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How to Test an AC Relay

Author: Apogeeweb
Date: 18 Jul 2022
bad relay



Ⅰ What is AC Relay

Ⅱ What is DC Relay

Ⅲ Configuring AC Relay Control

Ⅳ How to Test an AC Relay

Ⅴ The Signs Of A Bad Relay

Ⅵ Stuck AC Relay Switch: A Note

Ⅶ Diagnosing a Bad Car AC Relay

Ⅷ Testing the Air Conditioning Relay

Ⅸ AC Problems that Look like Bad Relays


1. How Much is an AC Relay?

2. Which is an Advantage of a Solid State Relay over an Electromechanical Relay?

3. Is Solid State Relay better?

4. What is the Function of Solid State Relay?

5. What does a Compressor Start Relay do?

6. How much does it Cost to Fix a Stuck AC Relay?

7. How do I know if my Compressor Relay is Bad?

8. How Do I Check an AC Clutch Relay?

9. What Happens when a Relay goes Bad?

10. What are the Symptoms of a Bad AC Relay?


The compressor is controlled by the relay in the air conditioning system of your automobile or light truck. A low-current switch can regulate the high-current electromagnetic compressor clutch via a AC relay. Because many of today's computer-controlled automobiles and trucks use the computer to turn the relay on and off, testing the relay requires the use of an automotive circuit tester.


Ⅰ What is AC Relay

The AC relay includes two coils that work together to form a transformer that keeps the core magnetic. The transformer primary is the wire of the conventional coil. The transformer's secondary is formed like a D-shaped copper washer or ring. Some of the primary coil's magnetism generates current inside the copper ring. To keep the core partially magnetic, the current in the ring, or transformer secondary, is delayed in comparison to the current in the primary coil. The lever will not chatter against the core because the continually magnetized core never releases it while the AC is flowing.

Ⅱ What is DC Relay

Direct current (DC) is constant and does not reverse. The electromagnet of a DC relay is made out of a single coil of wire wound around an iron core. When the DC coil is turned on, the magnetism generated in the core remains constant since the DC continues to run. As long as the DC is flowing, the lever is drawn by the constant magnetic. When the current is shut off and the iron core is no longer magnetic, the spring-loaded lever relaxes and the electrical contacts are reconnected.

 Configuring AC Relay Control

Configuration Requirements

  • The inverter communication board firmware (CPU) version must be 4.2xx or higher to use theAC Relay Control feature in inverters employing NetApp.
  • The inverter communication board firmware (CPU) version must be 3.18xx or higher to use the AC Relay Control feature in inverters with LCD and buttons. Contact SolarEdge support for an upgrade file and instructions.
  • When AC Relay Control is activated, all inverters in the system must be set toAC Relay Control mode.
  • If a Commercial Gateway is installed, the model should not be set to AC Relay Control.


AC Relay Control is turned off by default and should be enabled via the inverter menus.

 How to Test an AC Relay

When the air conditioning system is turned on, the compressor will not engage if an AC relay is not functioning properly. The capacitor relay and the starter are the two most important relays in a basic A/C system.

  • The compressor, blower motor, and exhaust fan are all powered by the capacitor.
  • The starting relay is responsible for transferring electricity from the capacitor to the compressor.

If an air conditioner won't start but makes a buzzing noise, the capacitor is probably broken. Fortunately, the starter and capacitor relays are simple to inspect and inexpensive to replace.

 The Signs Of A Bad Relay

How would you tell if a faulty relay is causing your air conditioner to malfunction? There are numerous reasons why your air conditioner may not be working.

It can be difficult to narrow down to a single explanation. Some symptoms, however, can indicate whether or not the relay is defective. They are as follows:

  • Inconsistent cooling will occur if the relay is not functioning properly. The relay's job is to relay the command you provide to the thermostat. If you see that your commands are not being received, you should immediately check on the relay.
  • A jammed relaycan cause the AC to remain on. A jammed relay can remain open, preventing some components from turning off. As a result, your air conditioner does not switch off at all.
  • The next sign is the inverse of the preceding one. If the compressor relayis faulty, your air conditioner will not turn on. Because the compressor will not function until it receives the necessary power. In this situation, you could say that there are problems with the relay.

Knowing these symptoms can assist you in determining the source of the problem. And, as you can see, with a little knowledge, you can fix the problem yourself.

However, if you lack knowledge, you must always rely on pros.

 Stuck AC Relay Switch: A Note

A relay's primary role is to open several electric circuits so that all of the components can function properly.

The fan is one of the critical components that the relay powers. Naturally, the relay should open and close to supply electricity to the component.

However, it can become stuck in one position at times. This interferes with the thermostat's and the fan's communication.

As a result, no matter what commands you issue through the thermostat, your air conditioner will not receive them.

 Diagnosing a Bad Car AC Relay



Defective Car AC Relay

There is a prevalent notion that if you hear the relay click, it must be working well. A poor automobile AC relay is depicted in the close-up photograph on the right. As the ECM activated this relay, it clicked.

