How does the Radiator fan clutch work?

I am pretty sure that my fan clutch is out. But i would like to know how it works before i replace it. I'm thinking what happens is that when the jeep gets to a certain temp, some kind of metal expands inside the clutch and this causes it to engage the fan. My engine temp goes up to 239 degrees when the A/C is running and when i turn off the engine and try and spin the mechanical fan it turns pretty freely. I would think that if my above theory is correct then it wouldn't turn at all.

Symptoms:

It overheats when the A/C is on.
It overheats at idle.
It overheats if the A/C is off and i go faster than 65. (the temp will slowly rise if i go any faster)

I've already invested close to $700 trying to track this down. I found out that it was missing the electric fan when i was 250 miles from home and the only place to stop was a Jeep dealership. $375 later i had a new stock elec fan but it still overheated. I have recently also replaced the upper and lower hoses, t-stat, and had it flushed but it still overheats.

Am I correct in thinking that it's the fan clutch?

It's a 91 4.0 HO

It's not a metal inside the fan clutch but you're close. There's a viscous fluid that thickens as it heats up, and on a hot fan it should be fairly tough to turn. If it's spinning freely, expect that it's dead and needs to be replaced.

So since i need to replace this, would it be better to just convert to an electric fan?

Negative; I think the only person that has gone to a dual-electric fan setup with any success has been GoJeep, but he ended up going through a few to make it work right. A good mechanical fan and clutch backed up by a solid electric fan should be all you need if the rest of the system is good. You're still using the early-model electric fan with 6 blades? The later (94+) is a drop-in and flows more air. It's easily identifiable by the curved blades.

Semper Fi,
Jim

I tried the dual electric fan route. My cheap second fan sucked and I didnt want to buy a 200 dollar flex a lite or search for the perfect fitting junkyard replacement.
I just bought an aftermarket clutch to replace my stock one. It was a bit large and required some engineering to make it fit. the OEM one was 129 bucks! Screw that Ill make the aftermarket one fit.
I just finished wheeling all day in VERY hot weather with an aftermarket fan clutch. I had no problems at all with my temps. I crept a bit over 210 on the longest climbs with the AC going full blast. Most of the time I was around 200 or less.
KH

Here is a cut and paste of the actual viscous fan drive test per the 97 XJ service manual. This really is the only way to test the drive unit and be sure if it is working correctly.

VISCOUS FAN DRIVE
LEAKS
Viscous fan drive operation is not affected by small oil stains near the drive bearing. If leakage appears excessive, replace the fan drive unit.

TESTING
If the fan assembly free-wheels without drag (the fan blades will revolve more than five turns when spun by hand), replace the fan drive. This spin test must be performed when the engine is cool.

For the following test, the cooling system must be in good condition. It also will ensure against excessively high coolant temperature.

WARNING: BE SURE THAT THERE IS ADEQUATE FAN BLADE CLEARANCE BEFORE DRILLING.

Drill a 3.18-mm (1/8-in) diameter hole in the top center of the fan shroud.
Obtain a dial thermometer with an 8 inch stem (or equivalent) It should have a range of -18°-to-105°C (0°-to-220° F) Insert thermometer through the hole in the shroud. Be sure that there is adequate clearance from the fan blades.
Connect a tachometer and an engine ignition timing light (timing light is to be used as a strobe light)
Block the air flow through the radiator. Secure a sheet of plastic in front of the radiator (or air conditioner condenser) Use tape at the top to secure the plastic and be sure that the air flow is blocked.
Be sure that the air conditioner (if equipped) is turned off
WARNING: USE EXTREME CAUTION WHEN THE ENGINE IS OPERATING. DO NOT STAND IN A DIRECT LINE WITH THE FAN. DO NOT PUT YOUR HANDS NEAR THE PULLEYS, BELTS OR FAN. DO NOT WEAR LOOSE CLOTHING.

Start the engine and operate at 2400 rpm. Within ten minutes the air temperature (indicated on the dial thermometer) should be up to 88° C (190° F) Fan drive engagement should have started to occur at between 74° to 82° C (165° to 180° F) Engagement is distinguishable by a definite increase in fan flow noise (roaring) The timing light also will indicate an increase in the speed of the fan.
When the air temperature reaches 88° C (190° F), remove the plastic sheet. Fan drive disengagement should have started to occur at between 57° to 79° C (135° to 175° F) A definite decrease of fan flow noise (roaring) should be noticed. If not, replace the defective viscous fan drive unit.


