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Car AC blows cold then blows warm

Car AC blows warm with low airflow

If your car AC blows great cold air for a while and then gradually turns warmer and you notice a decrease in airflow, the problem is usually caused by a low refrigerant charge. Here’s what’s happening.

Low car AC charge causes evaporator freeze up

How can a low car AC charge cause the evaporator to freeze up?

AC blows warm

Evaporator coil

You would think it would be just the opposite; that a low charge would cause it to never get cold enough to freeze. To understand why that’s wrong, you first have to understand how refrigerant works.

As the liquid refrigerant is dispensed into the evaporator (the cooling coil inside the passenger compartment), the liquid refrigerant picks up heat from the cabin air flowing across the coil. That heat causes the refrigerant to boil and change from a liquid to a gas. All the refrigerant needs is just enough heat to cause that “change in state” from liquid to gas. Removing that amount of heat from the cabin of your car is what causes your car to feel cool.

The proper refrigerant charge will cause the evaporator to remain slightly above the freezing temperature and produce and air temp or around 40°F

Once the refrigerant has changed states to a gas, it should flow out of the evaporator and into the compressor to complete the cycle.

But when a system is low on charge, the gaseous refrigerant continues to remove even more heat from the cabin air passing over the coil. The refrigerant near the evaporator exit becomes superheated. This additional heat removal causes the evaporator temperature to fall below freezing. Humidity in the passing air to freeze freezes on the evaporator. Over time, the moisture builds to the point where ice blocks the flow of air across the evaporator. That shuts off the AC and you no longer get cooling.

Clogged cabin air filter can cause evaporator freeze up

Just like the situation with low refrigerant charge, low airflow caused by a clogged cabin air filter causes the refrigerant to linger too long and pick up

Cabin air filter

Cabin air filter catch all the debris from fresh air, like leaves, dust, and pollen

too much heat from the cabin. If your car has a cabin air filter and you think you might have an evaporator freeze up problem, start by checking the condition of your cabin air filter.

How to check for evaporator freeze up

Evaporator freeze up usually happens most often on humid days after a long drive.  So you can have perfectly good AC one day, but evaporator freeze up the next time you drive for longer periods or on a more humid day.

If you notice lower airflow and warmer car AC temperatures, pull over and turn off the AC. But leave the blower fan on high with the engine running. Wait about 15-minutes or until the airflow picks up from the ducts. Then check under your car for a large puddle of water. Find it? That water puddle is what’s left of the ice that was frozen on your evaporator coil. It’s proof of evaporator freeze up. The fix? A recharge.

Another cause of AC blows cold then warm

An overcharged car AC system can start out by pumping cold air. But that cold air turns warm fairly soon after because the high side pressures build to the point that the high pressure switch shuts off the compressor clutch. Once pressures equalize, the high pressure switch allows the compressor to start up again. The cycle repeats over and over, providing a short period of cold air, followed by warm air. The fix? Evacuate and recharge the car AC system.

Yet another cause of AC blows cold then warm

If your car AC blows cold air when driving at highway speeds but turns warm at stop lights or when driving at slow speeds, you may have a bad radiator fan or radiator fan relay. Many cars are equipped with two radiator fans, while others have one fan that can run at different speeds. In cars with dual fans, one fan is designed to run at full speed any time the AC is on. If that fan isn’t working properly, it can’t cool the condenser enough to remove enough heat from the gaseous refrigerant to get it to condense back into a liquid. That also causes high pressures that can shut off the compressor. The fix? Check the radiator fans to make sure at least one fan turns one when the AC is switched on.

However, some cars operate both fans at low speed while driving and switch into high speed mode when the AC is running. So get tricked into thinking your fans are ok simply because they’re running. They must run at high speed in order to draw enough air across the condenser to make the system cool properly.

©, 2017 Rick Muscoplat

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Posted on by Rick Muscoplat

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