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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?

R.. GitHub STOP HELPING ICE
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2 Answers2

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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.

Scott Hillson
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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 02 '13 at 18:24
  • Reportedly the fan pulls 9.5A. – R.. GitHub STOP HELPING ICE May 02 '13 at 22:14
  • Normally the AC compressor will take more power from the engine than the fan itself. – Mauro May 03 '13 at 07:16
  • Yeah - the power requirement for AC is almost all the compressor. The fans on these things are low power as they don't need to move air fast, just keep it moving. – Rory Alsop May 03 '13 at 08:26
  • 9.5A @ 12v = ~115W = .115kw = .15hp. – Mike Saull May 03 '13 at 14:22
  • Obviously the compressor uses a lot more power than the fan, but the difference in optimal MPG with or without A/C is pretty big (about 36 vs 42 if everything else is in good shape for best mileage) and so even if the fan were just 10-15% of that it could still make a measurable difference. – R.. GitHub STOP HELPING ICE May 04 '13 at 01:21
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    The compressor is probably %99.9123782 of the parasitic load. – Mike Saull May 05 '13 at 19:23
  • @MikeSaull: Combining your last two comments, the compressor is using around 150 hp. That doesn't sound plausible considering the max output of the engine is below 150 hp. – R.. GitHub STOP HELPING ICE May 06 '13 at 19:42
  • According to this thread, I'm seeing estimates in the range of 1-5 HP. So at the low end, the power consumed by the fan could easily be 10%. At the high end, it would be fairly negligible. So it really does matter where in that range the actual consumption falls. Also, I would expect a vehicle which uses dual fans or one large fan for both the radiator and condenser to pull twice as much for the fan, making it up to 20-30%. – R.. GitHub STOP HELPING ICE May 06 '13 at 19:51
  • @R.. Lol yeah I was exaggerating :p. Go ahead and try it hookup a relay to power disconnect the ground of the fan and run a switch to the cabin and let us know if you see any noticable improvements. Just don't forget to turn the fan back on when you slow down. – Mike Saull May 07 '13 at 14:44
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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.

Pᴀᴜʟsᴛᴇʀ2
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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.. GitHub STOP HELPING ICE Mar 11 '16 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 '16 at 15:49