Just curious, because I hear this humming sound come on only when the engine is hot climbing up long grades. Just driving over normal streets and freeways, it seems the fan doesn't come on 99% of the time.

 

your in the right area to hear it, when under heavy load, your temp will climb, the fan clutch is sensitive to the increase locking the fan to pulley speed will create an audible whail of air flow

 

I am never aware of mine coming on, don't know if it is just quit or I am just deaf...Mark edit: Maybe it is because, around here, it is always locked at start-up and just dis-engages without sound...

 

Please correct me if I'm wrong, but doesn't the clutch ALWAYS dis-engage at high rpms because the ram air through the radiator is much more efficient than the fan? This happens regardless of ambient temperature, right?...Mark

 

I am pretty sure that it should not. I am going to check the FSM in a minute and see what it says. My understanding is that ram air is much more efficient than the fan, so it dis-engages. Not arguing, just my understanding of it...Mark

 

Well, according to the FSM, it operates on temp only: "if the fan does not operate when the coolant temp increases, the engine will run hot". I guess it is a mix of things, with the efficiency of the rammed air, the coolant temp should not rise enough to lock the fan...Mark

I found this on a generic automotive info site:

A fan clutch is used on some engine driven cooling fans, to disable the fan at higher speeds. It consists of a fluid coupling containing a silicone based oil. Most are controlled by a thermostatic spring that controls the amount of fluid that flows through the coupling. At higher engine speeds, the fan will spin slower in order to diminish the amount of load on the engine. The fan clutch is designed to provide greater airflow at low speeds and to limit fan rotation at higher speeds.

A fan clutch can lock up or seize after a period of time, causing a loud roar at higher speeds due to the fan not disengaging. They can also leak fluid and become ineffective at lower speeds causing engine overheating making repair mandatory. In some cases, a fan clutch can become excessively loose at the shaft outlet where it bolts to the water pump. Simple visual and physical inspection when the engine is off, can help to diagnose fan clutch related problems and should be the first step in an auto repair project involving such a component.

 

I don't believe mine has ever fully locked in the time I've had the truck but I generally don't work mine like yours is being worked. From your description I'd say it's working just fine.

 

Ok, from what im getting from this thread, normally the fan is viscously coupled to the pulley via a thermostatic coupling (fluid, spring, etc), so even at slow speed or cold engine the fan still turns but a much slower rate. When temp is high (ie engine very hot), the coupling locks and the fan turns at pulley speed, thus producing that roar that i hear. Makes sense?

 

Yes, but it should freewheel when temps are good...Mark Sorry,I read your post again, it should freewheel when the engine is cold...Mark

 

Important to remember that a good fan clutch on a cold engine SHOULD offer a fair amount of resistance to turning by hand. Shouldn't spin loosely but should not be seized as well.

 

I believe I have not seen my fan "free" wheeling even at dead cold engine or at warm normal operating temp. Theres always a significant fluid resisance, unlike the electric fan when it's off and being free-wheeled / sucked in front by the mechanical fan. I wouldnt dare hold the mechanical fan still like i can with the electric fan when off.

 

Ok OK, bad choice of words......I mean freewheeling as best it can. If you could feather or remove the fan completely at speed, you would get better cooling, that is the point of the fan clutch...Mark You didn't think I was telling you to reach into the fan and grab it did you?....

 

FWIW I've installed 3 different aftermaket fan clutches and I never heard any of them kick in, never knew what it sounded like. Then I bought a 2001 OEM clutch which is bigger and finnaly heard the roar everyone talks about. I tow about 10k which is really too much for a 518. My temp would go to 210 and that clutch would engage and drop my temps to 200 within 45 sec. Wow I finally bought something that worked. The OEM clutch cost about 3 times as much but it was worth it for me.

 

In 3 months I have owned my truck 15 years. I have had the fan kick on three times. All three times were the same hill in MO. All three times it was right at 100 degrees. All three times I was running a GVCW of around 15k.

If your cooling system is in good shape. It takes a lot of heat and a heavy load to kick the fan in. If you fan comes on when empty running down the road. You have a problem.