Is the fan clutch only engaged by the thermostatic coil attached to the clutch? Or are there other inputs that determine when the fan should run? Have these thermostatic coils been known to go bad?

It seems my fan runs too often. Now that the weather is getting warmer, it seems that my fan will often be running when just accelerating through the gears. Even if I just started and my water temps haven't reached operating, you can hear the fan roaring. But it's not something that happens consistently. I'm running the 180* thermostat so my water temp never gets above 190*, so I wouldn't expect the fan to run very much at all. So when I start hearing it, it makes me wonder.

 

I'll almost bet that your rad is plugged up with dirt and debris, look in between the rad and the intercooler, and in by the fan shroud, the "mist" from the crankcase vent collect on the rad which picks up dust and the next thing you know the rad is pretty much completely blocked off! I just cleaned mine last week, my fan roared constantly, mine had so much dirt in it that I removed the rad completely and took it to the car wash, and used a can of engine de-greaser on it. it took 10$ to finally get clean water to pass thru the rad. I tried washing it in the truck but was not sucessful just because it was so plugged up. also mine used to run a needles or two above 190 and on ocassion (with AC on and + 30C) it would creep up to the red zone. now when it idles (even with AC on) the needle drops to just under 190.

because air movement thru the rad is restricted the thermo on the fan picks up the heat, locks the clutch and tries to cool the rad, once it is cleaned out the fan does not lock on, as the air movement is now cooler than before.
Because its a pain to clean I went to the hardware store and got some window screen to put in front of the rad to keep the crap out, I also spent an hour straiting out AC fins,

 

Radiator and intercooler are clear. Gave it a thorough cleaning end of last summer when we had a N. Texas meet since Charlie had made a tool for cleaning in between. And I rinsed it all again about a month ago and everything was still clear. No oil buildup from the breather bottle as I keep it wrapped with a sock.

The water temp never deviates from the normal, unless I'm working it hard. I'm just trying to figure out why it locks in even when everything is cool.