Two Reasons Your Car AC Blows Warm Air

When a car AC blows warm air the root cause could be anything from a simple malfunction to a major expensive problem. Here we’ll discuss two common issues that can cause driver complaints of poor AC performance on hot days.

AC Evaporator Freeze Up

The air-conditioning evaporator is the part that gets extremely cold. It’s located in the HVAC compartment often very close to the blower motor. If operating properly, this device will maintain a temperature just above the freezing point. The interior blower motor forces air across this block of delicate heat exchanging fins. This process turns hot humid air into cool dry air by leaving the heat and the water in the evaporator coils. During normal operation gravity naturally removes the water droplets and deposits them outside of the vehicle. This is the water you see on the ground after running the cars AC. When the evaporator gets too cold this moisture can freeze around the evaporator coils. In this situation instead of air passing through the heat exchanging device it’s forced around the outside. This type of operation can cause increased temperatures of the air exiting into the vehicle. A visual sign that there is a problem in this area is when it takes an hour or so for the puddle to form under the car after the AC was run for a while.

What Causes Evaporators to Freeze

The component responsible for metering the Freon into the evaporator is either an expansion valve or an orifice tube. The automobile will be equipped with one or the other, but not both. In the case of an expansion valve, a temperature probe is mounted to the evaporator coils. This device controls the amount of refrigerant passing through the valve. When the probe detects icing conditions it can halt the flow. If the temperature probe becomes detached from the coils, or the valve malfunctions are two reasons that can cause ice to form. Vehicles that use the orifice tube type metering device don’t use a temperature probe. They are simply designed to allow the right amount of Freon to flow into the evaporator assembly. If a frozen evaporator is found, it’s possible the wrong size orifice was installed during service. A partially occluded orifice tube can also cause an icing situation at the evaporator coils.

Improper Freon Levels

When it comes to the output temperature of a cars air-conditioning system one of the most critical areas to investigate is the amount of refrigerant charge currently in the system. An individual automobile is engineered to run with a very specific amount installed. Just a couple ounces in either direction can affect how the air feels coming out of the vents.If the Freon is low enough it can cause the compressor to cycle on and off because there is not enough in the system to maintain the proper pressure readings. In this situation the compressor is turning itself off for its own protection, because the lubrication it needs is contained in the refrigerant. When the Freon level drops to a point where it’s dangerous, the compressor will not engage at all.

Too Much Refrigerant

On the other side of the spectrum is an overcharge situation which can also increase the output temperature of the air entering the passenger compartment. This is a common situation when drivers stop in the local parts store and buy a can of Freon with a self install kit or hose. Installing a little bit too much Freon won’t do any harm to the system but it can cause undesired results. When a do-it-yourself person installs way too much refrigerant this can lead to compressor damage. A telltale sign of an overcharge is a buzzing sound when the compressor is engaged and warm air discharged from the AC vents.

How Can You Tell How Much Freon is in the System

This is a great question and it’s something you need to know before you begin going down a logical path of diagnosis. The only way to know for sure how much is installed, is to remove it all, and weight it. The measurement the factory uses for full charge is in pounds and ounces on domestic cars. There will be a label in the engine compartment near the service fittings with the specs. An AC service machine can pull all of the Freon out and provide a measurement of how much it removed. This can be compared with factory specifications and is a great first step in solving complex automotive air-conditioning problems. The next step is to use the machine to pull a vacuum on the AC system. After its pull down to 30 inches of mercury it can be turned off and sealed to see if it holds this negative pressure. This is a quick and dirty way of seeing if the system is leaking. If it passes this test then the present vacuum can be used to draw in the recycled or fresh refrigerant charge. This assures no air is injected back into the system during the recharging procedure.