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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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Old 03-22-2002, 11:02 AM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784
Before changing the a/c high pressure switch, have the freon level/charge checked.. You more likely have a slightly low charge than a bad switch. The high side pressure never gets to the cut-in point of the switch if short on freon.

The test for the fan is to simply jumper the switch.
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Old 04-02-2002, 09:00 PM
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Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
"2) Air conditioning ON scenario:

Is this accurate? It is my understanding that

1) low-speed fan operation is NOT directly coupled to AC operation but rather is intended to come on IF AND ONLY IF refrigerant pressure is higher than a certain threshold;


2) the purpose of the low-speed aux fan circuit is really to protect the COMPRESSOR rather than prevent engine overheating.

In any event, my low-speed fans DO NOT come on every time the AC is on but do cycle on and off as refrigerant pressure rises.

Maybe the 560 engine is different in these respects from mine.
'93 400E
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Old 04-02-2002, 09:50 PM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784

Which is the norm.
The high side pressure switch has a cut-in point that has to be met in order to turn the low fan on. It is a safety switch.
The design of the cut-in point spec. is to let the a/c do it's job
until there is a demand to keep excessive high side pressure down.
[ air across the condenser does this]. This happens when the ambient temps and humidity are high enough to tax the system.
So, it is quite possible to have normal a/c and NO fan.
An example would be a/c operation on a cool day.
However, if you do have taxing conditions [ hot/humid, etc] and the fan does not run, before condemning the sw ,
[ assuming the fan runs with the jumper test], it is very possible to be JUST short on freon. This shortage will limit the compressors ability to ever reach the pressure switch's cut-in point.
So it is important to check freon level [ either with the high-side eye-sight or better yet, gauges].
But , you say "my a/c seems to work, so I must have freon".
True, but not enought to trip the switch, but still enough to cool on a NORMAL day.
The engine coolant fan [ high speed] kind of works the same way, but is eng. temp sensored. Again, it is a safty device and only comes on when the cooling system is TAXED beyound normal load. [ hill climb, towing, etc.]
I think a lot of the confusion on the fans comes from the fact that the a/c is actually a physical load on the engine and it's operation in itself will cause the engine coolant temp to rise enough to trip the coolant high speed fan. [ around 105-107C].
A fairly good DIY a/c high side pressure test is to let the car idle on a fairly warm day with a/c on and watch for low fan to cut in. This may take a few minutes. If it does not , take a peek at the eye-sight on top of the drier. Lots of bubbles/foam ???= low freon
That's the basics.....
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Old 04-02-2002, 10:02 PM
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Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
Nicely put, Arthur!

Definitely the most cogent and clear explanation I've heard for this confusing system.

'93 400E
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Old 04-03-2002, 11:05 PM
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Join Date: Jul 1999
Location: Youngstown, OH
Posts: 150
I had a VERY similar problem on my 1991 300E. I had the manual to figure out the LOGIC of all the sensors in the cooling fan system. A post here suggested that I lightly tap on the suspect fan with a stick (handle of a hammer) because sometimes the fans get stuck and won't start up. I did this about 50,000 miles ago and it fixed the cooling problem and I've yet to changed a sensor or fan.

Just a suggestion before someone slaps down money and time.

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Old 04-04-2002, 09:54 AM
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Intruder - in my case pulling the aux fan leads off the sensor and shorting them together caused the fan to run - so the fan is not suspect - just control circuitry.
Arthur Dalton - Sounds logical. I'm taking my 560 out of winter storage this weekend. Outside temps here are about 35F during the day, 25F at night. If I engage the compressor with this outside temp. will I still see foam in the spyglass if I am a little low on freon? I would appreciate your input. On my car, even if outside temp is over 90F the engine would not climb to over 105C under worst conditions. If however, I put the air cond. on the temp would climb over 5-10 110C at which point I turned the air off. I now realize that by turning the air off the engine temp sensor which kicks in at 115C did not get a chance to kick in the aux fan on high. However, the freon pressure switch never turned the aux fan on low at any time. The fan was dead as the temp. rose. Temporarily, last year, I left the freon pressure sensor wires shorted together so that the aux fan was running all the time and there was no overheat - engine ran at 105C MAX with 90-100C normal on real hot days with air on. That is why the sensor became suspect. Your low freon makes sense. All I need to know to save evacuating the air cond. system and replacing the drier and sensor - is will the low freon show the frothing in the spyglass regardless of outside temp. I am ashamed to say I have never checked the spyglass - I know now I should have.
Thanks for your input. If I am low on freon, I will find out this weekend and report back.
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