Symptoms of a Bad A/C Compressor Relay

The last thing you want on a hot day is to notice that your car’s air conditioning (A/C) compressor won’t turn on. There can be multiple reasons the A/C won’t cool, but this article focuses on the A/C compressor clutch relay.

If the relay is defective, the electromagnetic clutch can’t activate. If, for some reason the A/C relay has failed to engage the compressor clutch and nothing else is wrong but the relay, you may notice the following symptoms:

A/C Not Cooling or Freezing Up

The most obvious symptom of a faulty A/C compressor relay is that your A/C won’t function normally. In fact, in most cases, your A/C will stop cooling entirely. Nothing but warm or room-temperature air will be dispensed.

If you turn on the A/C and the compressor doesn’t spin, the relay is usually an easy place to start, even though the cause of a non-engaging compressor isn’t typically due to a faulty relay. But here’s how you can check to see if the relay may be the issue:

If your A/C unit bounces between blowing out cool air and releasing warm air, it might be freezing up. This intermittent cooling is often the result of ice forming in the system after air and moisture has built up.

A bad A/C compressor relay could be the culprit in both cases. The relay is part of the electrical circuit of the compressor, and anything that goes wrong here spells disaster for your A/C.

Blinking A/C Light

A blinking A/C light means that the clutch on the A/C compressor isn’t spinning. This doesn’t immediately indicate that the relay is to blame, but the relay is part of the system that keeps the compressor turning.

This feature isn’t available on all vehicles, but it’ll help you know right away that something’s wrong.

A/C Compressor Not Spinning

If the A/C relay isn’t functioning, the compressor might not turn at all, meaning the clutch doesn’t open and you get no cool air even when the A/C is on.

Clicking Noises

Clicking noises are normal whenever the A/C’s cooling cycles turn the compressor on and off. However, if you hear clicking outside of this normal cycle, that means the A/C relay or part of the A/C controls are malfunctioning.

The lack of clicking noises is also a cause for concern if you have your A/C on. No clicking noises means that the car’s A/C compressor isn’t switching on and off at all, which it should be doing to follow the cooling cycles.

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: If the A/C is “short-cycling” and it’s a hot day, the system may just be low on refrigerant. This won’t have anything to do with the relay.

P0645

A bad A/C compressor relay is a common cause for Diagnostic Trouble Code (DTC) P0645 on some platforms. This code means “A/C Clutch Relay Control Circuit.” This typically refers to the primary or trigger side of the A/C relay or its circuit. The secondary side is the load-carrying part of the relay.

Does the A/C Compressor Relay Fail Often?

The A/C compressor relay is controlled by the PCM and when the PCM triggers the A/C relay to close, the power that energizes the compressor clutch is delivered by the secondary load carrying terminals. When the relay tells the compressor’s magnetic clutch to activate, the clutch hub is pulled against the spinning pulley that is driven by the belt. Since the hub is connected to the compressor shaft, snapping it hard against the spinning pulley will cause the pulley to drive the compressor by way of the hub and its shaft. This drives the inside of the compressor where the important work is done.

The A/C compressor relay is controlled by the PCM and when the PCM triggers the A/C relay to close, the power that energizes the compressor clutch is delivered by the secondary load carrying terminals.

– Richard McCuistian, ASE Certified Master Automobile Technician

The truth is the relay is very seldom the problem. Usually, the relay is only doing what it’s commanded to do, and there are many reasons why the PCM might not engage the relay.

How Much Does a Replacement A/C Compressor Relay Cost?

Depending on the model of your vehicle’s sources, the cost of a replacement A/C compressor relay ranges from $10 to 150. For certain cars, a multi-purpose relay will do the trick just fine. Others, however, might need a specific relay to work with their system. To be safe, consult a mechanic or your vehicle’s manual before making a purchase.

Where Is the A/C Compressor Relay Located?

The A/C compressor relay is located in the engine compartment along with a bunch of other fuses and relays. The cover of the engine compartment usually has a guide to tell you which fuse or relay does what. The A/C compressor relay can be found by searching for the anagram MGCLT, which stands for Magnetic Clutch Relay.

At the end of the day, a bad compressor relay can cause a lot of problems with your A/C system. It’s the one of the first things you should check whenever your car’s compressor won’t turn on.