Rick's Free Auto Repair Advice

Fix your car AC yourself

How to diagnose and fix car AC problems yourself

Auto AC systems are fairly simple and so is the way to troubleshoot auto AC. The most common reason they stop pumping out cold air is because the system has a leak and is running low on refrigerant or is out of refrigerant. You can buy a DIY AC recharge kit at any auto parts store, but if you don’t diagnose the system properly, you can actually cause more damage. See this post on the dangers of using an AC recharge kit.

So, before you run out and buy a recharge kit, take a minute to perform these checks first. To do these tests, you’ll need the proper tools.

Must have tools for diagnosing and fixing car AC problems yourself

Manifold gauge set

You simple cannot diagnose a car AC problem with just the low pressure gauge on  R-134a recharge kit.

AC manifold gauge

Manifold gauge set with hoses

First, they’re inaccurate. Second, you need to know the high pressure readings to fully evaluate what’s going on.

Temperature probes — This one is really important if you’re getting some cooling but not really cold air. In order to diagnose this, you have to test the temperatures in certain locations around the AC system. Non-contact infra-red thermometers don’t work well for these tests.

digital thermometer kit

Digital meter with thermocouple temperature probes

Analog dial or digital thermometer — This one goes in the center vent to get an accurate read of how well the system is working.

ac not cooling

AC duct thermometer

Where to start diagnosing your car AC problems

Find out what type of refrigerant your vehicle uses

Most cars and trucks from the 90’s until around 2017 used R-134a refrigerant. That refrigerant is inexpensive and easy to buy at any auto parts store. But late-model vehicles use a more planet-friendly R-1234yf that you CANNOT BUY WITHOUT A LICENSE. If your vehicle is filled with R-1234yf, there are still a few things you can check, but if the system is low, you’ll have to take it to a shop.

If you own a hybrid vehicle, find out if uses an electric AC compressor

Conventional engines power the AC compressor with a drive belt, but some hybrid vehicles have an electric compressor. Older hybrid systems still use R-134a refrigerant but they use a different oil. If you try to recharge those systems with a DIY AC recharge kit that contains conventional oil, you can cause all kinds of damage to the electric compressor and even possibly electrocute yourself In the process. If you’re hell-bent on recharging the AC in your hybrid that’s equipped with an electric compressor, you MUST purchase refrigerant that doesn’t contain oil.

Next, find out if your system uses a magnetic compressor clutch

All older systems use a magnetic compressor clutch to disengage the compressor when the system is off or cooling needs are low. But some newer vehicles use a “clutch-less” variable displacement compressor. The pulley and compressor shaft on those compressors rotate any time the engine is running, even if the AC is turned off. That may seem like a waste of energy, but this type of compressor has a vacuum or electrically operated valve that only allows refrigerant flow into the compressor when the system is turned on. When the valves are off, the compressor is basically free-wheeling and using very little energy.

If you don’t check the clutch type ahead of time, you can easily misdiagnose the system by thinking that a clutch-less compressor is actually pumping refrigerant when it’s actually free-wheeling.

Next, find out if the system in your vehicle is an orifice tube or expansion valve design

Orifice tube and expansion valve systems differ in how they meter high-pressure refrigerant into the evaporator coil. Some of the testing routines I refer to below are system-dependent; in other words, the temperature checks for an expansion valve system are different than the temperature checks for an orifice tube system.

How to tell if you have an orifice tube or expansion valve system

The images below show an accumulator and a receiver/drier. Orifice tube systems use an accumulator and it will always be located near the evaporator exit at the firewall of the engine bay. An expansion valve system uses a receiver/drier instead and it’s located right next to the condenser in front of the radiator. You’ll find it in the high-pressure line between the compressor and the condenser.

expansion valve versus orifice tube AC system

Step 1: On a clutch style compressor, turn on the AC and check for compressor clutch engagement

If the compressor doesn’t engage, see this post. If it does engage, move on to the next step

Step 2: Using a probe thermometer, check the temperature in the center duct temperature following these steps:

1. Engine running at normal operating temperature
2. Set AC to MAX and blower to RECIRCULATE
3. Close all windows and doors.
4. Set blower to highest speed
5. Leave the thermometer in place for at least three minutes and record the temperature.

The center duct temperature should be 20° – 35° lower center duct temperaturethan the outside air temperature (although ambient humidity will have a huge effect on your reading)

Step 3: Check the superheat temperature at the condenser, evaporator, and compressor

Most DIYers and many pros skip this step. That’s a mistake because these superheat temperature readings tell a lot more about how the system is operating than pressure gauges. Think of them like lab results from a blood draw that tells the condition of the patient.

How to test superheat on an expansion valve system, Click here

How to test superheat load on an orifice tube system. Click here

Then do a pressure test with a manifold gauge set. Click here

If the AC gauge readings are abnormal, see this post

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