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Discussion Starter #1
I have a 99 f350 superduty with powersrtoke and electronic tranny with overdrive, I haul a Cab over camper and tow a twenty foot boat. I have a question about something I hear while driving down the road. I keep trans out of o/d unless diving on level road and or 55 to 65 mph (also keep it out of o/d below 55, just to heavy a load). With the a/c on and on flat surface going 65 on occasion tranny will shift out of o/d due to a slight change in grade. Motor or ? will make a whirring noise (like a vacuum cleaner turning on and off) until it gets enough gumtion to throw itself back into o/d. It only does this with a/c on. With it off I do not hear the on/off noise of what ever it is. I live in AZ. so ambient temps are high. It has done this for years, not really a problem I'm just curious what it is that I'm hearing. I bought the truck new in July 1998 has about 98k miles, usually used to carry camper and boat. Any Idea's? Thanks
 

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you are hearing the fan. when the truck is not in overdrive the engine speed is high and therefore the fan is spinning much faster when it locks on. When ever the A/C compressor kicks on the fan clutch locks up for full cooling, when the A/C kicks out so does the fan. without the A/C on, your truck probably is not getting hot enough to actually need the fan at full speed.
 

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Thanks for the reply, but a have question or two. This is the second fan clutch in my truck, I was told back in 2006 that the original was bad, had them replace it. I think they took me to the cleaners on that deal. I kept the old one. Is the fan clutch activated by temperature or RPMs or both? What you say makes sense. If it is temperature actuated only, it must be pure coincidence that the fan turns on when the tranny kicks out of o/d. And the fan is engaging and disengaging every 5-8 seconds (thus the on/off sound) at that exact temperature. If its kicked on by rpm, then I would think it would stay engaged until rpm drops in o/d. How would the a/c kick on the fan? It is not electrically connected? I've never seen a fluctuation on my temp gauge ever, loaded or not, a/c on or off, it never changes (except on start up its cold at that point) never any change during operation. Thanks trvsdad
 

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Ok, yesterday I installed my old fan clutch. We'll see in a couple of weeks (next trip) if what I was hearing was indeed the fan turning on and off. I borrowed the "fan clutch tools" from O'Reillys auto parts. I'll post findings after next trip.
 
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