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How to Fix a Broken Car Air Conditioner

Updated on August 28, 2017
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Bud Scannavino is a Certified Master Automotive Technician. When it comes to modifying cars, Bud believes in excess.

When temperatures rise, the last thing you want to deal with is a broken car air conditioner
When temperatures rise, the last thing you want to deal with is a broken car air conditioner | Source

Has the air conditioning stopped working in your car? Or maybe it works, just not as well as it did last summer.

Perhaps your A/C makes a terrible musty smell. Or it rattles when you turn it on.

Whatever the problem is, nobody wants to deal with bogus car air conditioner when it gets hot outside. Worse, nobody wants to plop down hundreds of dollars getting the air conditioning fixed in their car.

Let's take a look some of the most common car air conditioning problems. Fixing these problems or preventing them from happening are easy DIY car maintenance projects anybody should be able to do.

Whatever the problem is, just hope you don't need a new A/C compressor. That's expensive.

The A/C Blows Warm Air and Is Noisy

Vehicle air conditioners operate on a chemical substance known as refrigerant. It's commonly called Freon, but there are other brands of refrigerant. The terms are used interchangeably.

In its gaseous form Freon will ever so slowly seep out of a car's air conditioning system. It’s normal. The entire system is composed of many parts made of dissimilar metals and materials. Those parts heat and expand, and get very cold and contact in various places, sometimes immediately next to each other. Something is going to leak, or rather seep, under those conditions. As Freon seeps out it takes refrigerant oil with it, because those two substances are mixed together inside the closed air conditioning system.

This oil serves several purposes. It seals the compressor vanes, so it compresses the Freon gas better, and keeps the compressor quiet as it does so. Lose Freon and you’ve lost precious oil as well, not to mention cooling capacity. If you make the common mistake of adding only Freon without adding oil at the same time, the system will produce cold air for a short time, then the compressor will fail dramatically (and expensively) from lack of lubrication.

The refrigerant oil also helps seal the system against seeps. The oil coats the various O-rings that seal the system, and lubricates the compressor’s neoprene shaft seal and keeps it soft so it can seal better.

So, don’t skimp on the oil. It is inexpensive. But be sure to get the proper type of oil, as different cars call for different types and weights of refrigerant oil. They don’t mix well with each other, and when they come into contact with each other will harden into stuff that does expensive damage.

So, look it up in a reference manual. If you don’t have one, ask your parts store to look it up for you.

The A/C Makes a Musty Smell Inside Your Car

The evaporator core in your car's air conditioning system is a delicate assembly that looks like a small aluminum radiator. It usually lives under the dashboard. That’s where the cooling action takes place, as the hot Freon in liquid form boils away into a gaseous form. This process removes heat from the car’s interior.

Evaporators are prone to corrosion from within, and it happens at the bottom where sediment and harmful chemicals accumulate. Gravity pulls those corrosive substances down there. Oil is heavier than Freon, so it settles on the bottom of evaporators and is forced out of the tiny holes that the corrosion forms.

So, the evaporator lives in a totally dark, confined area under your car’s dashboard. It never sees the light of day. It gets very cold and very wet as it removes moisture from the air as it cools it. You’ve seen that moisture dripping under your car on hot days when the A/C was running.

Cold, damp and dark is a perfect recipe for mold and mildew to form, especially if some dust is present, as it usually is. If you get a musty smell every time you turn your A/C on, there's your reason.

It’s cheap to fix. Sometimes it’s even free. The evaporator housing will always have a drain hole at the bottom to let the condensed moisture drain harmlessly away. The drain hole will have a soft neoprene hose, and that hose often will collapse with age. Find the hose and cut the collapsed end off of it to allow the trapped water to drain freely. Don’t be surprised if a quart of trapped water comes gushing out when you remove the hose. That confirms you found the problem.

A good way to kill the mildew is to turn the air conditioning compressor off a few blocks before you get home and let the evaporator air out. To do that switch the A/C to "vent" and leave the fan on. That will dry out the evaporator, but you should still have some cool air coming out of the dash vents from the residual coldness of the aluminum evaporator.

You could also buy an aerosol can of evaporator cleaner. It’s like the aerosol spray you use to disinfect your kitchen counter and bathroom surfaces, but it also contains a perfume to make your car smell good. Or you could just use the kitchen aerosol cleaner. Run the engine, turn the A/C to "recirculate" and spray the can of disinfectant into the A/C intake opening under the passenger’s side of the dashboard. That opening sucks air from inside the car, obviously, straight into the evaporator.

