to the pump pulley), and the radiator cooling fan itself is fixed on the driven disc. All this is assembled into a case and supplemented with several details, but more on that later.

And the whole process of normal operation of the viscous coupling is to glue these discs (so that the fan starts to rotate at high speed) at the moment when the engine temperature begins to rise above normal, and to glue the discs (master and slave) when the engine temperature is lower than normal operating temperature (slow down the rotation of the fan). I think everyone understands this simple principle of work, and gradually move on to the details.

Coupling device.

Sectional viscous coupling.
1 - plate on which the stem (valve) presses, 2 - locking plate in which there is a hole for the passage of liquid, 3 - bimetallic spiral, 4 - cover, 5 - bearing housing, 6 - drive disc, 7 - liquid reservoir. Rubber O-rings are shown in yellow.

Controls the entire process described above, hot air from a heated radiator. After all, the viscous coupling itself is fixed close to the radiator and for good reason. At its end, a bimetallic plate is fixed (usually on rivets), most often in the form of a spiral. When heated from the hot air of the radiator (after all, the plate is very close to the radiator), the plate begins to change its shape (bend), and changing its shape, it presses on the thin rod, and the rod presses on the plate valve (see photo below), which opens the hole, to get between the disks (leading and driven) a special viscous liquid, which increases the friction between the disks and, as it were, glues them together. From this, the radiator fan, previously spinning slowly due to the slipping of the discs, begins to rotate at high speeds and effectively cool the radiator, reducing the temperature of the coolant and, of course, the car engine itself.

And the main reason for overheating of a car engine comes from the above, that is, since the engine overheats, it means that the fan speed is insufficient, and if the speed is insufficient, then this very viscous liquid is not enough to effectively glue both disks - the master and the slave. The liquid most often begins to come out (gradually) through the dry sealing ring of the rod (in the figure in the section it is shown in yellow), located in the very center of the coupling, but under the bimetallic plate.

A viscous coupling plate valve, which is fixed on a round shut-off plate (disc) and this valve closes the opening for the fluid passage.

To get to the sealing ring, it is necessary to carefully grind off from the sides one of the heads of the two rivets (so that later you can rivet back), which hold the bimetallic plate (but the plate is on older machines, and on more recent ones it is in the form of a spiral). Next, remove the plate and remove the stock with a ring. You can look for a ring for a new dear one, or you can pick up some kind of carburetor from a repair kit (by the way, in our city, O-rings of any diameter are sold separately).

Restoration of normal operation of the viscous coupling.

Before replacing the stem with a new sealing ring and the bimetallic plate, it will naturally be necessary to replenish the spent fluid. This liquid is silicone based and very viscous (similar to epoxy glue). You can find it on sale in large cities and its labeling can be different, depending on the manufacturer, one of the examples of the PMS is 100.

By the way, when the fan rotates and slips (when the motor is almost warmed up and the plate valve supplying liquid begins to open - it is shown in the photo on the left), the disks fidget between themselves, and the liquid that has begun to flow tends to glue the disks even better from this fidgeting, so how

Rarely on which viscous connections there is a special hole for filling, as a rule it is not. Some advise you to drill a hole in the case, and then after pouring the liquid, screw the screw there on the thread lock, you can do it this way (I will write on which couplings this should be done just below). Still, it's better not to add extra weight in the form of a screw, even one gram. After all, the fan speed is very high and the extra imbalance from the screw is undesirable. You just need to pour the liquid with a medical syringe through the stem hole. Of course, this will take much longer, because the hole is small, and the liquid is very viscous, but you will not need to drill or cut anything.

On some cars, for example Mitsubishi, there are no rivets to remove the bimetallic spiral, and the rivet, the head of which must be ground off (only from the inside), is the stem itself (with two sealing rings) that pushes and opens the valve. It is much more difficult to disassemble such a coupling without damaging the stem, and in this case it is better to drill a hole in the body and cut a thread in it. Liquid will be poured through the threaded hole, and after pouring, a screw is screwed on the thread sealant.

Filling can take a long time, so you will need to fix the syringe (for example, tightly inserting it into the hole with a funnel as in the photo) and gradually add liquid to the syringe during the day. But you can only act with a syringe, creating pressure with a piston for faster filling, and when you get tired of doing this, then you can use the funnel, filling it full.

I can't say for sure the amount of liquid to be poured, excuse me, since it is different for different car brands, and besides, I don't know how much liquid is left inside your clutch. Maybe it's not there at all, or maybe half the volume. If desired, you can find it in the manufacturer's documentation for your particular car.

