Bad Car AC Relay

bad car AC relayBad Car AC Relay

A bad car AC relay remains a common problem for many years, make and model automobiles. We have a lot of car air conditioning articles posted on the site that aim at getting your mobile cooling unit working again.

In many of these articles we focused on some of the worst AC problems an old car can have. This time we get to share some good news with motorists.

What if the root cause of your air-conditioning problem came from a part costing around $15? Furthermore, what if the repair involved simply plugging in a new relay at the convenience center of your automobile? If this is the malfunction, it results in a repaired AC system.

Here we'll discuss how to diagnose a failed compressor relay and point to some of the automobiles that commonly enjoy this problem. Although it's never fun when the AC stops working, it’s the absolute best case scenario when all it needs is a relay to make it work again.

Cars with Bad AC Relays

I first stumbled on to this problem 10 years ago when a large amount of Jeep products rolled in to see me in the beginning months of the summer. Of course it wasn't long before Chrysler products of any kind started exhibiting the same issue.

They often share the same relays. Let me get more model specific. Some of the most popular Jeep models ever made like the Jeep Grand Cherokee, Wrangler and Jeep Liberty all use an electrical relay with a high failure rate.

In addition, these junk electrical components found their way into many Chrysler products like a decade's worth of Chrysler Sebring and Dodge Avenger models. Then I started to do some research about other models that also have a high rate of bad car AC relay parts.

The two foreign cars that jumped off the map included two of the most reliable Japanese cars.

Honda automobiles like the Accord, Civic and Honda Pilot make this list. Surprisingly, the once bulletproof Toyota product line also has a high rate of AC relay failure in the Camry and Corolla models.

Digging into the message boards I found that often these relays work intermittently before they give up altogether. It’s difficult to track down intermittent electrical malfunctions. We'll get more into this in the diagnosis section below.

Diagnosing a Bad Car AC Relay

defective car AC relayDefective Car AC Relay

There’s a common misconception that if you hear the relay click that it must be good. In the close-up picture on the right you see an image of a bad car AC relay. This relay clicked as it became energized by the ECM.

However, you can see a hairline crack in the brass contact strip that allows current to flow to the AC compressor. This means the relay clicked, but did not work as intended. This is an extreme example, but it just goes to show that you can't apply simple rules to every situation.

A quick and dirty way to test a car air-conditioning relay is to locate it and then swap it out with the same type of relay. Often car manufacturers use the same exact part number relay in many different locations. This applies to all the vehicles mentioned above.

Although it’s not a permanent solution, it remains one of the fastest ways to test it out. If you swap it out and the compressor is not kicking on you have a different problem. In isolated situations where the AC compressor clutch relay has a unique part number we test it the old-fashioned way.

Remove the component from the socket and test for a power signal and a good ground at the proper terminals. You can also use a jumper wire to jump the two terminals that pass electrical current onto the air conditioning compressor.

Testing the Air Conditioning Relay

You have to remember that an automotive relay is nothing more than a remotely operated switch.

When the relay energizes it creates a magnetic field and pulls the contact closed. There are many different reasons that a relay can fail to operate.

When the part itself fails, it might be corrosion buildup between contacts. This is a repairable condition. You can take an emery board or fine grit sandpaper and clean these contacts.

This often gets the relay working again and you have nothing to loose by trying to fix it. With that said, there are other situations where the relay cannot be repaired.

If there is a break in the coil of wire that creates the magnetic field the car AC relay is bad. This is easy to determine with an inexpensive voltmeter. First, you select the continuity check function.

Then apply the test leads to the ground and apply signal terminals. If it reads open the relay is bad. It should measure zero resistance or make a beep sound showing the continuity is good.

AC Problems that Look like Bad Relays

On the cars mentioned on this page like the Honda, Toyota and Chrysler products it’s the ECM that decides when to energize the air-conditioning relay. The most common cause of a relay that fails to engage the compressor remains a low system.

We have pressure sensors in the car air-conditioning system that prevents the relay from energizing when the refrigerant is low. The most common cause of this situation is a slow leak from a Freon O-ring seal.

In rare cases, but likely on some models there may be a fault in the automotive computer system that prevents the control module from applying the ground to the proper leg of the air-conditioning relay.

Homemade automobile short finderHomemade automobile short finder

When this happens the relay won’t energize. You hear no clicking sound in this situation. Which brings us to the final thought for this article.

Mechanics consider fixing car air-conditioning systems as a complex and difficult task. With that said, sometimes we get lucky and all it needs is a $15 compressor clutch relay.

Will this solve your problem? If you're lucky it will. However, you must remember that straight line logical diagnosis will always uncover the true reason the air conditioner will not blow cold air.