Please explain to me how the mechanical fan clutch works

[gone postal]

'97 XJ. I drive 98% city. So, it is my understanding that the mech clutch is engaged most if not all of that time and that it disengages when at highway speed b/c there is enough airflow that it's not needed. How does this work? Is it thermal? If so, since normal operating temp is 210, what is the clutch's temp set for?

So, since I just installed a 2 core radiator, I'm thinking of nixing the mech fan and putting in another electric one. I'm thinking that w/ 118k, my mech is probably going bad anyway and w/ the larger, better radiator, it might just make the 5 mile trip to work w/out even needing a fan, therefore increasing my fuel mileage. (I have a goal to hit 18-20mpg city - kinda personal vendetta).

If I do this, what should I set it for? At what temp does the aux elec fan kick on at? 220? I'm thinking of setting the elec fan at 215 and leaving the aux fan as is. This way I can see if it'll stay running at 210 w/out the aid of a fan, but if it starts to creep up, it won't get far before the fans kick in.

 

[ehall]

viscous fan clutch

 

[gone postal]

Thanks - didn't know that's what it was called. Well, that explains the operation, but is there a certain on/off temperature associated w/ it? Doesn't seem as if it'd be able to be that precise. What would you suggest I set a temp to if I do the elec route?

 

[jeepman121]

It spins it a rotational manner, the curves in the fins will allow it to blow air to your motor for cooling....sorry, but that was fun.

 

[ehall]

Thanks - didn't know that's what it was called. Well, that explains the operation, but is there a certain on/off temperature associated w/ it? Doesn't seem as if it'd be able to be that precise. What would you suggest I set a temp to if I do the elec route?

Will you have one fan or two? Does the fan you are using to replace the viscous unit have one or two speeds?

If you're keeping the stock electric fan and putting in a single-speed electric as a replacement, I would set it to come on at roughly 200 degrees. That is a little higher than the thermostat opens, but not as high as the stock electric fan.

 

[ehall]

Thanks - didn't know that's what it was called. Well, that explains the operation, but is there a certain on/off temperature associated w/ it? Doesn't seem as if it'd be able to be that precise. What would you suggest I set a temp to if I do the elec route?

Will you have one fan or two? Does the fan you are using to replace the viscous unit have one or two speeds?

If you're keeping the stock electric fan and putting in a single-speed electric as a replacement, I would set it to come on around 210 degrees. That is a little higher than the thermostat opens, but not as high as the stock electric fan. For highway driving it will stay off as airflow alone will be enough.

 

[gone postal]

It spins it a rotational manner, the curves in the fins will allow it to blow air to your motor for cooling....sorry, but that was fun.

I actually believe it's purpose is to pull air through the radiator resulting in extracting the heat from the coolant which cools the motor

Will you have one fan or two? Does the fan you are using to replace the viscous unit have one or two speeds?

If you're keeping the stock electric fan and putting in a single-speed electric as a replacement, I would set it to come on around 210 degrees. That is a little higher than the thermostat opens, but not as high as the stock electric fan. For highway driving it will stay off as airflow alone will be enough.

At first I thought you just had a double post, but you actually listed two different temps - 200 and 210.

My setup would be a single speed fan in place of the mech one. The stock aux elec fan would stay as is.

I didn't want to set the replacement elec too low otherwise it'd be running non-stop trying to cool the motor down to a point that it's not meant to run at - ie - under 210. I also don't want it too high, b/c I don't want it and the stock elec fan to come on at the same time - seems like it would be a large spike on my elec system at the fans' startup.

So, if I set it to 200, it would always run right? I would think that even at 210 it would always run?

 

[winkosmosis]

Doesn't the mechanical fan spin even when you just start up the engine and are idling? If so, isn't there a reason for that?

 

[gradon]

If you have a 195* t-stat and drive on the highway, the air keeps it cool to the t-stat temp, so you really don't need the fans on to keep it cool. Now when you drive stop and go in the city, the temps will/may raise due to less airflow. I would have the 2nd fan cut on at 210 and off at 195(if you can control the off, or know how many * below cut-on temp it cuts off).

 

[ehall]

I typo'd 200 and changed it to 210. Anyway I wouldn't go much higher than 210 since the secondary fan will kick on at 230 or thereabouts. 215 would be okay but no more.

BTW you're theory is correct--coolant removes heat from the engine, and air flowing through the radiator removes heat from the coolant. Driving down the road is usually enough air to do the job, so the fan is typically only needed when you are idling in traffic.

My Cadillac has dual electric PWM fans, which come on around 230 and then speed up as needed. They only come on when I'm at a red light.

 

[WB9YZU]

Doesn't the mechanical fan spin even when you just start up the engine and are idling? If so, isn't there a reason for that?

The clutch type is a viscous coupler as mentioned in another post.

Even if the lockup is not engaged, there will always be a certain amount of power coupled to the fan due to the nature of the coupler.

When cold, and at idle, there is not enough resistance generated by the fan to overcome the coupling, and the fan turns at just about the same rate as the pulley.

As engine speed and fan speed increases, the resistance generated by the fan overcomes the coupler and the fan appears not to be coupled to the pulley.

When the thermostat on the front of the coupler reached a value, the coupler locks and transmits full power to the fan.

The thermostat on the fan is set for less than the temerature of the water. However, when dealing withe electric fans, you want to use a thermal switch in the coolant.
Older Honda fan switches turn on around 185-190degrees, and newer fan switches likely turn on later. They also make adjustable thermocouple switches as an aftermarket item.