How To Refill A Mercedes Viscous Fan Clutch

[Todd Miller]

If you have an original Behr fan clutch that's no longer doing a good job of spinning the radiator fan, and the car is running consistently hot, or hotter than normal, I've just posted a video to my YouTube channel showing the process of putting fluid back into the clutch. I also show the disassembly of a clutch, so you can see what's inside.

https://youtu.be/wTl6UCOnH-I

[subforry]

sweet

[greazzer]

I just did this as well, and used 7,000 cST silicone oil. I could not find anything which said 5,000 vs. 10,000, so I landed in the middle. I got FT brand (Factory Team) directly from them, and their shipping was super cheap, like $3.50 for two bottles. A bottle is 2 ounces. 59 mL. so you cannot beat the shipping price.

[Todd Miller]

My suggestion is to fill in small amounts, in stages, re-installing the fan and going for a test drive for a while, and see what you think. If you put in too much fluid, especially using something heavier than 5,000, you might have a fan that is trapped in the "engaged" mode all the time, and it will be very noisy. Personally, I wouldn't use 7,000 because I think it's too thick.

The fan clutches that are the style of the one shown in the video are designed to coast when cold, at 25-30% of the water pump speed. Once there's enough hot air coming through the radiator to get the bi-metallic strip to begin moving outward, thus slowly letting up on the brass pin, which then slowly uncovers the fluid supply port, the clutch will slowly begin to have increasing internal friction, and will speed up the fan... but only as much as is caused by the amount of fluid that can get through the supply port. In this phase, fluid is continuously being recirculated back to the storage area at the front of the clutch, and then back into the work area where the friction disc is being spun by the water pump. So the clutch continues to slip. Once the bi-metallic strip gets to 62*C/145*F and is fully bowed outward, the clutch internals get full fluid flow, and the fluid continues to be recirculated. The fan's maximum speed is 3,000rpm, so if it reaches that speed, it will not spin any faster, even if the water pump is spinning faster than that... so even at full-fluid-flow, the clutch is still slipping.

[greazzer]

Do you have anything from the FSM, or other authority ? I looked everywhere and couldn’t find anything. What I found is folks using 5K and 10K and both stating it worked fine

[Todd Miller]

The big blue book only describes function, shows a nice cut-away, calls out that the fluid is silicone so that it retains it's viscosity at all times regardless of heat, and states that the clutch cannot be repaired.

I chose 5000 because it was very much like what I found inside the clutch, and I was concerned that anything thicker or using too much, would cause the fan to not coast.

[greazzer]

Many thanks for the follow-up.

I will keep looking around. I live in the sunny south and it begins to get "warmer" starting in late February or early March and definitely hot by the end of April. This past summer was especially toasty with many consequetive days over 100F. With the "real feel", on some days, over 110F.

[Todd Miller]

I have a much more detailed post with photos, over on Vintage forum in BenzWorld. I didn't post all that here, because it's more tedious to post photos. I was learning as I was going, and the site won't let me edit the posts so that I can refine it all, but the info is all there.

I'm in SoCal, between SF and LA, about 10mi in from the coast. Other than about 3 months, in the winter, and sometimes not even that, it's 75*F/24*C here every day, or hotter. It's not your level of (insane) heat, but it's hot enough that a car with a comparatively undersized radiator, no fan shroud, and fairly "generic/standard" fan blade shape, will definitely run hot.

I've owned my Coupe since early '92, and it's always had a pretty "active" temp gauge needle. By "active" I mean that it would always sit above "80" (it's a model sold in Germany), raise higher than that in slow, or stop-and-go traffic downtown, and would also creep up, sometimes significantly, on the freeway at speed in real heat, or when climbing our local freeway grade to head north.