Hope this helps

Bryan

Yucca-Man said:

It's not a metal inside the fan clutch but you're close. There's a viscous fluid that thickens as it heats up, and on a hot fan it should be fairly tough to turn. If it's spinning freely, expect that it's dead and needs to be replaced.

Click to expand...

On the factory unit and the "heavy duty" after-market units, there is also a bi-metallic spring that makes a positive lockup above a certain temperature. The light duty replacement I inadvertently got from Auto Zone doesn't have the spring and works as you describe.

Eagle said:

On the factory unit and the "heavy duty" after-market units, there is also a bi-metallic spring that makes a positive lockup above a certain temperature. The light duty replacement I inadvertently got from Auto Zone doesn't have the spring and works as you describe.

Click to expand...

Is the heavy duty version a straight stock swap?

Bryan C. said:

TESTING
If the fan assembly free-wheels without drag (the fan blades will revolve more than five turns when spun by hand), replace the fan drive. This spin test must be performed when the engine is cool.

Click to expand...

When i try and spin the fan it doesn't spin freely, but i am able to turn it without difficulty no matter the temperature. Does it still sound like it's bad?

rockethound said:

When i try and spin the fan it doesn't spin freely, but i am able to turn it without difficulty no matter the temperature. Does it still sound like it's bad?

Click to expand...

If in doubt replace it or do the factory test. Question, when you replaced the hoses did the lower hose have a spring in it ?? The spring keeps the hose from collapsing once the water pump starts seriously drawing coolant and starving the system causing an overheat. One of the reasons I buy OEM hoses, they already have the spring in them and I don't trust those hoses that the parts stores say are strong enough not to collapse....

rockethound said:

Is the heavy duty version a straight stock swap?

Click to expand...

If you get a Delco brand from, I think, Carquest -- yes. Some folks have reported that Haydens will fit. Don't recall the brand sold by Auto Zone, but their heavy duty has a deeper hub and will not fit if you have a 2-core or aftermarket 3-core radiator.

RichP said:

If in doubt replace it or do the factory test. Question, when you replaced the hoses did the lower hose have a spring in it ?? The spring keeps the hose from collapsing once the water pump starts seriously drawing coolant and starving the system causing an overheat. One of the reasons I buy OEM hoses, they already have the spring in them and I don't trust those hoses that the parts stores say are strong enough not to collapse....

Click to expand...

I had it done at Pep boys, so i don't really know. I will check when i replace the fan.

Another thing is that the thing will overheat when my speed is over 70 for long distance. the guy at the part house (auto zone) said that it may be the Cat convertor. there is a rattling underneath and he said this may be causing heat to build up due to exhaust.
what do you think?

If you are overheating going down the highway, I would suspect flow in your radiator is a problem. Since you have a '91, it just might be time for a new one.

ACE said:

If you are overheating going down the highway, I would suspect flow in your radiator is a problem. Since you have a '91, it just might be time for a new one.

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Well, i checked that today and it looks good. there isn't a whole lot of bugs in it really to speak of.

I can tell you that i never hear the Thermal fan kick on. Only the electric.

*usually* when you do a cold start in the morning you should hear the mechanical fan 'roar' for about a minute then go quiet. If it does not do that then that may be an indication that the goop that causes it to lockup may not be working anymore...
When he says 'flow' he does not mean air flow thru the fins, he means coolant flow thru the radiator. Might be time for a good flush, I'm partial to the 7 hour prestone flush, drain the system, add flush and top off with water, drive it for 7 hours and follow the directions. The prestone back flush kit is also a good idea. Then drain and refil with a 50/50 mix of coolant and DISTILLED water.... Also replace the pressure cap, they don't last forever, could also be your pressure bottle

ACE said:

If you are overheating going down the highway, I would suspect flow in your radiator is a problem. Since you have a '91, it just might be time for a new one.

Click to expand...

If your XJ over heats going down the highway chances are that you have more than a fan clutch problem. You need to go over the complete cooling system.
I like how so many peopls say that converting to all electric fans do not work. Well I have done that ten months ago and to date all I have seen are positive results both in Miami rush hour traffic with the AC on as well as driving on the highway. Bear in mind a fan replacement is not the solution to most overheating problems.