ok - here's the situation: I have been keeping track of gas milage on
my '96 tbird 4.6, and when I don't run the defroster, I get between 25
and 26 mpg. I ran a tank through with the defroster on and I get just
over 20 mpg. Is this signaling the A/C going out? Is this much of a
drop normal? I thought when you run the defroster, the A/C comes on as
well, but isn't supposed to cool anything. Am I on track or just
Normally, the compressor does run when in defrost. It does cool the air,
but the air is then sent past the heater core to warm back up. Cooling the
air squeezes out the humidity, which fogs the inside of the car. If your
side windows are fogged up, use the regular A/C mode, but turn up the temp
knob and aim the ducts at the windows. It'll dry them up real fast.
If you have a combined heat/defrost mode, it may not run the compressor.
It may work for light defogging.
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Air Conditioning doesn't really cool air, it removes humidity, cooling is a
byproduct, so yes, it's designed to run the compressor when you are in
defrost. A 4-5 MPG drop seems excessive. Do you notice any drop in engine
power? It's possible your A/C is over charged.
I suppose I have noticed some power drop, but not a whole lot. Maybe
I've had it all along but just never noticed it - the car was a city
driver for its first ~50k. (now at 162k)
I thought the MPG drop was a bit much- I have never had the A/C
serviced. I bought the car in '96 and for the first 15-20k and at least
the past 80k miles, I've just been taking it to the dealer and telling
them to do the "xxx,000 mile maint". I will probably say something next
time it is in (should be next month). I was worried that I might be
ruining the compressor- those are expen$ive.
AC 101....air conditioning does not cool the air....it compresses a fluid
which boils at a very low temperature. at 70 degrees most freon type liquids
are a vapor...they've boiled now boiling removes heat...put a liquid under
pressure past it's boiling point.....control the release into a small
radiator type thingy, blow air over it and voila...cooler air...now cold air
can't hold much water vapor so it's cool dry air...run the ac make cool
air...blow that over heater core, warm it up, blow warm dry air over cold
foggy windshield...voila...no foggy windshield.
with the miles on this puppy and getting 20 mpg+ with ac running I say quit
sweating the small stuff and make an offering to the car gods...this thing
is doing great some drop in power is normal as the air conditioner does
require from 10 to 25 horsepower to run, depending on outside air
temperature, blower setting, whether it's set on "normal" or max and it
takes gasoline to produce that horsepower. I think you are just seeing the
effect of 162 k on the motor...it's not blown up or anything but an old
horse just can't do all the stuff a young horse can. just follow normal
maintenance and keep on driving that puppy. might be nice to have somebody
put a set of gauges on the ac to verify system charge...over charged ac
system will cause high side pressure to be way high...300 psi or higher in
the r12 days not sure with r134 and the harder that compressor has to work
to move the 'fluid' in it the more horsepower it's gonna need...more
horsepower for the ac=less horsepower for the motor...put the engine under
more than normal load and it's gonna burn more gas I know that's a very long
convoluted answer but I'll probably get a few comments about where I'm not
exactly right but then It works for me...
I guess I'd never thought of it that way before- the car had 205hp
(according to the reviewers) when it was new, and it probably doesn't
have that now. The 10 to 25 hp the A/C uses is now a more significant
percentage of total engine power than it was when the car was new. Not
to mention that the A/C is just as old as the rest of the car and
probably isn't running at peak efficiency any more. I'll have it
checked the next time I take it in. (should be next month at 165k)
Patrick Gilmore wrote:
Actually he does NOT have it backwards. YOU do.
#1. There is no such thing as cooling air. You can only remove heat.
#2. When you remove the heat, the moisture in the air sticks to the
evaporator, hence you have dryer air.
#3. See #1 and #2.
Other way around. Expansion of gas in A/C system lowers the temperature,
cooling the air. The air is cooled below the dew point so the moisture
condensates, removing the humidity as a byproduct, it is then sent past
heater core and warmed back up.
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