Aircon only during long descents? Will mileage stay the same?

[Synchronicity]

Just wondering, if I'm going down a long hill and not pressing the accelerator pedal, it won't hurt mileage to turn the A/C on will it?

I routinely travel down some pretty long grades between 5 & 10% gradient here on the island of Tenerife. For a 500m /1500 ft hill, this equates to between 5 & 10km of continuous downhill driving. For the 5% grades I often coast in neutral and I can maintain my speed (75mph or 120km/hr) with the engine at idle. Other steeper descents it is better to do in gear as otherwise the brakes can overheat. So the engine revs are there but you're not using any fuel (apart from that used to make the engine idle I suppose) since you aren't pressing the accelerator pedal...

I'm wondering: does the air conditioning run from the alternator or it's own drive belt...?

I've been switching on the aircon on the downhill sections and turning it off for long uphills. I'm guessing this will improve my fuel economy? It seems that this is the only way of recouping or regenerating some otherwise lost energy when driving downhill...

For those that are wondering, outside temp in the Summer ranges between 20 & 30°C and my passsenger compartment stays cool for up to 5 or 10 minutes afterwards as I've had it fully insulated.

[MGB=MPG]

running the AC on steep descents should increase the engine brake effect AND cool your cabin w/o using fuel

some machines have one serpentine belt runs everything, others have independent V Belts

out side temps here have been 30-35 C and in the cabin up to 45 C,. roast chicken in there i think, im gonna run the AC.. or die

[GRU]

If you leave it in gear on a decent then the a/c wil not use any extra fuel but if the car is auto trans or you put the car in neutral it will use more fuel

[euromodder]

Originally Posted by Synchronicity

Just wondering, if I'm going down a long hill and not pressing the accelerator pedal, it won't hurt mileage to turn the A/C on will it?

When you're having to use engine braking while going downhill in gear, the AC will be driven by gravity, and won't use any juice.

I'm wondering: does the air conditioning run from the alternator or it's own drive belt...?

They usually have their own belt & clutch to engage / disengage the AC compressor.
Though I seem to recall some cars having electric AC , and with stop/start systems the AC keeps running despite that the engine is off ... so they either recycle the air, use an electric compressor, or would have to restart the engine to power the AC.

I've been switching on the aircon on the downhill sections and turning it off for long uphills. I'm guessing this will improve my fuel economy?

It should - unless the engine load is so high during the climb that the AC would be disengaged anyway, in which case it wouldn't matter.