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Car AC blows cold then blows warm

Car AC blows cold then warm

If your car AC blows cold then warm, the cause can be a low system charge, moisture in the system, a malfunctioning expansion valve, clogged evaporator fins or an overcharge condition. Here’s how to nail down the cause.

Low car AC charge causes evaporator freeze up and low airflow

A low system charge can cause the evaporator coil to so cold that it freezes the condensation on the evaporator fins. The ice builds up until it blocks airflow. Then the AC system shuts off the compressor and you get warm humid air.

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 metered 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 an air temp around 40°F

Once the refrigerant has changed states to a gas, it should pick up a maximum of 10°F more heat and then flow out of the evaporator into either the compressor or accumulator to complete the cycle.

But when a system is low on charge, the gaseous refrigerant stays in the evaporator longer and removes even more heat from the cabin air passing over the coil. The refrigerant leaving the evaporator is superheated by more than 10°F. The additional heat the refrigerant removed from the cabin air causes the evaporator temperature to fall below freezing. Humidity in the cabin air 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 also 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 remove too much heat, causing evaporator to freeze.

Cabin air filter

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

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.

Moisture in the AC system can also cause an AC blows cold then warm condition

Any time you lose refrigerant from your car’s AC system, that leak allows air and moisture to enter. If you simply add more refrigerant every year without fixing the leak, you build up too much internal moisture.

The moisture can freeze into ice crystals right at the orifice tube or expansion valve, clogging it and stopping the flow. The symptom appears as: blows cold, then warm. If you shut off turn on the heat for a while, that’ll melt the ice crystals and the system will start cooling again. But it will still freeze up.

How to tell whether your AC has low charge or moisture?

When an evaporator freezes due to low charge, you’ll also have decreased airflow because the ice is blocking the air. If you shut off the vehicle and leave it for a half hour or so, you’ll see a huge puddle of water under the vehicle. That’s the sign of a low charge. All the ice has melted and drained out the condensation tube onto the ground.

If the AC blows cold then warm condition was caused by moisture in the system, you won’t notice a decrease in airflow and you won’t see a puddle of water after shutting off the vehicle.

Another cause of AC blows cold then warm

An overcharged car AC system can start out 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.

Most common reasons car AC won’t cool

©, 2017 Rick Muscoplat

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Posted on by Rick Muscoplat

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