Rick's Free Auto Repair Advice

Fix car AC

Fix car AC that’s blowing warm air

It may seem pretty simple to fix your car’s air conditioning, you simply add a bit more refrigerant and that’s it. Right? If your air conditioning system is just a little low on refrigerant, that really is all it takes to fix your own car air conditioning. But many times, adding more refrigerant isn’t the fix. In fact, sometimes adding more refrigerant can actually make your car’s air conditioning worse. Follow these steps to see why the AC is blowing warm air in car. I’ll walk you through how your air conditioning system works and give you some tips on how to diagnose it. Keep this page open as you click on the individual links to side topics. They’ll open in a different tab. Then come back to this master article and continue where you left off.

Step 1: See if the AC compressor clutch is engaged

Car makers use an electro-magnetic clutch to engage and disengage the compressor. When power energizes the clutch coil, the magnetic field pulls the clutch in to engage the compressor shaft. If the clutch doesn’t engage, you’ll have to perform an AC compressor clutch diagnosis to find out why it isn’t working. Because if the clutch doesn’t engage, the air conditioning compressor can’t pump refrigerant. So start your diagnosis by determining if the compressor clutch works.

Turn the air conditioning controls to MAX AC and start the engine. Then look at the front of the compressor clutch.

compresor clutch, wiring diagram, recharge air conditioning system.

Air conditioning compressor and clutch assembly

Both the pulley and the clutch should be spinning. If only the pulley is turning, there’s a problem somewhere in the system.

If the clutch doesn’t engage, the problem could be a blown fuse, an open in the wire to the compressor clutch coil, a bad compressor clutch coil, a short in the compressor clutch coil, a poor ground, or a low pressure lockout.

Air conditioning compressors need a constant supply of oil to prevent catastrophic failure, car makers install a low pressure switch to prevent the compressor from running if the system is low on refrigerant. NEVER try to bypass the low pressure switch. Doing so will force the compressor to run with inadequate oil and can destroy the compressor.

If the compressor clutch engages, but the vehicle doesn’t cool properly, continue with the following checks.

To learn more about air conditioning compressor clutches, sample wiring diagrams, and instructions on how to test them, read this article: Air Conditioning Compressor Clutches

Step 2: Decide if it’s really an air conditioning problem

All car air conditioning systems are designed to work side-by-side with the car’s heating system. In some cars, the heat is regulated by a heater control valve that controls the flow of hot engine coolant to the car’s heater core. If you get cold air at idle, but it gets warmer at highway speeds, suspect a problem with the heater control valve or the blend door.

blend door actuator, blend door motor

Check blend door actuator movement

The heater control valve is operated either by a cable or by a vacuum motor and engine vacuum switched by the heater control in the passenger compartment. Check under the hood and follow the heater hoses from the engine to the firewall to see if your vehicle uses a heater control valve. To learn more about heater control valves and how to test them, read this article: Heater Control Valves

Most late model vehicles use a blend door system to regulate heat, along with a heater blend door actuator. Hot engine coolant always flows to the heater core when the engine is running. The heater blend door actuator operates a door in the heater box. The heater blend door actuator opens and closes airflow through the heater core. If the door binds and cannot close properly, or the heater blend door actuator fails, the door may remain open, heating the air enough to cancel the cooling effect of your air conditioning. To learn more about heater blend door actuators and blend door systems, read this article Blend door Temperature Control.

Step 3: Check for proper evaporator airflow

The evaporator coil is located under the dash in your vehicle. Any reduction in airflow across the evaporator coil can cause the condensed moisture to freeze and restrict airflow even more. If the expansion valve temperature sensing bulb or the evaporator freeze sensor detects a freezing condition, it will shut down your air conditioning. So you may have a situation where the AC in car blows cold then warm. If your vehicle is equipped with a cabin air filter,

cabin air filter, blower motor, blower motor resistor

The cabin air filter catches all the debris from fresh air, like leaves, dust, and pollen

check it before suspecting a problem with the air conditioning system. To learn more about cabin air filters and how to check them, read this article Cabin Air Filters

In many cases, debris can obstruct the airflow across the evaporator coil, especially in systems that don’t have a cabin air filter. To check the condition of your evaporator coil, simply remove the blower motor resistor and use a mirror and flashlight check for leaves and debris on the evaporator coil. Remove any debris to restore proper airflow.

Step 4: Check for proper airflow across the condenser coil

The condenser coil sits in front of the radiator and is prone to clogging with leaves and insects.

AC doesn't cool, car overheats

The condenser coil sits in front of the radiator, so it catches all the bugs on the road and reduces airflow

Those clogs reduce airflow and prevent proper refrigerant gas condensation back into a liquid. Pop the hood and check the condenser coil. Remove any debris with a soft nylon brush and soapy water. Then flush the condenser with your garden hose.

Check the radiator fans

Proper radiator fan operation is critical to the operation of your vehicle’s air conditioning. If you’ve checked the condition of the condenser coil and radiator and are sure both are free of debris, make sure the radiator cooling fans are operating properly. Many vehicles have two radiator fans. Both should turn on the instant you activate the air conditioning. However, even if both fans run, that doesn’t always mean they’re operating properly since many car makers employ variable speed mechanisms to adjust the fan speeds.

Many late model vehicles have electric radiator fans that operate with relays. If your electric fans aren’t operating properly, your vehicle will overheat and the AC won’t cool properly. Here are two articles that describe how to diagnose fan problems.

