Viscous fan clutch--- WHY?! (1 Viewer)

[jcolegrove]

Hi all,
Worked on cars most of my relatively short life- had mostly honda's and mazda's, etc. Now have the lexus and LC and both have the viscous fan clutch, which is new to me....

I guess my question is: why are these used and is that viscous coupling a temperature controlled clutch, like the fancy diff, etc..?

Or is it just a buffer to help smooth the engine speed differences? We all see the fan turn at start up on a cold engine, so, what's the point?

don't think this has been discussed? thanks.

 

[Heffenoche]

The viscous fan moves more air than an electric fan can and this is why it is probably used. It is temperature sensitive so when it is not locked up there is less drag on the engine.

 

[cruiserdan]

The theory behind a viscous clutch is to reduce parasitic drag on the engine from powering a static mounted fan blade. A properly operating clutch (somewhat fickle in an 80 for some reason) will "lock up" when the radiator temperature rises in order to pull more air and then "relaxes" when the temperature drops.

D-

 

[jcolegrove]

thanks c-dan. that's kind of what i thought and makes sense.... so, how/why does it seem to run at engine speed on start up? seems to be made to lock up progressively as temps get hotter, so on a cold engine, why the lockup?

 

[chip7238]

I think it's always going to run - just not at "full speed".

From what I've gathered, it operates on the same principle as a clutch in a manual tranny - input shaft to tranny runs at the same speed as engine output when clutch is fully engaged. When the clutch is only partially engaged, the input shaft on the tranny runs at a speed somewhat less than the engine output because of the slipping clutch.

I say all that to say this - I don't think the fan clutch will ever completely disengage. Those in the know please correct me if I'm wrong...

Thanks!
Chip

 

[Darwood]

thanks c-dan. that's kind of what i thought and makes sense.... so, how/why does it seem to run at engine speed on start up? seems to be made to lock up progressively as temps get hotter, so on a cold engine, why the lockup?

I second this question. If the fluid locks up when it is hot, then I don't see what could cause it to lock up at startup. Is the fan whoose sound just a result of the higher startup rpms? Also if this is the case is the startup method only accurite because the fan doesn't even have the resistance it should have when it's cold?

 

[alia176]

The fan will lockup at startup until the viscous clutch determines that you don't need the fan. In which case, it'll release and your fan will simply free wheel. You'll need to move air over the viscous coupling spring thing to activate it.

Try this: startup the engine in the morning, pop the hood open, reach over and revv the engine a little bit, then listen for the sound of the air. If everything is working well, the air sound will go away and all you'll hear just the engine only. If the air swoosh sound is not diminishing, then perhaps your viscous clutch is not releasing. Assuming of course, the temp outside isn't greatly hot!

Just a non scientific test!

 

[jcolegrove]

have to re-iterate my question about it locking up at startup....
If it's temp controlled/sensitive and locks only when hot, then why the full-lock (even if it's a few moments) at startup....
As they say... "How do it know?"

 

[turbocruiser]

have to re-iterate my question about it locking up at startup....
If it's temp controlled/sensitive and locks only when hot, then why the full-lock (even if it's a few moments) at startup....
As they say... "How do it know?"

Just taking a goofy guess but suppose the viscous fluid itself is all settled in the lower half of the fan clutch causing an unbalanced situation that momentarily causes the fan to lock on start up? Once the fluid disperses evenly the clutch releases UNTIL that fluid then gets hot from running the engine??? Again, just a goofy guess but whaddya think???

 

[e9999]

this may one time when one of these blasted IR thermometers might come in handy to figure things out....

 

[sjcruiser]

I think this is all due to the extra slippage condition at start up (standing fan - rotating core) that temporarily solidifies the viscous fluild.

Just my nickle,
Frank.

 

[Beowulf]

I don't have that section of the NCF handy but the function of the viscous fan clutch is clearly explained in the manual. Maybe Raven can post that section before I get home?

-B-