Why Your Car’s AC is blowing hot air? Causes and Solutions

Troubleshooting: Why Your Car’s AC is Blowing Warm Air?

There’s nothing quite as frustrating as turning on your car’s air conditioning on a hot day, only to be greeted by a blast of warm air. Understanding the possibilities of why your car’s AC is blowing hot air can help you diagnose the issue and potentially save on repair costs. Let’s explore the main reasons behind this malfunction and what you can do about it.

Why Your car’s AC is blowing hot air—The 8 Most Common causes

1) The system is Low on refrigerant due to a leak—This is the most common cause of a car’s AC blowing warm air. Refrigerant removes heat from the cabin, and when you lose some due to a leak, the remaining refrigerant can’t remove enough heat to make you feel comfortable. When refrigerant levels drop too low, the system can’t cool because the AC compressor is disabled.

2) The compressor clutch is not engaging due to a low refrigerant/oil charge— The most common cause of a compressor clutch not engaging is because the low-pressure cut-off switch has locked out clutch engagement. It does this to protect the compressor. Here’s why. The refrigerant circulates oil to the compressor. When your AC system leaks, it loses refrigerant and oil. Even if it doesn’t lose a lot of oil, there’s still too little refrigerant to carry the oil back to the compressor. In this condition, the compressor can self-destruct. So, the low-pressure cut-off switch is a safety feature that prevents the compressor from failing due to a lack of oil.

Other possible causes of a compressor clutch not engaging:

• Blown clutch coil fuse
• The clutch coil has an open or short-to-ground condition
• The ambient temperature sensor used in a climate control system has failed. That will prevent compressor activation.
• The evaporator temperature sensor is out of spec or inoperative. That too, would prevent compressor engagement

This post provides a more detailed explanation of why a compressor clutch won’t engage and possible fixes.

3) The flow control valve (used on clutchless compressors) isn’t working properly— Variable displacement compressors used on late-model vehicles don’t use a compressor clutch. The compressor shaft spins the entire time the engine runs. Instead, a flow control valve mounted in the compressor varies internal refrigerant pressure to move a “swash plate” or “wobble plate” that varies compressor displacement. When the system isn’t calling for AC, the swash plate spins in the idle position, using little to no energy.

Because this is a newer technology, it’s not uncommon for the control valve to fail. When it does, the swash plate remains in the idle position, and the system can’t cool. For more information on variable displacement compressors and how they work, see this post.

5) The expansion device is clogged with debris or stop-leak products or is frozen due to moisture in the system— The expansion valve is what controls the amount of refrigerant entering the evaporator coil and, thus, the level of cooling in the cabin. The valve can get clogged by debris circulating through the system. The most common debris is sludge caused by moisture in the system. Moisture in an AC system reacts with the refrigerant and refrigerant oil to form acids and sludge, which then clogs the valve. As mentioned above, the moisture can also freeze in the valve, blocking refrigerant flow or preventing the valve from opening or closing.

Other system debris, such as degraded rubber hose material, stop-leak product, or metallic particles from a worn compressor, can combine with the refrigerant oil to coat the system with “black death,” a slimy black film that reduces cooling and clogs the expansion valve. These are all reasons why it’s never a good idea to add a stop leak product to your car’s AC system or to add refrigerant to a leaking system.

7) Worn AC compressor— A worn compressor can’t produce high enough pressures to make the system work properly.

8) Inoperative radiator/condenser fan— The compressor produces hot, high-pressure gas and sends it to the condenser to be quickly cooled and then condensed into a high-pressure liquid. If the radiator/condenser fans aren’t working properly, the high side pressures will rise, and the AC system will shut down.

9) Clogged condenser— If the exterior condenser fins are clogged with bugs and debris, they can’t pass enough air through the condenser tubes to condense the high-pressure gas. The refrigerant can’t cool and condense, so the system can’t cool the cabin. In addition, if the the internal condenser channels are clogged with debris, the refrigerant can’t condense, causing a significant drop in cooling.

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.

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 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

If your ac clutch isn’t working then your ac won’t blow cold air

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’s AC is blowing hot 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. Its 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.

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.

Using a manifold gage to check system pressures is mandatory if you want to find out why your car’s AC is blowing hot air

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

Use a manifold gauge set to read the pressures on your system and then refer to this chart to find out why your car’s AC is blowing cold air

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