Chrysler radiator fans

Diagnose an overheating condition

Check the shrouds and seals

If you’ve ever replaced a radiator or had work done on the front end, double check to make sure the seals and shrouds have been replaced. The seals prevent air from bypassing around the condenser coil and radiator, and the shrouds ensure the radiator fans pull as much air as possible through the condenser and radiator. Seals and shrouds are VERY important to the proper operation of the radiator fans. If they’re missing or broken, you can experience reduced air conditioning efficiency and engine overheating.

Step V: Find the refrigerant problem

At this point you’re going to have to invest in heat load test equipment so you can conduct a maximum heat load stress test. The heat load test will put the system in a state where you can more easily find the root problem.

You’ll need a digital readout thermometer and a temperature probe. One inexpensive option is the AGPtek Dual Two Channel Digital Thermometer 2 K-Type Thermocouple Sensor.

multimeter with thermocouple leads. AGPtek Dual Two Channel Digital Thermometer 2 K-Type Thermocouple Sensor

Digital temperature meter with two thermocouples

and a PYLE Meters PCTL01 Pipe Clamp Temperature Lead. With these thermometers you can check the level of refrigeration charge, the operation of the evaporator and condenser, as well as and restrictions in hoses. Hey, if you want to save the cost of a shop conducting these tests, you simply have to own or have access to the right tools, so stop whining. There’s no cheap magic bullet to fix your car’s air conditioning system.

clamp style temperature sensor

Clamp temperature sensor






Set up a maximum heat load test

1) Park the vehicle outside in direct sunlight

2) Open all doors and windows

3) Start and run the engine up to full operating temperature

4) Let engine run at idle

5) Set AC to MAX

6) Set the blower on High

Then refer to the actual tests procedures below. Pick the test that’s right for your vehicle. If you’re not sure which type your vehicle has, consult a shop manual or visit an owner’s forum online.You may have a thermostatic expansion valve (TMX) system, or an orifice tube style system. Click on the appropriate link.

Find out what type of AC system is in your vehicle

Diagnose an AC problem in a TMX system

Diagnose an AC problem in an orifice tube system

Step VI: Proceed with the car AC fixes

If the system is low on charge, there’s a reason for it. This is a sealed system. So a low charge means you have a leak. However, there are leaks, and then there are LEAKS. It’s not uncommon in an older vehicle to leak a small amount of refrigerant over the winter months when metal parts contract and seals age. If your air conditioning was working fine last fall, but is low on charge the following spring, it’s probably due to winter leakage. You can choose to add more refrigerant to top it off, AC pro refrigerant refillor you can replace the most likely parts to prevent this from becoming an annual event.

However, if you add refrigerant and get the system cooling properly for a while and then find that it’s low on charge during the same cooling season, you’ve got a non-winter leak and that must be fixed. You can try replacing the parts listed below. But your leak may require the replacement of expensive parts like the condenser and evaporator coils. Those are not parts you want to replace unnecessarily. So it’s best to have a professional find the leaks for you. If the leak is in a major component, leave the job to a pro.

Fixing minor car AC refrigerant leaks

Refrigerant leaks occur most frequently at hose-to-evaporator, receiver/dryer, accumulator and condenser fittings. You must evacuate the system to replace the special HNBR O-rings used at these fittings.

HNBR O-ring assortment

HNBR O-ring assortment

And, once you evacuate the system and open it to ambient air, you must also replace the receiver/dryer or accumulator. Then you must pull a vacuum for at least 45-mins. before refilling it with refrigerant.

If you opt to replace the O-rings, it’s a good idea to also replace the orifice tube or TMX valve. You can buy a complete O-ring kit for less than $15. Orifice tubes cost less than $10. TMX valves cost about $25. Most professional auto parts stores carry these parts. However, if you discover sludge anywhere in the system, stop. A sludged system requires a thorough professional flush before it can be recharged.

HNBR O rings

Buy a kit containing all the O-rings and seals for your system

low on freon, orifice tube

Orifice tubes are color coded. Buy the same color if you replace yours.



How to recharge your car’s AC

Recharging your car’s air conditioning system poses some risks to the vehicle and to your safety. It’s actually pretty easy to get frostbite from refrigerant, so wear leather gloves.

goggles to protect your eyes when recharging car AC

Protect your eyes from frostbite with safety glasses

And wear safety glasses as well. Trust me, I’m not kidding about this.

gloves for recharging car AC

Wear leather gloves when handling refrigerant

The biggest mistake DIYers make when recharging their car’s AC is to add too much refrigerant. In most late model vehicles, the tolerance for over/undercharging is 0 to 1-ounce. That’s it! If you overcharge, you’ll REDUCE cooling. In fact, just a 2-oz. overcharge results in a 15% drop in cooling. So more is NOT better.

The same holds true for adding refrigeration oil. Excessive oil coats the inside of the evaporator and condenser coils, insulating them and reducing heat transfer.

The key to DIY recharging is to add as little refrigerant as possible to get the maximum cooling. Following the instruction with the recharging kit, make sure you add refrigerant SLOWLY. It takes time (up to 5-mins) for the pressures to stabilize after adding refrigerant.

Invest in a digital thermometer and insert it into the center duct while you charge.

digital thermometer

Shove the digital thermometer in the center duct and check it while you recharge

Consult the gauge often. Quit charging once the thermometer falls below 50°F. Let the system stabilize for 5-mins. and then check the pressure reading on the recharging kit. If the gauge reading is still low, add small amounts at a time and wait for the gauge to reflect the changes.

If the can gets cold during the recharging process, place it in a bowl of warm water.

Can you jump a low pressure switch to get the AC compressor clutch to engage?

© 2014 Rick Muscoplat






Posted on by Rick Muscoplat

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