Auto Air Conditioning Trouble
Reader Question:
My air conditioner is not cold enough at idle sitting at a stop sign,
but gets colder when I get on the freeway. Is this a sign that my
compressor is weak?
Thank you,
Bill
Dear Bill,
Let me explain in layman's terms how the
air conditioning (AC) system works and what could be happening to your
car.
Like your body, the air conditioning compressor
is the heart of the AC system, and Freon is the blood. The compressor
pumps Freon throughout the AC system, either the older type R12 which
costs as much as gold it seems these days, or the new
environmentally-friendly R134A Freon. This Freon is a gas and liquid
combination that is compressed and circulated throughout the air
conditioning system. The compressed Freon is pushed through the system
under pressure and is passed through different sized metal and rubber hoses
and a special valve called an expansion
valve that cause the gas to expand and contract.
This expansion and contraction makes the
Freon gas very cold. This cold gas makes its way via metal lines into
the dash area of your vehicle to the evaporator core. This evaporator
core is like a small radiator, except it has cold Freon circulating
inside and not hot antifreeze. A small fan (the AC blower fan which you
control from the control panel on the dash) sits in front of the
evaporator core and blows air across this cold evaporator and then
through the vents inside your vehicle.
The other objective of the air
conditioning system is to remove the heat from inside the cab of the
vehicle. This heat is removed by the Freon with the help of the AC
condenser located at the front of the car (usually in front of the
radiator). The Freon coming back from the evaporator carries the heat
from the cab to the condenser via rubber and metal hoses. Just like your
radiator, the condenser is lightweight aluminum with many internal
winding coils.
The Freon travels through these coils, and in between
these coils are small slits or fins that the Freon is forced through.
The condenser will have an electric cooling fan mounted in front or
behind it to push or pull air through these fins to remove the heat from
the Freon. Some vehicles still use the old fashioned fan blade driven by
the engine to pull air across the radiator and the condenser.
Now I know that is just a tidbit of
information on how the air conditioning system works, and it is very
general, but I wanted you to know what to look for to give you insight
as to what might be happening with your vehicle.
A few causes of low cooling efficiency or
no cooling at all at idle are:
- Lack of air flow across the
condenser. Make sure the electric cooling fan motor near the
condenser is coming on, or in models that are equipped with a fan
blade make sure this fan is turning and is turning very fast.
- Low Freon levels. Freon
level and pressure should be checked by your certified air
conditioning mechanic.
- Overheating. If the
engine is running hot or overheating, it can have a noticeable
negative affect on the air conditioning system. Some cars have two
electric cooling fans, one for the air conditioning condenser and the
other for the radiator. Make sure they are both working properly.
Usually at idle on a hot day with the AC on both fans will be on.
When the vehicle is traveling at freeway
speeds, the compressor is pumping the Freon throughout the system much
faster and harder than at idle. There is a dramatic increase in air flow
across the condenser due to 55 mph winds, and the engine is usually
operating at a cooler, more efficient temperature as well, thus allowing
the air conditioning system to operate efficiently.
Note: An air
conditioning system that is somewhat low on Freon can still feel
comfortable at freeway speeds due to the added air flow across the
condenser which can overcome the ill effects of slightly low Freon.
Periodic air conditioning performance checks by your mechanic are the
best way to keep the system in great shape.
Sincerely,
Austin C Davis
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