3 Reasons Car AC Compressors will not Engage

When drivers turn on the air-conditioning and nothing but warm air comes out, a good first step in diagnosis is to figure out if the AC compressor is engaging. Here we’ll discuss three reasons a car AC compressor will not engage. Having a basic understanding of what could be wrong and being able to discuss it with your mechanic can improve your chances of a successful repair.

How to Locate the Compressor

On most cars the AC compressor is visible. The part can be identified by the air-conditioning hoses attaching to the compressor. These are large rubber and aluminum tubes often covered in insulated sleeves to protect them from the engines heat. Where these hoses attached to the compressor, there is an aluminum block called the manifold. Therefore, when you see a belt driven accessory with an aluminum block on the back or top that leads to hoses that run throughout the engine compartment you have found the compressor.

Is the Car’s AC Compressor Engaging

When the air-conditioning is turned on from the climate control panel a strong electromagnetic coil is energized. This pulls the metal clutch plate against the drive pulley that rotates all the time with the engine running. Therefore you can see the pulley spinning yet the compressor may not be engaged.The first sign that the clutch is engaging would be an audible click sound as the clutch plate moves back towards the magnet and drive pulley. With the hood up an assistant can turn the AC on and off, so the driver can listen for the click and look for movement at the air-conditioning compressor pulley.

Low Freon Disables the Compressor

Many people don’t realize the air-conditioning compressor is lubricated with an oil mixture that is co-mingled with the Freon charge. If the Freon level is low so will be the amount of oil making its way to the compressor. For this reason vehicle manufacturers have a low pressure cutoff switch that disables the compressor from turning on when the Freon is low. Air-conditioning Freon leaks can be hard-to-find. Most mechanics will have a halogen Freon leak detector to help them locate small invisible gas leaks. Even with the special tools it will take time and patience to seek out and repair refrigerant leaks.

Defective Compressor Switches

Another area that can prevent an air-conditioning compressor from engaging properly is one of the many switches that are used in an average AC system. As touched on above two of these switches have to do with the current refrigerant level. If the pressure is too low, the low cutoff switch will prevent the compressor from engaging. If there’s too much Freon or a system blockage the pressure could be too high and again the compressor has a protection device called a high pressure cutoff switch.There are several switches in the other automotive systems that have the capability of disengaging the AC compressor. As an example there is a wide open throttle switch that automatically turns it off, so full engine power can be utilized. When an engine is idling with the AC on, sudden loads can cause the engine to stall. Many manufacturers use a power steering cutoff switch to turn off the compressor in a parallel parking situation to prevent engine stall. Testing these electrical switches is straightforward with an automotive multimeter and the cost of replacement parts reasonable.

AC Relay and Fuses

When the driver turns on the air-conditioning they are sending power to the compressor. These circuits are protected by fuses and controlled by relays. Most automobile manufacturers will use a separate standalone fuse for the air-conditioning system. When a driver verifies the compressor is not engaging this would be a good area to start checking. Consult your owner’s manual for the location of the fuse panel on your specific car.

The AC compressor circuit is almost always controlled by a relay. When the owner activates the air-conditioning they force the relay to close and current flows to the compressor. If this relay malfunctions the compressor will not engage. Some models vehicles like the Jeep Grand Cherokee are known for problems with AC relays, but malfunctions with this part on any car are possible.AC compressor relays are usually located in an engine compartment convenience center, along with many other relays, usually of the same part number. Often mechanics will swap out the relay with another one of the same identical part number as a quick test to see if it solves the problem. Automotive air-conditioning is a hard to repair complicated system. These three reasons a car AC compressor won’t engage is a great starting point.