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Problems within AC electrical circuit and wiring connections are the most common reasons for the AC compressor not turning ON. Car AC compressors can suffer failures such as leaking seals, bent valve plates, or worn-out valve plates. But in those cases, you will notice that the AC compressor will still engage and turn unless the refrigerant circuit is empty.

A key component of any car air conditioner system is the compressor. One of the most common problems in the car’s AC systems is that the compressor isn’t working properly or not working at all.

When you turn your AC on, in most car models you will hear a “click” sound that comes from the AC compressor being switched on. If your AC is not blowing cold air and you don’t hear that “click”, it could be a symptom that your AC compressor is not engaging.

How an AC compressor is engaged

We are not going to go deep into the mechanics of AC air compressors, but it’s important to know how a car’s air compressor is engaged and disengaged. This mechanism is pretty simple and easy to understand.

Compressors have a pulley that rotates along with the crankshaft pulley by the accessories drive belt (also called “serpentine belt” or “poly-V”). This belt transmits the crankshaft pulley rotation to the air conditioner compressor, power steering, the alternator, and other accessories of the car.

The air compressor pulley rotates at all times when the engine of the car is working. Compressors have an external pulley that is connected to the drive belt, and an internal pulley that is linked to the compressor’s input shaft (compressor works by its rotation). This prevents from engaging the whole mechanism when the AC is OFF.

Do you want to see what is inside of a compressor? Here is your chance:

The internal pulley is disengaged by default. When the air conditioner of the car is switched to ON, the internal pulley connects to the external pulley by the action of a clutch (which is activated by a magnetic coil).

When the magnet receives electrical current, it attracts a clutch drive plate assembly. By its friction with the external pulley, it is being attached to the internal pulley and to the compressor’s mechanism.

The magnetic coil is connected to an electric circuit that is also connected to the air conditioner ON-OFF switch. Series of sensors send an electrical current to the magnetic coil or prevent electricity from reaching it.

Why is the AC compressor not turning on?

Any car air conditioner compressor has basically, two possible states: engaged and disengaged from the drive belt. Knowing this fact, we are ready to see what could be the main reasons that make a car AC compressor stop working.

The first thing that has to be determined is whether the problem is located in the AC electrical circuit or in the compressor.

Checking AC electrical circuit

  • Fuse(s). Sometimes a blown fuse can be the source of the problem.
  • High or low-pressure switch. A pressure switch controls the pressure of the refrigerant inside the AC circuit. If the pressure gets too high or too low, it could damage the whole system. When a lack of pressure or an excessive amount of it is detected, this switch makes the compressor stop working. Sometimes, these switches can break and start throwing false readings. Replacing a defective switch will make the compressor start working again.
  • Refrigerant level. If it’s too low, it will cause the compressor to stop running.
  • WOT (Wide Open Throttle). Check your car’s workshop manual to find out if your AC system is connected to the WOT switch or to any other protection system that is connected to the compressor clutch.
  • AC system relays. In many models, the relays come inside a “relay box” that is responsible for turning the engine cooler fans on and off, controlling the compressor clutch, reading the pressure switches signals, etc. Sometimes these boxes get damaged and can disable your AC system completely.
  • Wiring and connections. Try to find bad contacts or burnt cables, loose connectors, and signs of corrosion.

Checking for compressor problems

  • If the compressor is stuck, the clutch will engage but will start to slip. This can burn the clutch and damage the coil. In this case, you will need to have the compressor repaired or changed.
  • The compressor magnetic coil wiring can be defective. You can check it by testing continuity between the two ends of the coil. If the circuit is open, then the coil wiring is cut somewhere. You will need to get the magnetic coil replaced, which in most cases would be cheaper than having it repaired.
  • The compressor clutch could be slipping. This can happen for different reasons. The first one is oil leakage. If oil falls into the clutch disc, it will get slippery and it won’t have friction enough to move the inner pulley of the compressor. The second reason could be the result of a worn-out clutch disk. You should get the clutch assembly replaced in both cases.
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Conclusion

This article intends to give you a small introduction to car AC compressors and the reasons why they can stop working. In order to find out why your AC compressor is NOT turning ON, I recommend that you, first of all, check the components that are outside the compressor first (AC electrical circuit), and then start checking the compressor itself.

You need to know these things before you take your car to the shop and go for the most expensive solution, which would be replacing your compressor. Very often small fixes like replacing a blown fuse, a bad relay, or a broken pressure switch could resolve the problem a lot cheaper.

Here is how you check for leaks and replace a damaged compressor yourself:

This article is for informational purposes ONLY and is NOT a replacement for professional advice!


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