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Pelican Technical Article:

Replacing Cooling Fan Clutch

Mike Holloway


30 minutes30 mins


$160 to $600




10mm wrench, slot face screwdriver

Applicable Models:

Mercedes-Benz R107 (1972-89)

Parts Required:

Fan clutch

Hot Tip:

Wash the engine compartment down

Performance Gain:

Better performing engine from thermo-management

Complementary Modification:

Replace hose, water pump, thermostat

Before you do any work on your car it is important that you wear safety glasses, work gloves and dispose of all fluids in a safe manner. Coolant is poisonous and should be treated with great care. Animals and small children have been known to die from ingesting coolant. If you have to jack up your car, make sure to use jack stands and chock your wheels as well as applying the parking brake. Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

The Mercedes Benz 450SL is considered by many to be one of the most attractive cars Mercedes produced. It is a stylized combination of power, grace, and comfort and was sold under the model designation R107. At over 3,600 pounds and designed to meet (and actually exceed) strict safety regulations, the 450SL was nicknamed "der Panzerwagen", which means "the armored car", by the engineers who designed it. The 450SL was produced from 1973 through 1980, after which the R107 became known as the 280, 380 and 500 SL.

The SL variant was a 2-seat convertible/roadster with standard soft top and optional hardtop and optional folding seats for the rear bench. The designation SL derives from the German Sport Leicht, or Sport Lightweight and was first applied to the infamous gullwing 300SL. The 450SL was the third generation SL. The SLC (C107) derivative was a 2 door hardtop coupe with normal rear seats. The SLC is commonly referred to as a 'SL coupe', and this was the first time that Mercedes-Benz had based a coupe on an SL roadster platform rather than on a sedan, replacing the 280 and 300 SE coupé.

The robust, V-8 powered coupe is a joy to drive and many drivers test the power of the 450SL. While the speed off the line is questionable, the top end performs smooth and responsive giving the driver a sense of security. The 450SL cruises comfortably at speeds in excess of 75 mph. Of course that means that the engine will heat up. That heat has to be managed and that is where the cooling system comes in.

While overheating may be due to a failed water pump or thermostat, it may also be due to a failed cooling fan clutch. A slipping fan clutch can be the cause of overheating. After you have checked the coolant level and before you replace the thermostat or water pump, check the condition of the belts. There are a few ways to quickly check the cooling fan clutch. You may find references of the set-up referred to as a Viscofan.

The fan clutch can fail for a few reasons. The fan clutch can slip, which would prevent the fan from cooling the engine at low speeds. Or the drive can be seized, which can be noisy as well as waste engine power because its turning when not required. The fan speed should decrease as the engine RPM decreases and should increase as the engine RPM increases.

There are a few ways to find out if the clutch needs to be replaced. The fan is not designed to spin upon startup when the engine is cold. When the engine heats up the fan clutch engages. The fan clutch uses a special type of metal combination that changes its shape as it heats up. As the hot air coming through the radiator reaches certain temperatures it causes the bimetallic strip on the front of the fan to change shape and allows the oil in the clutch to circulate and turn the fan. This allows the fan to move with the engine speed. If the fan clutch is not working correctly, then the fan will either not cool correctly or spin at high RPM when not needed. Either way it is not working correctly and needs to be fixed. A bad fan clutch won't engage and the engine temperature will increase.

There is an easy way to see if the fan clutch needs replacing. Try this simple process. It requires a single sheet of newspaper.

Start your engine.

While the engine is cold, take the single sheet of rolled up newspaper (roll it up so that it is several layers thick, but not so stiff as to support the weight of a small child).

The fan may be moving, if so try to stop the fan with the rolled up newspaper page. Keep in mind, using USA Today may stop anything moving: its been known to do the same with folks checking out of a hotel only to be distracted by the front page and the interesting graphics located in the lower left corner. Best to use the New York Post. Not many folks will mind the shredding.

The fan should stop turning when the motor is cold. That suggests that the special bimetallic strip has not changed in volume due to increased temperature. When your engine reaches operating temperature, you should not be able to stop the fan with the paper. Instead, the fan should cut the paper, tearing it. If you decided to use a page from the New York Post, you can only hope a shower of confetti will be created. While this isn't a celebration of discovering that the fan clutch has failed, take solace in knowing that you have done a small part in removing yet another page from circulation.

If you don't want to take the chance of tearing a sheet of your favorite daily read, there is another method that has been used to establish fan clutch functionality. Try this:

place a portion of cardboard in front of the radiator to block the air flow through the radiator.

Start & run the engine until the engine temperature reaches above operating temperature. This may take several minutes.

After the engine has come to operating temperature, turn off the engine and see how easily the fan turns.

If the fan turns freely or without much drag, the clutch needs to be replaced. If the fan is hard to turn, but will turn by hand pressure, it's ok. Keep in mind that if you feel some play in the fan that is not an indication it needs to be replaced. If there is no more then a quarter inch of play on the outer blades and it passes the newspaper test, then that's acceptable.

There are four 10mm bolts holding the back of the Viscofan to the front of the cooling pump. You do not need to remove any of the drive belts to replace the fan but you will need to remove the fan shroud. It's easy to remove: refer to the article on removing the fan shroud.

Using a 10mm wrench, loosen the 4 bolts that hold the fan clutch to the engine.
Figure 1

Using a 10mm wrench, loosen the 4 bolts that hold the fan clutch to the engine. You will need to hold the fan in place while unscrewing the four 10mm bolts. While there are lots of different tools made to do this, the simple approach is to use a large screwdriver between the fan center and one of the bolts. They are not held on under a lot of torque and should easily come off. Do NOT attempt to place anything between the fan blades to keep it from turning, as you can easily damage the fan and yourself.

Figure 2

With a gentle tap of a rubber mallet, the Viscofan will pop out of the engine bay

Using a 10mm wrench you can undo the four 10mm bolts holding the fan to the clutch assembly and replace the clutch.
Figure 3

Using a 10mm wrench you can undo the four 10mm bolts holding the fan to the clutch assembly and replace the clutch.

The clutch is now free.
Figure 4

The clutch is now free. Installation is the reverse of removal. 

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Comments and Suggestions:
Graham Comments: Could not edit my question, but actual centrifugal fan rpm is of course dependent on ratio of drive pulleys. But so far as I know, temperature is not a factor. Just cuts out audibly at about 3500rpm engine speed.
June 27, 2016
  Followup from the Pelican Staff: See my earlier reply. - Nick at Pelican Parts  
Graham Comments: Nick - That would mean that depending on temperature, the fan does not always run at same speed as engine. Suggest that you check that out and report back.

Might be an idea to remove the part about sticking newspaper into fan - you could get sued if someone injures themselves.
June 27, 2016
  Followup from the Pelican Staff: correct. engine temp changes fan operation. That is what a fan clutch does. Allows slip or grab depending on temp.

use the articles at your own risk. - Nick at Pelican Parts
Graham Comments: Article seems to indicate that clutch is temperature controlled. However, MB Shop manual section 20 says M117 engine clutch is speed controlled part 1162000522. Even when engine is hot, fan ca be turned by hand when engine is stopped.

It is a very bad idea to put a piece of newspaper into the fan. People can get hurt if they do! I did!
June 7, 2016
  Followup from the Pelican Staff: Temperature changes the state of the viscous fluid in the clutch. That is the temp control portion. - Nick at Pelican Parts  

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