Still learning my way through owning an older vehicle. I have a 97 Lx450. I watched a video on youtube about how a viscous fan clutch works and understand that there are different temperature tolerances built in to tell it to kick on...my question is,
isn't it always on?
Maybe because I live in Las Vegas where the temps are almost always above 60 ambient (except winter), every time I open my hood, that fan is spinning...and it seems crazy to me that there would be a time when it doesn't spin..... essentially putting no air flow across the radiator when stopped in traffic or parked. What am i missing here?.
In cold weather, or below certain temps, does that mean the fan technically shouldn't spin?
appreciate any insight as I can't test here, it seems it always too warm.
The point of the viscous coupling is not to stop the fan from spinning. No one cars if it spins freely. The point is to reduce the amount of drag that the fan creates... the amount of power it uses moving air. When the engine temps climb and the airflow through the radiator created by the forward motion of the vehicle is not enough to cool the radiator, then the hot air passing over the coupling opens the thermostatic valves and the oil is allowed out of the reservoir and into the small clearance "interaction" zone (my term). The drag of the oil makes the two sides spin together and the fan comes up to engine speed (from the idling/coasting/freewheeling speed it had before). With the fan moving at engine speed, "locked" if you will, it pulls lots of air (and also creates more drag/uses more power).
Once the increased airflow brings the engine/radiator temps down, the cooler air passing over the coupling no longer causes the valves to open and the oil that is constantly being pumped back to the reservoir area is trapped there. With no oil in the interation zone, the fan returns to freewheel status.
It still spins... but at a slower speed as the drag of the air overcomes the minimal friction in the coupling. This limits the spinning of the fan and... the important part... how much drag it creates and how much power it sucks up.
To visualize what is going on, when the fan is not being called on to move large amounts of air to assist in cooling you can use something soft and flexible (like a piece of heater hose) to stick into the fan and stop it from spinning. Once stopped you *could* easily hold it in place with your fingers (I don't recommend this little trick). When the engine is hot and the fan is locked, it can still be stopped... but it will beat the hell out of anything you stick in there and you will want to keep fingers far far away even if you did make it stop against it's resistance.
A better way to see the difference between a locked and unlocked fan is to watch when you shut the engine off hot and the fan is locked. It will stop spinning right along with the engine. When the engine is cooler and the fan is not locked, it will freewheel for a bit after the engine stops spinning.
When you first start your rig in the morning, with a cold engine, the fan will always be locked for a few moments until the fluid that has seeped out of the reservoir is pumped back. You can hear the louder roar of the fan on startup, and hear it quiet down as it begins to freewheel and stop blowing a gale of air under the hood.
If you stand beside the engine bay with the hood open and rev the engine, a locked fan will blast air at you ... an unlocked fan will do almost noting. You can see the difference in fan speed as you do this.
Mark...
