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Mechanical Cooling Fan Clutch Free-Wheel Failure Mode


Aug 2, 2004
Salem MA
Anyone else ever had a 240 or 740 mechanical cooling fan fail in the off position?

I always thought these things failed in lock-up mode.

but alas, my 240 was just really quite obviously hotter than it should be and the fan spun freely.
When the engine is off the fan should spin by hand. You should feel some slight resistance and it shouldn't spin more than about a turn or two. When idling warmed up you should be getting a lot of airflow. That's why I like the stock belt driven fan. Lots of cool air when you need it.
The viscous clutch, like the nature of the name implies, locks up progressively. It also fails slowly and progressively most of the time. What I have seen a lot of is the fan clutch gets lazy and as the clutch wears, there isn't the nice "in-between" between full (or nearly full) lockup and being able to stop the fan at idle with your finger. They start to whoosh and lockup completely once everything is badly heatsoaked already or get lazy and don't lockup at all. On cars that see mostly highway driving in the PNW, the fan clutch can last a very long time...it isn't asked to do a whole lot so it wears pretty slow and the temps aren't extreme. On a fan clutch that sees more extreme temps and is saddled with more rev up/down and more temp change or load, the clutch can be as short lived as 70-100K miles.

There is a tropical heavy duty version of the clutch that works well for more heavily used cars. You can also twist the bi-metallic spring coil on the front of it a little tighter and re-crimp it for more airflow, though that will make it less efficient, particularly if the clutch is already worn. I've done that on 240s where I wanted the A/C icy cold haha. It works well if you are well practiced at it, but I'd call it a "hack," not a solution with absolute distinction.

I will NEVER put an electric fan on a street car with a longitudinal motor that originally used a clutch fan like a 240. Constant variable rate cooling that reacts instantly without everything getting all heat soaked works a lot better than letting everything heat up and cycling an e-fan on and off. The clutch fan with a new clutch works very well on a 240T at keeping everything cool with minimal noise and fuss. Again, my opinions are my opinions. I've done a lot of HGs on e-fan cars, seen how hot everything gets in the e-fan 940s and like the traditional clutch fan a ton better. I'm also a firm believer that if a car isn't cooling well enough or making enough power with low cooling system pressure and a clutch fan, you are doing something wrong.

On a turbo aluminum head motor I also like a margin or error...when you are hammering on the car things get HOT...I'm ok with using a little extra power to have a little margin rather than ruin a motor by ruining a head, pinging it, heating the hell out of the engine bay/hardware etc. I'd rather pay a little in power than save that 1mpg city and sacrifice all of the above.

I run a 160 stat in k-jet and carbed cars in summer. 180 most of the rest of the time or LH cars with the temp sensor. In the dead of winter I sometimes will step up to 188 where overheat risk is basically nil, hot heat is nice, and some heatsoak in the engine bay is a good thing.
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