vicious viscous fans

This is a query about cooling on the D310 Merc engine. The electric fan on our 1994 S555 cuts in regularly on motorway journeys at 50-55 mph even though the external temperature is well under 20C and the engine temp gauge is only around or maybe just above the halfway mark. Is this normal or is there some way I can adjust the thermostat on the fan? I wouldn't mind except that it's really noisy. Thanks for any advice.

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  Christian Skeels Is it an OM602 engine? As far as I am aware, the original fan was some sort of viscous coupling that engaged when a set temperature was exceeded. That's certainly what I have on my S550. These were problematic though and it's quite possible it has been replaced with an electric fan during the van's life. I am not aware of any electric fans being fitted as standard pre-1995. Happy to be corrected though!

  5 hrs · Like
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  Richard Benson Should be a 602 engine Christian, and unless modded it would have a viscous coupling. Mine only comes on on long climbs in the mountains when it's rate ot

  5 hrs · Like
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  Ron Bentham Unless you can see what is obviously an extra electric fan bolted on the front of your radiator - then you don't have one! What you do have as CS says, is the original Merc viscous coupled fan - it just sounds like an electric fan! I am very familiar with this problem - it drove me nuts for years on my old S700, which is basically the same engine. The fan looks like it is driven by the fan belt, and it is, but if you look closely you will see that the aluminium centre of the fan has what looks like cooling vanes. Inside there is a clutch which is operated by a fluid. When the fluid gets hot, it expands, engages the clutch, and power from the belt is transmitted to the fan blades, which speeds sucks more air through the rad, and cools the water, which cools the engine, which then runs cooler, so the air flowing through the rad gets cooler, which disengages the clutch in the fan..... And the noise you hear like an aircraft engine is the noise the fan makes when it is under full load. And unfortunately the VCF is a bit of a blunt instrument, and with no way to adjust it - so over it's life the point at which it cuts in and out changes slightly, and if you are unlucky, as you are and I was - it can cut in and out at cruising speed.
  
  You will notice the temp gauge needle rising and falling slightly as the cycle happens.

  3 hrs · Edited · Like · 3
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  Ron Bentham So what you end up with is a positive feedback loop - the fan cuts in, the temp drops, the fan cuts out, rinse and repeat.
  
  The bad news is you can do nothing about it. There is nothing to adjust. The options are either to completely replace the fan assembly, which is what I did, at a cost of around £400, or the other option is to add an electric fan on the front of the radiator, with an adjustable thermocouple which senses the engine coolant temperature, and powers the fan accordingly. This should have the effect off keeping the engine temp down below the threshold at which the original fan does it's annoying in and out shake it all about routine. It would be even better to have the thermocouple control in the cab, so you can adjust it as you go. This is a reasonable DIY project if you fancy it - the fans are on Ebay for less than £50.....and the other bits are cheap - you could do it in a day if you know what you are doing.
  
  You may ask why did the otherwise infallible Mercedes design team do it this way in the first place - the answer is that when the fan is operating, not only is it noisy, but it takes quite a lot engine horsepower - which you may have noticed as a drop off in speed when the fan cuts in, especially on those long motorway hills - often necessitating a change down in gear. So they tried to design a system that keeps maximum horsepower to the wheels, and only to the fan when necessary. And it does work, usually you don't even notice. But unfortunately in this case the fan system hasn't aged as gracefully as the rest of the engine. The problem is even more pronounced sometimes with those vans that have been turbo'd and intercooled, as the intercooler sits in front of the main radiator and upsets the airflow even more. Basically it would have been better if merc had fitted an electric fan as standard.