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Car Air Conditioning Troubleshooting

Auto AC system

The air conditioning system on your car is one of those things that never goes out of commission at a convenient time. It's always in the middle of the day, when it's hottest, and when you need to keep cool on your way to something important. Right?

Troubleshooting a broken A/C system can be just as frustrating since the systems are so spread out and contain refrigerant that you can't legally handle without a license. The first step is understanding the parts of the A/C system, which we covered in our article on Automotive Air Conditioning Parts. We also talked about how they work.

Now let's look at a simple way to walk through that system and diagnose a problem.

Step 1 - Look at the process

If you refer to that previous article, you'll see the process that your system goes through to send cold air into your car's passenger compartment. This is also a good reference for walking through the cycle of the refrigerant so you can see where things go and then maybe where they could fail.

Step 2 - Check for leaks

Most of the time, the problem is because refrigerant is leaking or has all leaked out. This leads to low or no pressure and causes the entire system to fail. It's at once the simplest problem and also the most frustrating to fix because there are yards of line, any point on which a leak could have formed.

The first thing to do is make sure there is some kind of pressure in the system. If not, you have either a blockage or (more likely) a leak that's vented all of the R-134a or R-12 refrigerant. Using a gauge (NEVER VENT TO THE ATMOSPHERE), put it on the low-side valve and see how many psi are in the system. Your owner's manual will detail the amount that should be there. If there is sufficient pressure (or close to it), then you'll need to test for leaks as it may only be leaking when the system is running.

The easiest way to test for leaking is to purchase in-line dye for A/C systems from your auto supply store. It's fairly cheap and allows you to pinpoint a leak without having to be licensed to drain the refrigerant from the system. The dye will appear on the hose or pipe wherever the leak is. Most likely this will be at a joint, connection, or other juncture. Those leaks need to be repaired.

If you aren't licensed to drain the refrigerant, you'll have to take your car in to someone that is. Many auto parts stores have certified employees and the right equipment and can do it for you for a nominal charge. Once the system is drained, mark your leak points and either purchase replacement parts or otherwise stop the leaks with a permanent fix. Duct tape does not count.

Once the leaks are fixed, do another pressure test. Use the dye again (another of a different color might be helpful) and pressurize the system with compressed air to just below the factory recommended level. Watch for leaks. If there are any, vent the air and fix those trouble spots, then do another test. Once all is well, have your certified friend refill the system with the proper refrigerant or purchase the DIY refill bottles and put it in (often the cost is roughly the same).

Step 3 - It's not a leak

In this case, it's likely one of the system components is no longer functioning. Start the engine and turn on the A/C inside the cabin. Watch the engine and see if the air conditioning pump is turning. If not, shut off the engine and check the belts and try to turn the compressor by hand to see if it's seized. If it is turning, put your hand on the evaporator (shut the engine off if you can't do this safely with it running). If the evaporator is cool, then your problem is likely in air delivery. If it's not cold, then you aren't sending pressurized refrigerant to it and the problem is somewhere before the evaporator.

Some newer systems have a lot of electronics on them, especially in the operator's controls in the cabin. Check the switches and fuses connected to those - refer to your owner's manual for a schematic. Often a little fifty cent 15A fuse might be the culprit.

Finally, if all else fails, take it to a mechanic for a diagnostic.

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