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I have a 96 Camry, 2.2L Automatic with very basic AC system. AC is either on or off. No temperature setting. Fan has four speed settings. My question is this. With AC on, does the speed of the fan have any impact on FE? I have read about better FE with AC on low, but isn't the load on the compressor the same regardless of fan speed?

It really doesn't make a difference what fan setting you use. There's a tiny extra power draw to run it faster, but nothing you'll be able to measure in mpg.

Set it to max and click the fan from 4 to zero to 4 to zero as needed. Set the hot/cold mix to max cold - any warmer and it's just mixing in some engine heat. The AC is still running at max. Pulse and Glide the AC.

I just got my scangauge and I just tried it for all of 1 minute.

Idling, but at fastest fan speed, my GPH was .33
Idling, but at lowest fan speed, my GPH was .30
For a 10% reduction.
And since there is a slight margin of error at +/- .01 anyway due to engin fluxuations, that's even more insignificant.

As another test, I did change the temperature setting. MAYBE a .01 change, if at all (and that could have been due to fluxuations as well).

I'm guessing you're better off running it at full speed, then shutting it down for a short while. 'Course, if you're in Florida in the summer, you might burn out your A/C by doing that too offten!

-Mike

I ran with nothing but the fan at max on a tank and my scanguage showed a difference of 0.03gph as well. But when I went to fill up my tank, the sg was 3% off of what it estimated I had gotten.

I ran with nothing but the fan at max on a tank and my scanguage showed a difference of 0.03gph as well. But when I went to fill up my tank, the sg was 3% off of what it estimated I had gotten.

Hmmm. Quick back-of-the-envelope calc. At an avg speed of 40 mph, and an avg FE of 40 MPG, you'd expect an average burn of 1 gph. So 0.03gph difference would indeed be a 3% difference. Sound right?

I think that the A/C compressor is controlled by a non-adjustable thermostat to prevent freezing up the evaporator inside the HVAC blower. This activates an electric clutch on the compressor drive belt pulley so that it cycles on and off. Therefore the more hot air being blown through the evaporator coil (looks like a small radiator) the less often the compressor will cycle to off. In very warm conditions, even on a slower fan speed, the compressor may tend to remain on.

In more moderate conditions you will likely find that there would be some savings by running the fan at a slower speed due to the compressor resuming its more normal on-and-off cycles.

One thing I found useful on my '91 Accord with manual A/C was to avoid using it going up long grades because of the extra load on the motor but then switching it back on when going down long grades. It was almost like downshifting and saved on the brakes.

Keep cool! The humidity in your area can be miserable this time of year.

Cheers,

Roger

Thanks guys. Good technical explanation Roger. I thought as soon as I hit the AC ON button, the compressor stayed on continuously, but obviously, this is not the case. So, in summary, by passing more cabin air past the evaporator, theoretically, I heat up the refrigerant more, causing the compressor to cycle on longer, resulting in lower FE. Makes sense. If Nissynis's calcs are correect, it looks like about a 3% reduction in fuel economy when switching from low to high fan speed. Very interesting!

Shutting the AC off uphill and turning it back on when going down hill also makes sense - especially if you are applying brakes to limit speed on the down hill. Thanks for the additional tip.

I think that the A/C compressor is controlled by a non-adjustable thermostat to prevent freezing up the evaporator inside the HVAC blower.

Would that be the cold equivilant of a thermal circuit breaker?....something really designed not to go off unless the system has problems or is abused?

Experience with older cars suggests that lower fan speeds result in colder output air (due to longer contact time), but that air isn't distributed as completely through the car. Plus it can be almost painfully cold if blowing onto your hands. Turning the air speed up, results in warmer output and better mix of air...I know when I had it set low, I tended to turn the A/C on/off more often than when I set it to high. With passengers in the back, high or one notch down was needed to keep them cool without freezing myself. Much happier now with temperature based A/C controls.

Hmmm. Quick back-of-the-envelope calc. At an avg speed of 40 mph, and an avg FE of 40 MPG, you'd expect an average burn of 1 gph. So 0.03gph difference would indeed be a 3% difference. Sound right?

Not sure if I follow. At idle, SG showed 0.30gph with the fan off or on low, and 0.33gph with the fan on max. I normally have my sg calibrated to report 5% less but this tank when I filled up I had to adjust it to 8% less, so it was 3% off of its predictions.

So instead of using 0.33gph it was actually using 3% more than that, which was 0.34gph.

Not sure if I follow. At idle, SG showed 0.30gph with the fan off or on low, and 0.33gph with the fan on max. I normally have my sg calibrated to report 5% less but this tank when I filled up I had to adjust it to 8% less, so it was 3% off of its predictions.

So instead of using 0.33gph it was actually using 3% more than that, which was 0.34gph.

Oh, I'm proceeding on two assumptions:

1) that you don't idle the whole tank away, but actually go somewhere, and therefore your gph doesn't stay in the 0.30s :)

2) that the gph draw of the fan remains constant independent of whether you are idling or gassing it

And you are right -- I misunderstood your prediction. I thought you meant that you had a projection in mind based on past experience that was off by 3%. I don't know what would have caused the SG calibration error.