How to Test HVAC Relay with Multimeter – A Definitive Guide

how to test hvac relay with multimeter

Relay are devices that help you control the low power logic signals where there are high power circuits. It works by disconnecting high power circuits to protect the low power circuit by giving the logic circuit an electromagnetic coil for control. But do you know how you can test relays on an HVAC system using a multimeter? Is there any particular skill you need to do this? Read on and understand more.

How To Test HVAC Relay with Multimeter Explained

  • The first thing you need to do before you test the relay using a multimeter you’ve got to check on the relay schematic sheet. Usually, a relay has a standard pin configuration, but you need to check out the schematic to know the number of pins, but this information is always on the relay.
  • The next thing you need to do is check on the pin’s information, which entails the voltage and current ratings, which helps you do a worthy test. It would help if you also inspected the relay with your eyes to determine any possible damage, melting, or blackening. But since most relays have LED lights, it will be easier to know if the relay is the problem.
  • Note that you should make any electrical connection after disconnecting the device from the power source. You should also check on the capacitors in the circuit as they can have some charge after disconnecting the connection.

Testing a Coil Relay Using multimeter

Before you start testing the coil relay with multimeter, you’ve got to ensure you know the coil requirement. After that you should;

  • Look at the manufacturer’s part number on the case. Also, check on the valid datasheet to determine the current and voltage requirements of the coil.
  • After that, look for the control’s coils diode protection, either on the poles or the drawings.
  • Then study the configuration of the relay, which is either on the case or datasheet. The device can have either one or more poles and is shown by drawing a single line switch that connects to one pin on the relays. It would help if you then used a multimeter to test the disconnected condition of the contacts on the relay.
  • It’s advisable to use digital multimeters to monitor the resistance between the poles and the normally open (NO) or the normally closed (NC) contacts. All the NO contacts should be showing an infinite, while NC should indicate 0 ohms.
  • After that, you should stimulate the relay with an independent voltage source according to the coil’s rating. You will hear some clicking sounds if you energize the relay. Since you’ve energized the relay, you should check the energy condition of its contacts.
  • Use the multimeter to test the resistance between the poles and the NO and NC contacts. The NO should be reading 0 ohms while the NC show infinite resistance.

Testing a Solid-State Relay Using Multimeter

  • If you want to test a solid-state HVAC relay, you should use an ohmmeter when checking the relay on NO terminals after disconnecting it from the source of power.
  • After that, you should use the digital multimeter to review the device’s findings while on its diode-test mode.
  • The multimeter will transmit some voltage enabling the semi-conductor to conduct and see the display’s voltage. If your relay is faulty/damaged, it’ll read 0 or OL, but if it’s okay, it’ll indicate 0.5 for a germanium transistor but 0.7 for a silicon transistor.

The best thing about the solid-state relays is that they’re affordable and easy to troubleshoot, and durable when well maintained. Note that testing an HVAC relay with a multimeter is not hard as you might think. All you need to do is to follow the above process keenly.

What Causes a Relay to Fail?

Relays fail mostly when mechanical switching elements refuse to close, making them unable to carry any current. The mechanical failure of relays is mostly because of contaminations from various chemicals used in the manufacturing process. The contamination can be metallic, which leads to shorted conditions or blockage of the physical movement of mechanical elements. Or non-metallic, which causes open circuits after several deposits on the contacts.

Another reason can be mechanical damage of internal switching elements. This mainly results from high rush currents, high voltage spikes, or high sustained currents.

How Do You Troubleshoot a Relay?

If you want to troubleshoot a relay, you’ll require an ohmmeter. Then disconnect all the connecting wires on the relay. After that, measure the 1 and 2 terminals to know whether the resistance is zero as these are usually closed contacts. But if it’s infinite, the connections are stuck open, meaning you should remove and throw them.

If the NC contacts between both terminals are dirty or pitted and arched, the ohmmeter is likely to show a resistance more than 0 though less than infinity.

How Do I Know If My HVAC Relay Is Bad?

If you want to know if your HVAC relay is bad, check if the clutch and AC compressor are getting a sufficient power supply. When this happens, your HVAC relay will not be cooling as it should. This is because it’ll not be able to produce enough cold air. The AC compressor may also not be able to turn on because of the low power supply. Usually, when you turn the HVAC, the compressor will produce a clicking sound that is impossible with a bad relay.

How Many Ohms Should a Relay Have?

The relays get a 12V direct supply from the vehicle battery. However, the electrical resistance of the coil is not the same and depends on the relay manufacturer and the relay type. A standard relay should have between 50 ohms and 200 ohms.

How Do You Test A HVAC Start Relay?

If you want to test an HVAC start relay, check its coil by putting the ohmmeter leads across the second and fifth terminals. If the ohmmeter displays a high resistance of between 3000 and 18000 ohms, it shows the coil of the relay is electrically fine.


Testing the HVAC relay with multimeters might at first seem hard, but you’ll realize it’s an easy thing to do. You need to follow the above process without skipping any steps. But know that there are instances when the relay may fail, forcing you to troubleshoot it. You should also be able to know when your HVAC relay is bad and act accordingly.

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