How to properly check and recharge your AC system


First off you need to understand a little bit on how an automotive air-conditioning system works. Click here to learn about all the components of your AC system.

How your car’s AC system works

First open the hood of your car and locate the AC compressor. A flashlight will be helpful. It will be located on the front of the engine where all you belts run (the front of the engine can face either side of the engine compartment if it is a transverse engine. Look for the drive belts and you have found the front of the engine). The compressor will have two metal tubes coming out of the back or side/top of unit. Most vehicles, the compressor is located at the lowest point of the engine. It will look something like this.

how car ac system works

You will notice that with the car running and the AC off, the pulley ran by the belt will be spinning but the front hub will be stationary. With the car running, turn the air conditioner on and see if the clutch engages the hub on the AC compressor (everything on the front of the compressor will be spinning). If the clutch never engages, there could be one of two problems. Either you are too low on freon or you have an electrical issue and the compressor is not getting power or ground.

If the clutch does engage, let it run for a few minutes. Next, you are going to find the low side pressure hose. Look at the two service ports. The low side can be identified by the service cap that is blue or has the letter L on it. If both ports are missing their caps, the low side port is the smaller of the two ports. Once you find that hose, and the vehicle’s been running for a little while with the AC on, take a temperature reading of that hose near the service port preferably on the metal part. The temperature of that hose, if the AC system is working correctly, should be around 30 to 40°. If you get this reading from your AC system, and it’s still blowing hot air out of the vents, you have a vent issue not an AC Freon issue.

We have gone as far as we can go without any tools, but we should have narrowed down where our problem is with the above steps. For the next testing procedures you’ll need a set of AC gauges, a voltmeter, and/or test light. Select your symptom below to get your testing:

No AC compressor operation

Verified good freon level, still no compressor operation

Compressor turns on, but I have no cold air

Compressor turns on, low side hose is cold, but no cold air at my vents