Out of all the things I've done to make sure the cooling system was working at it's maximum capacity, (having the radiator rebuilt twice in that time, 80*C OEM thermostat, replaced the cylinder head with a much nicer head that I had rebuilt, flushed block with head off, etc.) this is the first time I've had a temp gauge that sat on "80" just at the top of the "0", and was rock steady other than when climbing the local freeway grade (it moved to just under the next white painted box).

I actually tried an aftermarket replica fan clutch about 2 years ago, and that clutch seemed too active, because it made a ton of noise above 2,000rpm, all the time. But, the temp was finally showing in the normal range on the gauge. I took that clutch off, because I couldn't stand the roaring engine sound, put my OEM fan back on that I'd purchased in the mid-2000's when you could still get them for normal money, and as expected, the gauge returned to reading high.

That sent me on a little quest to learn more about how the clutch worked, which lead to learning about "dripping" silicone fluid into it over a loooooong period of time. I purchased the 5000 that you saw in the video, and never did the job, because I was driving the car so infrequently these past couple years, and only just around town for short drives. Finally put my big-boy pants on and decided to tackle it once and for all.

I did the drip method a couple weeks ago, and that resulted in a temp gauge needle that was about half-a-needle width, to a full-needle-width lower, and the fan was not any noisier. Huge improvement! So I decided to take apart a junk fan that I had, and see what magic makes them work, and see if I could figure out a way to refill them without it taking forever. And so here we are.

The way the fan works (in my opinion) makes it tough to diagnose, both whether or not it's failing due to fluid leaking out (they leak out of the pin and out of the input shaft) or whether the amount of silicone put in them is enough. That's why I suggested filling yours in stages.

Unlike how the clutched fan works on something like a Chevy small block (I have a 1990 GMC Surburban), these aren't really noticeably active at first start, and then become quiet. The Big Blue Book says they should do that, but mine never has, even now that it's filled. The fan in my Suburban roars on first start, and it takes about 1/2mi of driving before it will suddenly become silent. It's not heard again until another really cold start after sitting a long time, OR until the outside air temp gets to about 100*F, OR if towing at a lower speed, with less air flow, like up a winding 2 lane road, going camping, and the outside temps are 90-something. Then it will roar again.

These fans (the fan clutch) are silent all the time, but above about 2,500-3,000rpm they roar, even when cold. And on a hot day, you can pull over, shut the engine off, open the hood and try to rotate the fan, and it's freely spinning with almost no friction.

The Big Blue Book says the bi-metallic strip is fully activated (bowed fully outward toward the radiator, thus releasing the brass pin, thus fully opening the fluid supply port) at 62*C/145*F of heat exposure from the air through the radiator. So you'd think that at 180*F coolant temp in the top tank of the radiator, that would likely produce full traction inside the clutch. It doesn't seem to. And with a long 6 cylinder, the back 2 or 3 cylinders are going to be running much hotter than the front of the engine, so it seems risky to push the coolant temp, just to see if the fan clutch will truly engage.

So what I finally decided to do, was to just use a syringe, force in about 3ml of silicone (along with what I had dripped in), and see what happens. Hence the video.

I know have a working fan, but I don't know: did I put in too much, or too little.

[greazzer]

What I did was perhaps overkill. I put the clutch in the oven with my wife's approval and with the fill hole opened up (cover screw removed). I heated it to 170F which was the lowest I could get it. I did the drip method. I took my 59mL bottle and put that in boiling water so I know it was very near 212F. Decent gloves. I put maybe 1/2 the bottle but then noticed it stopped being "sucked" into the body of the fan.

Not sure how to "know" how much to use, but there is probably a way to take a junk one, and measure very accurately the inside volume. From there, you would know the max. However, I am in the same boat where I do not know how "much" to put in it. I am positive there is an optimal amount, just like filling your crankcase.

As for what cST, I know there is a range when it comes to what folks are using. I have seen as low as 3K and the high is 10,000K. I arbitrarily picked the middle ground. I do know this -- It will not be easy to remove whatever I put in it.

