Sounds like nobody actually knows how to test the fan clutch.

 

Exactly.

Currently, know one knows a method that is:
1) Measurable (how much is little, a lot, or no "resistance"?)
2) Objective,
3) Practical (meaning does not require special equipment in an environmentally-controlled lab setting), And
4) In the future would help others without guesswork.

 

The issue is the hysteresis. You have no idea if the clutch is locked and in the process of cooling the rad down, or is unlocked and freewheeling while waiting to heat back up.

I doubt it finds an equilibrium point of some % engaged since the heat load varies with driving conditions.

I would think the only real way to get meaningful data is to measure the fan speed with a tachometer, while you are driving and plot the speed curve. Of course, this is a lot of work, and nobody really knows what it's supposed to spin at.

 

Perhaps we're over-thinking this.

Keep in mind that the fan is only to cool the engine when the truck is not moving. Why do I think this? Assuming the fan blade is 24"diameter, at idle the tips will be moving 14fps or about 10mph. If the blade tips had a 45° twist (actually, not quite), at any speed above 10mph the air flow would be driving the fan, not vice versa. (Put another way, since air is not a "perfectly viscous" medium, the air flow through the fan at idle is probably no more than 5mph through the tips.)

That's why the clutch disengages unless it's REALLY hot. No point having the engine spin the fan when it can't possibly help with air flow.

So if you have a cooling problem while you're driving, the fan has nothing to do with it.

 

You're assuming the fan doesn't have a radiator and grill in the way, the actual air volume hitting the fan, even at 70 MPH isn't going to be a ton after fighting its way through all those fins.

Easy test, take the fan off and go for a drive on the highway. You may well be right, it would be interesting to see what the "critical speed" is for natural cooling.

 

I agree with Scope on cooling issue while driving being more than a fan issue.

I am concerned only with monitoring fan engagement while stopped, idling or at low speed because I have no issue while driving; only while creeping up a steep mountain road in cool spring weather when we rely on the fan for cooling.

Temp gage reads obviously higher than usual, but stays there, and never reach "red" zone. I would not know if this is normal because I do not know how hot a brand-new first-gen 4Runner ran in this condition back then. If anybody who owned a brand-new first-gen remembers, pls chime in.

Valid test but I would not volunteer mine - LOL!

 

"CHECKING THE FAN CLUTCH

A good clutch should offer a certain amount of resistance when spun by hand (engine off, of course!). But if the fan spins with little resistance (more than 1 to 1-1/2 turns), the fan clutch is slipping too much and needs to be replaced.

If the fan binds, does not turn or offers a lot of resistance, it has seized and also needs to be replaced.

Fan speed can also be checked with an optical tachometer, by marking one of the fan blades with chalk and using a timing light to observe speed changes, and/or listening for changes in fan noise as engine speed changes.

You should also try to wiggle the fan blades by hand. If there is any wobble in the fan, there is a bad bearing in the fan clutch, or a worn bearing on the water pump shaft. A bad water pump bearing will usually cause the water pump to leak and/or make noise, but not always. Remove the fan clutch and see if the play is in the water pump shaft. If it feels tight (no play or wobble), replace the fan clutch."
http://www.aa1car.com/library/cooling_fan_clutch.htm

How about that?

Mine is of an unknown vintage, cold it spins only about an eighth of a turn, but I don't know if that helps any.