M117 Fan clutch

The other day I was driving my 560SEL on a fairly hot (30C) day in heavy traffic.  The engine temperature got quite warm when sitting in traffic.   It was well over 100C in traffic, which would eventually cool down when I got moving.   The car didn’t overheat, but I did hear coolant boiling in the radiator hose after I finished the drive.

While the temperature itself is well within spec, given the day wasn’t really that hot compared to days in the mid 40s we get here in Sydney, I wanted to do something about it.   Additionally, the air conditioning doesn’t cool properly when the engine is at those sort of temperatures.   This is presumably due to the R134A gas not being as efficient as R12.

The first step was to replace the radiator cap as it is likely not holding pressure properly.    This is a simple fix.   I know the radiator works properly as the engine was running too cool in winter before I had the thermostat changed.   In hot weather the car has two systems to keep cool.   The first is the M117 fan clutch.    The second is the electric auxiliary fan.  The focus of this article is the fan clutch.

On older cars, the fan is connected directly to the water pump and spins at the same speed as the engine.   This wasteful as most of the time this much air flow is not needed.   The fan clutch assists here by allowing the fan to spin slower under normal circumstances.  However, when the temperature reaches a prescribed level the fan can spin faster.    It generally won’t spin at a 1:1 ratio but will be limited to a particular RPM that is efficient for max airflow.

My 560SEL is equipped with such a fan clutch, as are other W126 models.   When the engine is hot, you should be able to hear the fan roar when you rev the engine and I never heard this.   Additionally, if you turn the car off when the engine is very hot, it should be hard to spin the fan.   On my car it wasn’t.   As the M117 fan clutch in my car was the original unit, it seemed like a good idea to change it.

It is a very easy job.   The M117 fan clutch is held on by four 10mm bolts.   There is no need to remove the radiator or even the top radiator hose.  All that is required is to loosen the shroud so the fan assembly can be removed.

Once the fan assembly is removed from the car there are four more 10mm bolts holding the fan to the clutch.  I used a rubber mallet to remove the old clutch from the fan.   The new one looks a bit smaller and doesn’t have the plastic protector like the old one.   It is made in China, so I hope it lasts 33 years and 328,000km like the old one did.

The picture above shows the original M117 fan clutch, with the 1987 date on it.

Once the new clutch is installed then the installation is the reverse of the removal.   One trick I picked up on some forums was to put the lowest of the four bolts in first.   The fan assembly doesn’t attach through hole – there are cut outs.   This allows the fan assembly to slip onto the bolt to locate it before the others are installed.   The photo below shows the whole assembly removed before re-installation.

I have not been able to test the success of this job yet.   It was 15C and raining today.   This is probably the last cold snap before a hot summer, so I doubt it will be long.   I also need to test the operation of the electric auxiliary fan.