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Old 03-22-2002, 10:02 AM
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While I talked to Mercedes Canada Airbag Compliance - the gentleman was very very nice - he asked me how I liked my 560sel. I told him about this forum and how some say 80-90C is normal in hot weather and how some say 100-115C is normal. I told him that over 90F outside temp. with air on doing 60 mph - mine climbed to 110C at which point I shut the air cond off and it drops to 100C. He did not have an answer but said he would would have a Mercedes technical cooling specialist call me. True to his word - I got the call yesterday. The call LASTED 45 MINUTES- THE TECH WOULD NOT HANG UP UNTIL HE WAS SURE I UNDERSTOOD WHAT HE WAS SAYING!! According to Mercedes Canada the normal operating temperature of any Mercedes Benz is 80-100C depending on outside conditions. 105C is considered the high end of normal operating temperture. He clarified this by saying that any number of things, including parts failure, can cause the temperature to climb higher. The highest temperature the engine should be allowed to climb to - even for a short period is 120C - anything higher can cause engine damage. According to this fellow -there are two scenarios to consider.
1) No air conditioner on scenario:
the flex fan should be enough to cool the engine to below 105C. There are exceptions though-- If you - for example - are towing a trailer up a hill - outside temp over 100F - the temperature can climb higher than 105C. This is normal. Once the temperature hits 115C - the coolant sensor will kick in the aux fan on high for extra cooling and the fan will not shut off until the engine temp falls to 105C or lower. Nothing to worry about - it's totally normal.
2) Air conditioning ON scenario:
What I did not know--- AS SOON AS YOU TURN THE AIR CONDITIONING ON - REGARDLESS OF OUTSIDE TEMPERATURE - THE FREON PRESSURE SENSOR WILL DETECT THAT THE COMPRESSOR IS COMPRESSING THE FREON AND THE SENSOR WILL TURN ON THE AUX FAN ON LOW SPEED TO ENSURE THERE IS NO EXTRA HEAT LOAD ON THE ENGINE CAUSED BY THE LOAD OF THE AIR CONDITIONER. If it is really hot outside (over100F) and depending on driving conditions - the engine temperature could then start to climb higher than 105C. When it hits 115C - the coolant sensor will kick the aux fan on to high and not shut off until engine temp hits 105C or lower, if the air cond is still on the aux fan will kick down to low speed.

When you know how things are supposed to work (logical theory) it makes diagnoses easier. On my 560sel - I was partially the cause of my problem as I was so-o-o-o worried about overheat - I turned the air conditioner off before the engine temp. hit 115C which should have kicked in the aux fan on high. However, I now feel confident that I know my problem. When I put my air conditioner on - MY AUX FAN DOES NOT COME ON AT ALL. IT IS THE FREON PRESSURE SENSOR!!! As soon as the snow disappears in a few weeks - I will be changing the sensor and report back.
There has been tons of threads on this overheat subject with many many opinions of what is and is not normal. I hope this new thread will help many who have high temp problems or even just high temp worries.
I cannot praise the Mercedes Canada Corporate Head Office enough. They do what they say and they do it in a very friendly manner that I seldom find in large corporations anymore. They did not treat me as being a nuisance or treat me as a number or anything. Even though my Mercedes is old - they treated me as a VALUED CUSTOMER. TWO THUMBS UP FOR MERCEDES CANADA
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