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Re: What is thermostat, fan clutch and aux fan? (archive)
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Posted by Henkka 750i on June 17, 2001 at 06:44:17:

In Reply to: Thermostat - What does it do exactly? posted by Ken on June 16, 2001 at 21:49:32:


I'm still tracking down my overheating problem. What does the thermostat do? It keeps track of the temp and when the temp reaches X-degrees, does it make the fan run faster? Or what does it do?

THE THERMOSTAT

Like Rich said, it's a valve. It controls the cold water flow from the cooler to the engine. And nothing else.

When you start your car, it is shut and the same coolant rotates around the engine block(s). As the water in the block gets warmer, it will cause the thermostat to open up a bit and let in the cold water. If the water in the block is about to underheat, the thermostat closes and lets the blocks get warmer.

It's not an "on/off" switch but it can be in any state between fully open and fully closed, OR STUCK in one of the abovementioned positions thanks to debris in the coolant or just old age.

It is not operated by electricity or anything; it operates because certain metal shapeshifts at certain temperature... Well, I'm no engineer and don't think I need to explain the details (or that I would know the correct english words).

THE FAN CLUTCH

These cars of ours have a hydraulic fan clutch, situated in the middle of the fan.

It is in contact with the water pump and the water pump warmth makes the hydraulic fluid enlargen and the hydraulic clutch to get a firmer grasp on it's axle. Therefore it is able to give a bit of extra punch to radiator cooling when needed the most yet save energy (=fuel) when unnecessary by slipping more.

I don't think the fan EVER moves at the exact same speed as its axle does, especially at 5600rpm...

The reason I'm asking is, if it's supposed to kick on the auxiliary fan when it gets too hot, I don't think it's doing that. Someone said that the fan should sound like a jet engine when the engine gets hot. Mine doesn't. Is it the main fan supposed to run at varying speeds?

AUX FAN:

There is a separate temperature reading unit on the right side, in the upper section, of the radiator. It gives the aux fan the temperature reading, by which it is determined whether the fan will run at full speed, half speed or at all. This is determined by the unit that is attached to the side of the aux fan. When you engage the A/C then the aux fan should be running as long as it's on.

SHORT:

The main fan operates in different speeds determined by the viscous clutch (inbuilt) temperature, transferred from the water pump.
The aux fan operates in 2 speeds, determined by its temp reader and control unit, override when using A/C.
The thermostat controls the flow of clod water from the radiator to the water pump.


Henkka
Car not functioning thanks to a broken $200 water pump

ps: you sound like you could really use the Bentley manual ;-)

I recommend the Barnes & Noble, good, fast, reliable service and decent pricing.




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