Weird question about viscous fan operation (1 Viewer)

[desertwampa]

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.

 

[jonheld]

There is no time or temperature where the fan doesn't spin. There is always some fluid between the clutch halves.

 

[IdahoDoug]

A fan takes incredible energy, so as Jon says it is always pushing SOME air. But it's designed to save power/fuel when not needed and one way to look at it versus the old days when the fans were belt driven and always at max flow/fuel sucking drag, is that these are heat activated to better match the fan speed to the engine's needs.

 

[desertwampa]

A fan takes incredible energy, so as Jon says it is always pushing SOME air. But it's designed to save power/fuel when not needed and one way to look at it versus the old days when the fans were belt driven and always at max flow/fuel sucking drag, is that these are heat activated to better match the fan speed to the engine's needs.

aha, ok..so it's always on..but goes faster the hotter it gets?

 

[IdahoDoug]

Technically, the hotter it gets, the more closely it matches the engine speed. If you have any doubts, cooling issues, or belief it's not up to snuff, just get a blue model - available here I believe from Landtank. Both my 80s have these and we tow heavy. You'll lose a fraction of MPG I suppose, but an excellent tradeoff.

 

[Marvelicious]

aha, ok..so it's always on..but goes faster the hotter it gets?

The fluid works on the same basic principle as the transmission torque converter where you have two sets of opposing fins and a fluid between the two. When you first start it, the fluid will have pooled in the bottom and if it is working well, it should roar for a minute until the fluid gets spun away from the fins and it will make that same noise if it's pushing much air during operation. Ideally, it really shouldn't be engaged when you're going down the highway: you should be moving fast enough to provide plenty of airflow, so pushing the fan is just wasted energy.

Edit: That's not to say it won't spin when not really "engaged". Even when it's cool, there is enough friction to keep it spinning, but if you don't hear it over the engine, it's not pushing hard.

 

[80t0ylc]

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.

There is soooo many threads and information on this forum about the 80's fan clutch with theory and things to do to improve the performance. All you need to do to glean massive amounts of info is a search on "fan clutch" specifying 80 series forum. Here's an example:

Search results for query: fan clutch

forum.ih8mud.com

You may have noticed, what we refer to, as the start up roar of the fan on your rig when it's started cold. It'll go away after about a minute or so, it takes longer, the colder the ambient air temp is. It's one mark of a healthy fan clutch, even on a 100+ day on a "cold" start.

 

[Mark W]

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...

 

[desertwampa]

Thank you all for your insights, this is exactly what i needed to know. appreciate it