But usually, for filling the viscous coupling of a machine with an average working volume, about 100 grams is enough. If you find the exact value in grams specifically for your car, then it will be necessary (so as not to overfill) to disassemble the vyaskuft, and carefully remove the remnants of the old liquid with a syringe and rags, and then collect and fill in a new one in the exact amount. Excess fluid is extremely undesirable, since if there is too much of it, then the centrifugal force will have nowhere to expel it when rotating from the cavity between the discs (when the motor is not warmed up) and your fan will rotate constantly at high speed, preventing the engine from warming up normally.

Vyaskomfta in the open state, and grooves for draining fluid from the cavity are visible (at revolutions).

By the way, when you disassemble the clutch, you will notice the impeller-shaped longitudinal grooves (see photo). These grooves are just needed so that when the engine of your car is not yet warmed up, and the liquid remains between the disks, and when the fan rotates, using centrifugal force, through these grooves, the liquid unnecessary at the moment is removed through these grooves (so that the disks slip between themselves and the fan rotated slowly while the car engine was warming up). And if you have already disassembled the clutch, then check if there is any dirt inside and in the grooves, and if there is, then rinse all the parts and assemble.

I advise you to tighten the screws holding both halves of the coupling onto the thread lock. We install the bimetallic spring in place, not forgetting to insert the stem with a new sealing ring into its place. If it is not possible to fix the bimetal plate in its place, because it often happens that the rivet head deteriorates, then you have to go to any car service and ask the argon operator Uncle Vasya to drop a drop of aluminum on the rivet and thus fix the plate.

After assembling and installing the viscous coupling in place, you should check its operation. This can be done in a simple way: roll a long roll of thick paper in advance, and then start the engine of your car. While the engine is not warmed up, substitute a roll of paper under the rotating fan (take care of your hands, after all, all craftsmen have gold), and try to stop the rotating fan with a paper roll. We managed to do it - the fan was stopped, so so far everything is fine.

Now let the engine warm up well to operating temperature, and then try again to stop the fan impeller with a paper roll. If this cannot be done, then everything is in order and the valve has worked, letting in liquid between the discs and gluing them together, which is why the fan began to rotate without slipping, at the same speed as the pump shaft and it now effectively cools the radiator, reducing the temperature of the motor. Overheating in this case is excluded. And the repairs have been successfully completed.

If someone finds such a repair difficult, and who does not want to bother with repairing the vyaskomft, or someone may not be able to repair it, then it will be possible to weld fasteners in the area of the radiator fasteners, and an electric fan can be fixed to these fasteners. But only in this case, it will still be necessary to fix the fan switch sensor in the radiator itself.

Not repaired yet.

And finally, a couple of tips. Many drivers, especially in the outback, will not be able to immediately find or order to bring silicone fluid, but it is very necessary to drive. Many "experts" can advise to drill a hole between the two halves of the part and thus fix the fan impeller and prevent overheating of the motor. It is categorically impossible to do this, since the parts deteriorate (the tightness will be broken) and if you manage to buy a liquid, then it will no longer help - this is in the first place. And secondly, after a few hours of operation, the iron bolt will simply cut off, and the parts will finally break and deteriorate, since jerks during fan operation are inevitable. The place of such a viscous coupling is only at the landfill.

Well, what to do if you still have to go, but there is no liquid? There is only one easy way to keep the parts until the liquid is found. We disassemble the clutch and insert the cut out circle of sheet rubber between the two discs, and then tighten both halves. Only the rubber should not be too thick, otherwise, when pulling the halves of the coupling, you can bend the discs. That's all - now, when the fan is running, you will have a soft (rubber) grip between the discs, which eliminates the breakage of parts from jerks.

But I want to warn you that this is a temporary repair, since by fixing the fan impeller in this way, it will now rotate at high speed with you, even when the engine is warming up.This means that the engine will warm up and reach operating temperature much more slowly, which is equivalent to a constantly open thermostat (or when there is no thermostat at all). There is no need to explain that this is not good and that trips on a cold engine are harmful, and you can read more about this here.

But I think that for any adequate driver, this will be only a temporary measure, and having bought a fluid, he will try to restore the normal operation of the viscous coupling, as I described in this article, or buy a new one - you choose; Good luck everyone!

I am writing a milestone time, about the fact that this topic was discussed milenon times!

