How to keep your car Air Conditioner working well

by Leon Percy, Mountain Air
leonp@modemss.brisnet.org.au

Leave the Fresh / Recirculate air vent open when you leave the car

The evaporator is situated under the dashboard, it's job is to remove moisture from the air, thereby cooling it. The water drains out a tube onto the ground, but the evaporator fins remain damp. When left closed up for long periods, this water with any dust and lint, smoke etc. can cause a musty smell. Opening the fresh air vent will let the case breathe, and dry it out to stop it smelling like a storm-water drain. If the car is parked in the sun, the open vent will also reduce the inside temperature, so the air conditioner will then work faster on startup. When you get into a hot car, open the windows and leave the fan on high and the vent on 'fresh' until it cools down a bit. This will flush the hot air out of the cabin. When the temperature equalises, close the windows and switch to 'recirculate' for best results. Familiarise yourself with the car owners handbook to help you figure out the controls.

The A/C performs best on 'recirculate' and lower fan speeds

The air temperature at the fresh air duct above the bonnet may be over 40° but the temperature in the recirculate duct in the foot well may be only 20° once it has been running for a while. So there is 20° head start by using recirculated air. The more time the air spends going through the evaporator core, the colder it will get and the less power it will use, so don't use a higher fan speed than necessary to keep you comfortable. If it is very hot, or if the sun is shining on you, increase the fan speed. Window tinting can dramatically improve the efficiency of the A/C system, by rejecting heat before it gets inside the car.

On the open road, with rear passengers, mix a little fresh air

Open the fresh air vent slightly, and open a rear window a fraction to improve the flow-through ventilation. This will extract some warm air from the back and draw the cool fresh air from the front into the rear seat, as most cars do not duct air to the rear seats, this will keep the rear passengers happier.

Don't expect good A/C performance at low speeds

The A/C compressor is not fully efficient below about 1800 rpm, so the system will not perform well at idle or in a traffic jam because of low compressor speeds and low air flow through the condenser out the front.

Don't restrict the airflow through the front of the car

The condenser (in front of the radiator) can only cool the gas down by a certain number of degrees. If the airflow through the grille is restricted by spot lights, a car-bra, the number plate hanging down over the grille inlet or flywire over the front, you are asking for trouble. The radiator needs very little airflow to keep the engine temperature under control, but the A/C needs plenty of constant flow to keep the pressure / temperature down. If the airflow is blocked for as little as 30 seconds, the pressure can treble.

Don't thrash the system

If you have a true temperature adjustable thermostat, or an 'economy mode' use it only as cold as necessary, to conserve power and fuel. "Climate control" systems tend to mix warm air with the A/C to adjust the average temperature where the sensor is, so you can't control how much the compressor is used, stick to about 21° for economy. On a regular "preset" system the vent temperature should be between 2 and 5 degrees when the thermostat cuts out, this means that the evaporator core may go below freezing briefly. If the temperature and humidity conditions are right, this can cause fog to blow out of the vents, in the short term this is not a problem. If this behaviour keeps up for 10 or 15 minutes, the system may form a sheet of ice over the evaporator and you will lose air flow and the air will go warm. If you suspect this has happened, turn the A/C off and put the fan on high, and let it thaw out for a few minutes. Then run the system on fresh air (forcing the intake air temperature up) until the conditions change. Get a technician to adjust it for you.

Keep an eye on the drive belts and electric fans

If the alternator belt is loose it will squeal when you switch the A/C on because of the extra electrical load. If the A/C belt is loose, you will get a resonant vibration above idle speed and it may squeal when the clutch cycles on at high speeds. Make sure you report any strange noises, the belts are often shared by more important things like the power steering.
If the electric fans stop, the high pressure switch will shut the compressor off for safety when the system gets to about twice the normal pressure, until the pressure drops off again. The symptoms are the compressor cutting in and out about every 15 seconds, this should be rectified as soon as possible. Many front wheel drive cars rely on both fans for the engine cooling as well.

Get it serviced regularly

From new, and A/C system may not need service for up to 3 years, but it should be inspected each year to keep the warranty valid. The refrigerant is very hygroscopic (it attracts water) and will absorb moisture through some hoses and connections. Minute amounts of moisture in the gas cause hydrochloric acid to form, and this will obviously corrode the metallic parts within the system. The filter/receiver/drier contains a desiccant to dry out the gas in operation, this should be replaced every 2 years, in accordance with manufacturers specifications, or whenever the system is opened for more than a few minutes.

Systems should be run for at least 10 minutes each week

There is a shaft seal in the compressor which requires an oil film to make it seal properly. Manufacturers recommend running the system at least ten minutes per week to lubricate this seal, or the gas will leak. The compressor relies on oil being circulated in the gas to return to the compressor for lubrication and the returning gas cools the compressor, if the system is not performing due to lack of gas, the compressor will overheat, and subsequently fail.

If the system does not cool properly, have it checked

There are many causes for cooling problems, some may not be directly related to the air conditioner itself. Many cars use vacuum controls for the airflow system, if a vacuum line perishes and leaks off, or is disturbed during service, it can cause the air vents to malfunction, or even bring the heater on. Electrical faults are common, with loose or damaged wires, faulty relays, switches and thermostats as leading causes. Electronic systems are controlled by a 'black box' which can fail. Not all these are difficult or expensive to fix, so have any strange behaviour checked.