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The general consensus is that an electric fan will not move the air that a stock engine driven one can. Electric is ok for light towing at most, but any heavier and it won't keep the engine cool. If you don't tow, there should be no reason why it wont work for you.
I had one on my Suburban with a 6.5 diesel, and it didn't work like the ads say it should. When towing my 31' camp trailer it was marginal at keeping the engine cool. At full speed it would pull 29 amps, which put a big load on the alternator. Overall I saw no fuel economy gains after running it for 2 years.
The biggest drawback was it is wired to come on any time the AC is on, even the defrost, so in the winter the fan was running almost all the time.
On mine the electric controller went out and it was $100 for a new one, so I just put the original fan back on.
I think they might work on a gas engine and help with HP and fuel economy, but on the diesel they can't keep up with the heat produced.
I have some math to help with the evaluation of hp savings with an electric fan.
The Flex-a-lite 278 fan you're looking at pulls 28-36 amps with their variable speed controller, according to how hot the engine is running. Taking the worst case scenario...
If: amps (A)= 36 and volts (V)= 13.8 (operating voltage of a 12-volt auto system) then A X V = 496.8 watts (W)
496.8 watts X .00134 = 0.67 horsepower
Plus, the fan isn't always on, and often when it is on, it's pulling 28 amps, not 36.
Sorry that pbr man didn't have a good experience, but a lot of other people say they feel a power difference and that they do see an increase in mpg.
The fan will always be spinning since there is always some drag from the clutch. Its a fluid clutch which never completely disengages. As the air coming over it warms up, the clutch slips less. I'm not sure if it ever completely locks up 100%, but any of the guys that tow often will tell you that when it kicks in, you will hear it. I believe they only last around 100k though (can't recall where I read that though) and as they wear out, the fan will slip less and run more.