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  #1  
Old 10-20-2002, 02:44 PM
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thermostat question

I am wanting to put an electric fan on my 420 and was told that I would have to modify or replace the pump thermostat to close bypass flow. Can someone clarify this for me, and what exactly would I have to do/buy in order to accomplish this? Also, the fan will be either a 2000 or 2500 CFM unit, activated at 100C, does this sound correct?
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Old 10-20-2002, 03:23 PM
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i am not sure about that line of thinking. the bypass flow is in effect when the thermostat is closed. the modification to close off the bypass is not clear to me because i don't know how the coolant flows when the main valve is not open. the bypass is closed off so where does the water pump draw coolant from? a lower temp thermostat such as a 79C might do the trick. there was a thread on this in the performance forum but the issue on whether or not the special thermostat was required was never resolved. here is a diagram of how the whole system is supposed to work. "A" is a cold thermostat . "B" is when it is partially open. "C" is when it is fully open. i know about removing the thermostat and closing off the bypass in tropical climates where the ambient temperatures are always high and even then it is not always desirable because of the possibility of over cooling. maybe someone can explain how that special thermostat is supposed to work...
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Last edited by jsmith; 10-20-2002 at 03:28 PM.
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  #3  
Old 10-20-2002, 11:15 PM
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Are you looking to replace the conventional fan with an electric one? I don't see how that would affect the thermostat.
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  #4  
Old 10-21-2002, 12:14 AM
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there was a thread in the performance forum about doing the electrical fan conversion. there was a statement to the effect that if you left the standard thermostat in the fans would be cycling continuously because the thermostat's normal operation keeps temps in too tight a range. blocking off the bypass makes the coolant go through the radiator getting the maximum cooling effect at all times.
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  #5  
Old 10-21-2002, 12:26 AM
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The bypass thermostat does just that. It causes the coolant to bypass the radiator by redirecting it back into the block allowing for quicker warmup of the engine (and heater if needed). The electric fan (not moving at all until preset temp. cut in) would make the bypass less neccessary, since the cooling of the radiator is diminished as compared to the mechanical one at lower temps.(the viscous clutch still has SOME coupling at low temps). It really shouldn't matter if you keep the bypass thermostat with an electric fan; it just become a bit (small) redundant.

A wide enough spread between the on/off set points on the sensor for the electric fan would prevent excessive cycling. (much like our A/C compressor cycling).
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  #6  
Old 10-21-2002, 01:08 AM
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i re-read the electric fan conversion thread:

electric fan

i think the thermostat conversion is to fix it so that the main is always open and the bypass closed (like "C" in the previous diagram). i still think that warm up time will take longer, specially in the winter and more so if the car is moving...
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  #7  
Old 10-23-2002, 10:00 PM
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Why oh why do people always mess about with proper engineering?

The only reason to modify the thermostat bypass would be to (modestly) increase the cooling capacity. You will get poor heater response, slow warm up, increased fuel consumption, and will fail emissions testing if the engine won't stay at operating temp. You WILL NOT get any performance gain, and will in fact tend to overcool the engine most of the time.

The only application I would suggest for modification would be racing, in which case fan cycling isn't an issue.

Leave it alone and let it work. The fan only needs to run when the coolant gets to 100C -- temp won't change that fast to be a problem. Besides, you won't notice the loss of the four or five horsepower the fan uses.

Peter
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  #8  
Old 10-23-2002, 10:28 PM
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I'm in agreement with Peter. Leave it alone, because you will create all the negatives he mentioned let alone the fact you will cause increased engine wear. Engines wear more during the warm up process than when at operating temp and what you plan to do will keep your engine in warm up stage far longer than normal.

Peter
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  #9  
Old 10-23-2002, 10:39 PM
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Peter:

You know more about this than I do, I'm sure, but would there be ANY coolant circulation with the bypass blocked and the thermostat closed, or would all flow have to be through the heater core? Could be a really big problem!