[Todd Miller]

I initially did the drip method, with it flat on the counter, fan still attached. When it stopped taking drips, and was sort of acting like it was full, I'd pick it up, holding it vertical by the water pump mounting flange, and spin the blades for a bit, and hold the blades and spin the hub. That was allowing more drips in, and seemed to work best if I held the assembly tilted face up, at about a 45-60 degree angle (guessing) while doing the spinning/rotating.

I could definitely use the junk clutch that I've taken apart, to measure. The first plate in the clutch is sealed tight against the face plate of the clutch, and the outer clutch housing, so that area becomes a sealed storage area for the fluid. The fluid remains in that area, until there is some amount of bow in the bi-metallic plate, allowing "some" amount of opening(uncovering) of the fluid supply port.

So it would be easy to just put the brass pin into that front cover, lay it face down, and fill it with the 2nd plate removed, until whatever I put it in, is about flush with where the 2nd plate sits. The only reason why I haven't done this, is because I don't know for a fact, that the storage area is completely filled. Maybe, with the clutch assembly held in the horizontal plane, as it would be in the car, the storage area is only filled with an amount that winds up being "half way full"... in other words, it's filled to just below the hole for the pin, so that fluid isn't always weeping past the pin, making an oily mess into the cooling fins on the clutch, which just packs the front of the clutch with oily grit. And that "half full" amount, translates to good "traction" in the working compartment, because it is slung outward and remains in the location where the friction material is on the working disc/friction disc. All the oil is doing is "filling" the very thin gap on either side of the friction material surfaces, in order to create traction/drag/friction. Those gaps are probably only about .25mm/.010" (just a guessing by what it looks like in there).

The mystery is: How does the fluid that's being slung into the circumference, regardless of the amount, get recirculated back to the storage area, which is what the clutch is constantly doing when the fluid port is opened at all. Based on the drawing in the Big Blue Book, which contains and inset detail off the fluid return port, the fluid "escapes" around the outer edge of the friction disc, gets to the forward facing side of the disc which is the side that has the narrow friction material, and then is channeled out a very tiny port and back into the storage area. The tolerance at the outer edge of the friction disc is very small/narrow, so it's sort of amazing that it works like this... and that's why I have concerns about using a fluid that's too thick; that the fluid will flow out of the supply port, which is a large diameter, and through the similarly large ports in the friction plate's middle area, but then get's trapped in the working compartment and can't return to the storage area, or winds up being extremely slow to return.

It cracks me up how simple these are inside, and yet how sort of mysterious they are, in how they work. They're really a masterful piece of engineering, and I'm guessing it's a concept that was probably from systems used in aircraft during WWII... "fliud clutches." But I'm just guessing.

I hope you'll report back on how your fan performs, both here, and in the comment section of the video, as it will help others.

[greazzer]

Again, many thanks. I recommend contacting one of the forum moderators and making your thread and research a "Sticky" You are perhaps one of the first individuals to do a deep dive into this.

[Todd Miller]

You're welcome! Hopefully it helps others who have cars that run consistently on the warm/hot side, and who don't want to spend the money for an OE Behr clutch.

I colored the factory diagram to show cold operation, and hot operation mode:

[Todd Miller]

Here are shots of the clutch disassembled. You can see the little return port off to the outer edge of the 1st disc, and it has a little black rubber plug with a spring in the center. The spring is pushed inward by the square boss of the front plate and if you look at that front plate, you'll see a clean mark where the spring has been touching. The plug doesn't come out of that hole. It stays there, and is being pushed onto the surface of the friction disc where I guess it might act like a squeegee and help fluid get through that little cut-out in the hole where the plug sits. The 4 big outer holes in the friction disc line up perfectly with the supply port in the 1st disc, so that might be how fluid gets to the back side of the friction disc to where the large friction material is bonded. I don't know what the other large "inboard" holes are doing, other than that they must also be involved in fluid transfer. The clutch is speed sensing in some way; maybe that's just in the amount of traction that the factory fluid/fluid amount provides vs. the wind load on the fan blades, or maybe it has to do with these inboard holes... I'm not sure.