The A/C Blows Hot Air and Engine Is Running Hot

The A/C Blows Hot Air and Engine Is Running Hot

If your car's air conditioner is blowing hot hair and the car's engine runs hot when the A/C is on, you've got a bad radiator cooling fan. This fan draws air through the A/C condenser, which is mounted in front of the radiator, then through the radiator. Since both parts are lacking air flow, both the A/C and engine are running hot.

Simple and cheap to fix. Replace the cooling fan. While you are at it, clean the condenser fins with a garden hose.

Weak Air Flow from A/C Dash Vents

A clogged cabin air filter can make the air flow from your car's A/C weak.
A clogged cabin air filter can make the air flow from your car's A/C weak. | Source

If the air flow from the air conditioning dash vents is weak, you've got a clogged cabin air filter. Do you park your car under a tree to keep it out of the sun? It's probably clogged with leaves and pine needles.

Never mind that tree sap is not good for your paint, and that trapped, wet leaves cause rust wherever they accumulate on a car. The leaves, twigs and debris also block the flow of outside air from the base of the windshield into the car.

Read your owner’s manual to find out where the cabin air filter is, buy a new one, and then install it. The old one will be as filthy as a vacuum cleaner bag.

And while you're in maintenance mode, it might be a good idea to look up how to change the fuel filter in you car if you haven't done so in a while.

Air Flow Alternates between Cold and Cool

You can keep track of the temperature of your car's air conditioning with a thermometer mounted on a vent
You can keep track of the temperature of your car's air conditioning with a thermometer mounted on a vent | Source

If your air flow alternates from cold to just cool to cold again, well, you've got nothing to worry about. A variation in the air temperature is normal as the compressor cycles on and off repeatedly. Why does it do that? To make the car less expensive to build, that’s why.

You could buy an A/C thermometer. It’s a very inexpensive tool that looks like the kitchen thermometer, except the A/C thermometer is graduated to read lower temperatures. Your car does not get roasting hot.

Place your new thermometer in the center dashboard vent. When driving at highway speeds, the thermometer should read as low as, say, 38 degrees, maybe a little less. Then the compressor will turn off, and you’ll see the temperature rise to, say, about 58 degrees. Then the compressor will switch back on to bring the temperature back down. The cycle will repeat itself continuously. Weird, right?

Years ago, cars had extra, expensive parts and heavy, energy-consuming A/C compressors that would run constantly, all to keep the air flow coming from the dash vents at a constant temperature. When the evaporator temperature would approach freezing, the valves would redirect some heated Freon back through the evaporator to prevent icing.

Then somebody figured out they could leave those parts out, and just have the compressor turn off when the evaporator approaches icing conditions. Cheap design, right? Luxury cars still have those valves to allow the compressor to run constantly and keep the vent outlet air at a constant temperature, just like in the good old days.

No Air Flow at All from the Dash Vents

A car HVAC blower motor
A car HVAC blower motor | Source

If you're not getting air from the dash vents, the HVAC blower motor has stopped running. A number of issues can create this situation. The inexpensive motor is broken, or a cheap relay that feeds it power is bad. Or the resistor that regulates the fan speed is bad, also not too expensive to replace.

If you know a little bit about electricity, you can test the circuit until you isolate the problem. Hopefully it's not something that's really expensive that needs to be fixed, like an electronic control module.

Car Air Conditioning Maintenance Tips

Run the A/C in the Winter from Time to Time

If you don’t use your windshield defroster during the winter, the A/C system will remain idle. That inactivity is not good for all the O-rings and it’s especially not good for the compressor. All those parts need to receive a bath of oil every week or so, even when the weather is cold.

Keep the A/C Condenser Clear

Here’s another tip. The A/C condenser, the part in front of the radiator, the one with all the fins that bend so easily when you touch them, absolutely needs to be kept free of leaves, dirt and even plastic grocery bags that can get sucked up from the road and lodge there. All that junk blocks the flow of air through the condenser, which causes the compressor to overheat and fail. Just look at the condenser every blue moon and make sure it’s clean. That's not asking too much.

Get Moisture out of Your A/C System

Even a small amount of humidity in the air is too much moisture to leave inside an A/C system after you open it up for repairs. Moisture will freeze and cause blockages, which is the least of your troubles. Worse, it will react with refrigerant oil and make it harden into a wax-like residue. That’s not good for expensive parts. You’ve got to get the moisture out. Don't skip that step. That’s what evacuation pumps do.

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