How / when does the fan clutch engage?

[whiteboyslo]

Pardon my ignorance, but mechanical fan clutches are bit of a 'new' thing to this youngin'. I know the clutch is a purely mechanical device, but are there some general rules of thumb about when the clutch does/should come on?

I ask because I was giving the truck a good workout yesterday and there seemed to be no rhyme or reason as to when it engaged. I was hauling a couple of sport bikes back from a trackday doing about 75 or so with a steady head wind. It was 90 degrees or so outside so I had the AC on. The drive was mostly downhill, but with a few decent uphill grades as well. As expected, the truck ran a bit warmer than usual, but nothing alarmingly high. What was curious, though, is that the fan seemed to engage at all sorts of places. Sometimes the temps would creep up as much as 15 degrees over what she normally idles/cruises at and then the fan would kick in. Sometimes she would barely be 5 degrees over and would engage. The fan itself does an awesome job. Once you hear that roar, the temps drop QUICK. I just wish I knew how to make it engage more consistently.

Mike

 

[chvycmnslvr68]

My fan comes on at exactly 210 everyt time ... but I have a cummins in my chevy .... from what I have heard about the IDI's the fan comes on a bit higher .. But the fan is engaged buy air temp coming through the radiator so your a/c can create the temp needed to engage the fan at times even though the engine temp is not that high ... This may be why your fan would come on at only 5 or so degrees above normal driving temp ...

 

[fordf350man]

what you could do if it would work is get a clutch fan off of a newer vehicle, most are electric now and use a temperature sensor with it do engage it when you want it to

 

[RLDSL]

If you had teh ac on, the fan clutch was not only sensing the temp off of the radiator, but it was also sensing the temp off of the ac condenser. It is dry out there and your ac doesnt have to work near as hard and will cycle well at highway speeds, this will cause quite a difference in the temperature being presented to the sensor spring on the front of the fan clutch, so if the ac had cycled off, then it would not be seeing that extra heat from the condenser, and an extra 15 deg in engine temp would be a min that I would expect to see before it would kick on since you were just carrying a light load like that.

A few things you can do to stabilize this, first off, ALL of these things should have a small electric pusher fan rigged to come on with the ac to cool the condenser, this will aid your ac performance in traffic when your fan clutch doesnt like to kick on, it will also help to stabilize the temp when running with the ac on.
THen go through and blow out or wash out your rad and condenser, then spend a day with a couple of pick tools and clean out all the tiny rocks that are blocking everything, you will likely find a min of 15-20 % air flow blockage from the combined junk, your fan clutch cannot function properly if the air isnt getting to it , and the engine cant cool if the air isnt getting past the rad