Until reading the description in the Big Blue Book, I guessed that the clutch might supply one side of the clutch with fluid, and then the other side of the clutch with fluid, in order to create traction either on the small sized friction material, for slower rotation speeds, and then to the larger friction material, for full-on hot running and highest rotation speed. But the book just says that "the working area" gets fluid on both sides of the friction disc, and that fluid creates an amount of friction based on how much fluid has passed through the port, which is based on how much the port gets opened by the bi-metallic strip.

If you look at the first 2 photos, especially the 2nd photo, you can "read" the fluid flow. The slow moving fluid creates a dirty environment, so in the 1st photo, showing the inside of the storage(front) compartment, the fluid, and the metal is stained dark, and this was sort of a coating when I wiped it all out. Then in the 2nd photo, which is the interior of the front half of the "working area"(what the book calls it), you can see that the fast moving fluid is keeping the back side of the front disc clean, except where there's not much fluid action: The very center area, AND in the "flow shadow" behind that rubber plug. It looks like the fluid builds up in front of that plug, and is sort of forced back into the storage area, through that little cut-out next to the hole for the plug.

On a related note about how much fluid these may have been filled with: That first plate has a large opening in the center, and that opening is not sealed against the protruding center hub of the friction disc. There's not only a gap all the way around, the hole has a bevel on the inner side, that matches the bevel on the center hub of the friction disc. So my guess is: The amount of fluid fill, in the storage compartment, is probably just what that compartment will hold, with the clutch horizontal(as installed on the engine), and the larger box area where the port's opening/closing finger lives up at the 12 o'clock position, and then the fluid level reaches just to the bottom of that large center opening. That way, when the engine is not running, fluid won't spill over through the center opening and into the friction clutch area (working area). But who knows. There's no check valve in that little rubber plug/spring part, so if the fan stopped with that in the 6 o'clock position, fluid would flow back into the working area. The Book talks about this indirectly, by saying that the fan will be engaged for 1-3 minutes, after a cold start where the car has been sitting for a while... and that at some point during that 1-3min, the fluid will be circulated to the storage compartment and the fan will become silent, and then only coast along at 25-30% of the input speed from the water pump.

[greazzer]

I always assumed because there is a spinning mass, with a slot which opens when the temp goes up, e.g., thermostatically controlled, the fluid would sling outwards. When the fan slowed down, I assumed that the fluid would "contract" and go back through the slot. Because heat would not dissapate that quickly, the slot would remain open long enough for the fluid to go back to its original location. Not sure if that is an over simplificaiton since the circular motion slings the fluid out, and the fluid "escapes" through a slot or opening which opens with increased temps. When the mass stops spinning, the fluid retracts back.

Again, many thanks for the very thorough research.

[Todd Miller]

I think fan clutches spin so fast at all times, that there is always a centrifugal force acting on the fluid. And based on what the book is saying, the silicone doesn't change its viscosity due to either hot or cold.

Based on what I see in the diagram, and in how the parts look when taken apart, the clutch may be using channeling to get the fluid to move into the working area, and then mass of the fluid and centrifugal force to return it to the storage area. And the fluid is always in "return" mode due to that. They just close the port via temperature, and stop that returned fluid from escaping, via the channeling.

So in function, the fluid is just "waiting" to exit the storage area, and once in the work area, it's being shoved back into the storage area. Then when things cool, and the port is closed off, there's no more "exiting" happening, and the fluid winds up being almost completely "drained" out of the working area. Then when the engine is off, there might be a little fluid creep and equalization between the 2 compartments, so the fan has some traction on initial start, but because the port is closed, all there is, is, "return the fluid" mode. It's an amazing little piece of engineering.