The clutch is not repairable! It is not known where to look for such oil.
It remains only to throw it away.

Toyota Mark II E-GX100
2.0 Grande 1G-FE - Sold Out

Renault Duster 2.0 4 × 4 waiting

My temple was simply jammed, spun and cooled even at -15 overboard, so I drove for a while without a fan at all - it was very noisy.

Today I decided to return everything back, for some reason, having previously repaired the clutch. To begin with, I figured out the device and the principle of operation.

It is arranged simply, I will not go into details, I will only touch on the locking mechanism. It consists of a bimetallic spring fixed on the clutch housing and on a movable outlet from the clutch. The spring has a spiral design, but when heated, it does not twist, and does not unscrew - but pulls the movable pin up (if it is necessary to block) and down (when unlocked). Accordingly, the malfunction was that the spinach was clogged with mud and did not move at all.

Solution: It is necessary to carefully remove the spring, or pry it from below, pouring the brake air onto the spinning pin and develop the spinning pin. And that's all. The liquid inside is eternal, if it has not flowed out - so everything is fine, you shouldn't even climb inside.

At the moment, the clutch is working normally, when it is cold, it turns on for 10-20 seconds, then it turns off itself and remains in this state until the need for cooling.
So go for it. It's simple!

Radiator fan viscous coupling theory

How is the Toyota fan clutch and how does it work? Since this topic still raises questions at times, let's try to figure it out.

A belt-driven fan, usually combined with a coolant pump, has traditionally been installed on most longitudinal powertrain models. If the fan impeller was rigidly connected to the drive pulley, then its rotational speed would be directly proportional to the crankshaft speed - such cooling would be excessively effective, especially at high speeds and at low overboard temperatures. Therefore, to adjust the intensity of the air flow passing through the radiator, a viscous clutch is installed between the pulley and the impeller.

At low temperatures, the fan speed is minimal, which allows the engine to warm up faster and at the same time reduces the noise from the impeller. As the temperature rises, the fan speed will also increase.

The clutch rotor is rigidly attached to the coolant pump pulley. Along the circumference of the rotor disc, oblique teeth are cut, which act as a pump for pumping oil. The clutch housing assembly (bearing housing and front cover) rotates around the rotor on a bearing.

On both sides of the rotor, plates are installed that separate the working chambers from the reservoirs. The front (with inlets A and B and the return channel) is fixed to the rotor cover, the rear (with the return channel) is fixed to the bearing housing.

1 - bimetallic spring, 2 - bimetallic plate, 3 - inlet B, 4 - inlet A, 5 - front chamber, 6 - return channel, 7 - return channel, 8 - rear chamber,
9 - front reservoir, 10 - rotor teeth, 11 - bearing housing, 12 - rotor shaft, 13 - bearing housing, 14 - rear reservoir, 15 - rear dividing plate, 16 - rotor, 17 - front dividing plate, 18 - front cover ...

Working chambers are “labyrinths” formed by ribs on the rotor and on the dividing plates. The torque is transferred from the rotor to the housing by means of “internal friction” in the silicone oil.

A bimetallic spring mounted on the outside of the clutch housing moves the plate, opening and closing the intake ports and adjusting the oil flow based on the air temperature.

1. Cold air.
When the rotor rotates, its teeth through the return channels “pump out” oil into the front reservoir from both chambers and the rear reservoir. As a result, its amount in the chambers decreases, the transfer of force through the liquid decreases, and the fan speed becomes much lower than that of the main rotor.

2. Warm air.
Centrifugal force forces the oil from the front reservoir into the front chamber through the open inlet A. “Viscous friction” between the rotor and the front plate increases and the speed difference decreases.

3. Hot air.
Both inlet channels open, after which oil flows into both working chambers. The volume of fluid in them and "friction" are maximum, so that the transmission of rotation through the coupling is also maximum.

Note. Since the speed control is due to a change in the volume of silicone oil in the cavities of the clutch, its leakage inevitably leads to a decrease in the fan speed and possible overheating of the engine.

Video (click to play).

Some couplings of the early design did not have a rear reservoir. Since after stopping the engine oil flows down to the lower part of the clutch, here its level in the chambers increased significantly, and immediately after starting the engine, when the “friction” between the rotor and the plates is high enough, the fan speed increased too much. If there is a rear reservoir, the liquid level in the chambers with the engine stopped is lower, and after starting it drops faster - as a result, the noise level from the fan is reduced.