A hairline crack in the brass contact strip, however, permits current to travel to the AC compressor. This indicates that the relay activated but did not function properly. This is an extreme example, but it demonstrates that basic rules cannot be applied to every case.

To test a car air-conditioning relay quickly and dirty, locate it and replace it with an identical type of relay. Car manufacturers frequently utilize the same precise component number relay in many locations. This is true for all of the vehicles indicated above.

Although it is not a long-term solution, it is one of the quickest ways to test it out. If you replace it and the compressor still does not turn on, you have a separate issue. When the AC compressor clutch relay has a unique part number, we test it the old-fashioned method.

Remove the component from the socket and check the terminals for a power signal and good ground. A jumper wire can also be used to connect the two terminals that carry electrical current to the air conditioning compressor.

 Testing the Air Conditioning Relay

Remember that an automobile relay is nothing more than a switch that may be controlled remotely. It completes the opening and closing operation with the help of magnets.

When the relay is activated, it generates a magnetic field that closes the contact. When the power to the relay is turned off, the field collapses, and the switch becomes naturally open. There are numerous reasons why a relay may fail to function.

When the part fails, investigate possible causes such as corrosion buildup between the connections. This is a fixable problem. You can clean these contacts with an emery board or fine-grit sandpaper.


This frequently restores the relay's functionality, and you have nothing to lose by attempting to repair it. Having stated that, there are some scenarios in which the relay cannot be fixed.

If there is a break in the wire coil that generates the magnetic field, the car AC relay is faulty. This is simple to determine using a cheap voltmeter. To begin, select the continuity check function.

Then connect the test leads to the ground and connect the signal terminals. If it says open, the relay is broken. It should read 0 resistance or generate a beep sound to indicate that the continuity is excellent.

 AC Problems that Look like Bad Relays

The ECM determines when to activate the air-conditioning relay in the vehicles described on this page, such as Honda, Toyota, and Chrysler models. The most typical cause of a relay failing to engage the compressor is a low system voltage.

In the automotive air-conditioning system, pressure sensors prevent the relay from activating when the refrigerant level is low. A slow leak from a Freon O-ring seal is the most common source of this issue.

There may be a malfunction in the vehicle computer system that stops the control module from applying the ground to the proper leg of the AC relay in uncommon situations, but likely on some models.


Homemade automobile short finder

When this happens, the relay will not turn on. In this case, there is no clicking sound.

Car air-conditioning system repair is regarded as a sophisticated and tough undertaking by mechanics. With that stated, every now and then we get lucky and all it takes is a $15 compressor clutch relay.


1. How Much is an AC Relay?

Replacement of an alternating current fuse, circuit breaker, or relay Cost The cost of replacing fuses, circuit breakers, or relays in an air conditioner ranges from $15 and $300.

2. Which is an Advantage of a Solid State Relay over an Electromechanical Relay?

One of the most significant advantages of solid state relays over electromechanical relays is their ability to switch "OFF" AC loads at zero load current, fully eliminating the arcing, electrical noise, and contact bouncing associated with conventional mechanical relays and inductive loads.

3. Is Solid State Relay better?

An SSR is unquestionably a faster device with a longer lifetime, which is critical in high-speed switching. Bretz: Solid-state relays are ideal for high-speed applications and those requiring millions of operations.

4. What is the Function of Solid State Relay?

AC output solid state relays are used in alternating current power systems to control the flow of electrical energy. The control voltages (corresponding to electromechanical relay coils) can be AC or DC.

5. What does a Compressor Start Relay do?

During the cooling cycle, the compressor start relay cycles on and off to maintain the ideal temperature within your home. The compressor will not be connected if the start relay is not present, and the AC unit will not be powered.

6. How much does it Cost to Fix a Stuck AC Relay?

It could cost you anything from $20 to $400 depending on whether you have a blown fuse or a broken AC relay.

7. How do I know if my Compressor Relay is Bad?

You can use the shaking and rattle test to determine whether you have a faulty compressor relay. Simply disconnect the start relay and shake it. If you hear rattling from inside, the start relay needs to be replaced.

8. How Do I Check an AC Clutch Relay?

Find the Relay. Trace the wire connection from the air conditioner compressor plug-in to the relay to find the air conditioner clutch relay.

Examine the Function. Connect the alligator clip to the negative terminal of the battery using a 12-volt test light.

Remove the Relay.

9. What Happens when a Relay goes Bad?

If your starter relay has failed, the electrical signal from the battery will never reach the beginning motor. As a result, no matter how many times you turn the key, your engine will not start. When you turn your car, a malfunctioning relay frequently generates an audible clicking sound.

10. What are the Symptoms of a Bad AC Relay?

The compressor of the air conditioner would not turn on due to a faulty AC relay. Furthermore, there will be no cold air. You may also notice inconsistencies in cooling.




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