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Originally Posted by JimF
bannanas! I was only talking about the VFC and when it "engages". Maybe we do agree but based on a bms that 'opens' at 100C, then the radiator water temp MUST be significantly higher than 100C for the bms to open at 100C..
The reason, based on physics, is based on the transfer coefficient between the radiator's 'metal' (aluminum) and the transfer medium, "air". As you may know, stagnant air is one thing but moving air is a severe complication b/c the air is mixed with cool air and the rad's hot air in and around the surface.
Since there's a goodly amount of air moving through the rad's fins, the air 'landing' on the front of the VFC (and thus bms), is much cooler than the rad's internal temp. Since the BMS responds to the air temp, it not unreasonable for the temperature in the rad to be 110 - 120 degsC so that the bms "sees" 100C.
You say that portion of the MB system was 'properly' engineered but as you can see, it's practically useless. Only when the rad temp is at a 'dangerous' point is there a chance that the 100C bms will operate.
This is not just my opinion but many learned people share the same opinion. But MB DID recognise the failure of this system and there is a vfc unit that is designed for the tropics that does operate at 85 - 90C (bms is 'bent' fully at 90C) or so. The discussion is an interesting thread here on the MSF.
I don't care where the temp sensors (B10/8) are on the car, since that has NOTHING to do with the operation of a vfc.
If the aux fans system is 'thrown' into the mix, then it certainly operates MUCH earlier than the vfc. For a "S" class car, the aux fans are set to fire off at 100C for the first stage (low speed).
As you can see, that dampens the operaion of the vfc since the aux fans 'cool' the air down further preventing the vfc operation.
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Although your "Physics" may be sound, some of your assumptions are not.
When Mercedes specifies an activation temperature of 100C it already took into account that the air temperature is cooler than the coolant temperature. So the Fan was designed to come on when the air temperature is approximately 90C-95C which roughly corresponds to a coolant temperature of 95C-100C. Exceeding that temperature should cause the Auxiliary fans to come on.
If the VFC would have indeed only been triggered at an equivalent coolant temperature of 110-120C as suggested, than it will always come on AFTER the auxiliary fans have already kicked in at their lower set temperature of 100-105. This is not the case. The VFC normally activates BEFORE the auxiliary fans do.
Phil