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On my vehicle (92 Civic) and most others I've seen, the condenser fan comes on whenever the A/C clutch is engaged. This seems like a waste of electricity (and thus fuel) when cruising at 60-75 mph, where the air flow from the fan is going to be dwarfed by air flow from the vehicle's motion. My car has separate radiator and condenser fans (they're mounted side-by-side rather than one in front of the other) so it seems like it would be even more of an issue on vehicles where a single larger fan or pair of fans turn on together.

Any idea if the electrical load is significant to fuel economy? Is there any easy modification that could be made to prevent the fan(s) from running when the vehicle speed is greater than a particular threshold?

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I'm sure you could manage to measure the amount of energy wasted by your condenser fan, but I promise you it's statistically insignificant. If you're trying to save an amp or two or power, it would make more sense to make sure you don't have any lights on or that you're not carrying any heavy objects you don't need in your trunk.

That said, the most effective way to make sure your condenser fan isn't turning on when you're confident that you don't need it to, would be to splice a relay into the wire running to it, and control that relay with switch near you in the cabin.

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3  
+1 Agreed with power insignificance and solution to OPs question. – Mike Saull May 2 '13 at 18:06
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I've never measured the power consumption, but I accidently left my intercooler fan (same size as a typical AC condenser fan) on for an hour and a half with the car off. No noticeable impact on the battery. So, it's certainly not as big a consumer as the lighting... – Brian Knoblauch May 2 '13 at 18:24
    
Reportedly the fan pulls 9.5A. – R.. May 2 '13 at 22:14
    
Normally the AC compressor will take more power from the engine than the fan itself. – Mauro May 3 '13 at 7:16
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The compressor is probably %99.9123782 of the parasitic load. – Mike Saull May 5 '13 at 19:23

Since nobody even went to this realm, I thought it might be a good idea to answer.

When driving the vehicle, the fans only provide as much air flow through the radiator as if you were moving ~35 mph (in most vehicles). Once you get past this speed, the fans are no longer a factor. Infact, in most cases, the air moving through the radiator is actually pushing the fans faster than they'd normally spin. What does that mean? It means while the fans are engaged (powered on), they then become a small (very slight) generator and are pushing power back through to the electrical system. They are actually providing current. When this happens, they are creating zero drag on your electrical system and in some small way may be helping reduce drag produced by other electrical systems.

I saw how this happens when I installed a set of electric fans (Flex-a-lite Monster Fan 270) on my '04 Suburban which came with a water pump mounted clutch fan. After I got them installed, I was turning one of the fans and the other fan would turn as well (unaided in no other way). The only way this other fan could have been turning is if the first fan were creating an electrical current and providing it to the second fan. While every car is going to be different and every fan setup is going to provide different amounts of generated current, it is a real phenomenon.

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Interesting. If that's the case then you actually want the fans to be "always-on" at speeds above ~35mph so that you can harvest the generated power (to take load off the alternator) rather than wasted, right? – R.. Mar 11 at 15:47
    
@R.. - I don't know if it really means you should want them on, just that it really doesn't matter if they are on. They are not drawing power from your system at highway speeds. – Pᴀᴜʟsᴛᴇʀ2 Mar 11 at 15:49

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