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Car AC blows warm air

Car AC blows warm air— How to diagnose and fix

How to diagnose an AC blows warm air condition

There are many other things that can cause even a fully charged AC system to blow warm air. There are certain steps you should take to diagnose why your car AC blows warm air. Here they are:

Step 1: Determine if you have a clutch or clutch-less style AC compressor

A clutch style AC compressor engages and disengages

Older vehicles have a clutch style AC compressor that engages and disengages the AC compressor shaft based on the needs of the AC system. The ON/OFF cycle can be controlled by the high/low pressure switches or an evaporator temperature sensor.

car AC compressor

A clutch-less style AC compressor spins all the time

A clutch-less AC compressor spins the compressor shaft all the time. That may seem like a waste of energy, but think of it more like leaving your transmission in neutral. Because unless the compressor control valve is sensing a need for refrigerant, the compressor shaft is just idling and not actually pumping.

Watch the angle of the “swash plate when it’s pumping and when it’s idling. The control valve uses refrigerant pressure to change the angle of the swash plate based on the cooling needs in the cabin. If your vehicle has a clutch-less style AC compressor, scroll down to Step 3.

Step 2: Check for AC compressor clutch engagement on a clutch style compressor

AC compressor clutch not enagaging

AC compressor clutch connector

Lack of compressor clutch engagement can be caused by low system charge, excessively high pressure, a blown fuse, bad compressor clutch relay, clutch connector or grounding issues, an incorrect clutch air gap or a failed compressor clutch coil.

So start your diagnosis by turning the AC to MAX. Start the engine and pop the hood. Check to see if the center AC compressor clutch disc is spinning. If not, read this post to diagnose a compressor clutch not engaging condition.

If the AC compressor clutch engages and immediately disengages, you may have a clogged orifice tube or expansion valve. To learn more about orifice tube and expansion valve systems, see this post.

If the AC compressor clutch is spinning but your car AC blows warm air, follow these steps

Determine which type of system the carmaker installed in your vehicle. Orifice tube and expansion valve systems operate differently and must be tested differently. Find the type of system in your vehicle in this chart

Learn how an orifice tube system works BEFORE you start working on it

An orifice tube system is pretty simple. The orifice tube is simply a refrigerant metering device that allows only a certain amount of refrigerant into the evaporator coil. Orifice tubes incorporate a filter to prevent crud from clogging the orifice. When the filter becomes plugged, it can’t allow enough refrigerant to enter the evaporator. If you don’t understand how an orifice tube system works, you can’t diagnose and fix it.

See this post to learn how an orifice tube system works

See this post to learn how to diagnose an orifice tube system

Learn how an expansion valve system works BEFORE you start working on it

An expansion valve system is a bit more complicated and slightly harder to diagnose. The expansion valve can be a “block” style” or a valve with a capillary tube. It’s job is to meter liquid refrigerant into the evaporator based on the outlet temperature at the evaporator or the temperature of the evaporator coil using a capillary tube. Expansion valves can fail due to clogging, a kinked capillary tube (especially if you’ve had work done under the dash or hood), or failure of the diaphragm.

See this post to learn how expansion valve systems work

See this post to learn how to test an expansion valve system

Click here for more information on how to fix your car’s AC

Most common reasons why car AC won’t cool

Step 3: Use a manifold gauge set to check high and low refrigerant pressures.

Some DIY forums tell you to use a small screwdriver to depress the valve in the pressure port. They say if you hear a hissing sound, that means there’s refrigerant in the system. That’s true as far as it goes, but it’s NOT a measure of how much refrigerant is in the system.

AC manifold gauge

Manifold gauge set with hoses

SO it’s an invalid test.

You can’t diagnose your car’s AC system without gauges. So get a

manifold gauge set and refer to this post for what the pressures mean.

Refer to this chart for a quick diagnosis

AC pressure chart
If the readings are a problem, conduct a superheat load test

Click here to learn more about SUPERHEAT and why it’s important in diagnosing your AC system

Click here for more information on how to conduct a superhead load test on an expansion valve system

Click here for more information on the symptoms of a bad expansion valve

Click here for more information on how to conduct a heat load test on an ORIFICE TUBE system

©, 2018 Rick Muscoplat


Posted on by Rick Muscoplat

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