Peter
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  #10  
Old 10-23-2002, 11:17 PM
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i think that would be a real problem if the thermostat were closed. that thread i pointed out indicated that it should be fixed open. i've also read somewhere that holes should be drilled so that there is always some flow to the main. i still believe this setup is only suitable for really warm climates.

i think the electric conversion is worth doing as long as you get it working right. after all, the newer models have this setup now...
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  #11  
Old 10-23-2002, 11:37 PM
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Peter,

I'll have to research that one.

Joe,

You're forgetting that the mechanical components in the newer models like pistons, etc. are designed to run at the higher temps and older engines are not. Tires on new cars are a good example of engineering changes. The theory has always been that the closer the rim gets to the ground the less air you have to ride on. This translates to the lower the profile the rougher the ride. Take a look at the new MBs. They have like 50 series tires. Put those on an 80s car and you will be riding in a buck board. The suspension is all new design. The same goes for engines.

Peter
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Old 10-24-2002, 12:22 AM
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what's higher temps? the m103,m104 & m119 generation are designed to run in the 90's for optimal burn and for epa requirements. at least those are the engines we are discussing here aren't we? it has been stated many times on this forum that 120C is safe on these motors according to MB. unfortunately one component breakdown will put you over the top and cause serious damage. i myself have only installed the cool harness - haven't seen temps over 100C so far...
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  #13  
Old 10-24-2002, 11:46 AM
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Joe,
Actually not. The starter of this thread is asking about a v8 from the mid 80s which is before the proliferation of the electric fan. I still don't think additional cooling is needed if everything is funtioning properly. Any design flaws should be taken care of by a recall. Remember these are world cars designed to operate in Siberia or the Sahara. Before the advent of rubber steering wheels when they were plastic, you could order with a white steering wheel for hot climates. Now that American Engineers will probably have a hand in design we may start getting crappy make it through the warrantee period design. That remains to be seen. Remember: A pessimist sees a glass as half empty. An optimist sees a glass as half full. A German engineer sees a glass twice the size it needs to be to insure no beer is spilled.

Peter
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  #14  
Old 10-24-2002, 01:23 PM
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Peter,

Okay, I thought it may have been an m119. Your statement about cooling systems on these cars in general tend to make me wonder there are so many "overheating threads". Without the cool harness mod, my car would warm up to 110-115C in the traffic I run into in the summer (maybe worse, but I don't intend to find out the hard way). Why does the viscous fan clutch lock at a temperature when the engine is practically overheated already? I contend that the cars here were engineered for places like Siberia where cooling is not an issue . Summers here in the metro NY area are regularly up in the 90's and will go over 100C at least a few times. I think your cooling system better be in top shape to deal with that. BTW I had an 84 mitsu that only had an electric fan for cooling which I also hard wired with an override switch. That wasn't exactly rocket science back then.

Sorry for sounding contentious but there are too many stories of hundreds and sometimes thousands of dollars spent replacing the water pump, fan clutch, radiator, thermostat, aux fans - all to no avail...
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  #15  
Old 10-24-2002, 09:44 PM
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The hotter the rad the greater the heat transfer. Letting the coolant get to 120C before turning the fan on saves fuel and allows greater heat transfer from the engine to the air. No risk to engine.

If the cooling air is already fairly warm (say, in the Sahara at 2 pm on a sunny day), if the rad isn't significantly hotter, you won't transfer any heat to the air, and the engine will instead get hotter. Make sense? I'm talking about the rate at which calories of heat are removed from the coolant by the air, not about the actual temperature.

This is the other reason not to screw around with the thermostat and bypass -- if you speed up the water flow too much, it won't stay in the rad long enough to loose any significant heat, and won't stay in the engine long enough to pick any up, either. The result is overheating! Coolant can't dump heat, and the metal gets much hotter than it should as the water races past.

Remember that MB uses fairly accurate gauges, not the funky non-linear ones you see in American cars. On those, by the time the needle gets close to the red, you've probably blown all the coolant out.

Peter
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