Testing a Control Unit Relay

If you suspect an issue with your Control Unit Relay opening or closing, you can test the relay with a multimeter. This article will provide information on doing this.

Confirm the LED display

Each Control Unit has four physical LED’s: Relay 1, Relay 2, Radio, and Power. When the Control Unit Relays are closed (circuit completed), the LED’s are illuminated. If you see the Relay 1 or Relay 2 LED illuminated, the relay circuit is currently closed. In the below image, both Relay 1 and Relay 2 circuits are closed.

Control LED's

Confirming in iMonnit Online

While the LED will tell you certainly whether the Relay circuit is closed, you can also review the iMonnit Online Portal. If the sensor is currently communicating with a gateway, the status shown in iMonnit Online should also indicate whether the Relay is On (closed circuit) or Off (open circuit).

Control Unit Relay On - iMonnit Online

Testing the leads for resistance

Monnit Control Units do not supply any voltage, but when the circuit is closed, you can test the leads for resistance.

To test the leads, connect a multimeter between the leads of each relay and check the resistance value.

  • If the Relay is On (closed circuit), you will see 100 mO of resistance for the 10 Amp Control Unit, or 50 mO of resistance for the 30 Amp Control Unit.
  • If the Relay is Off (open circuit), you will see 8 (infinite) resistance.
    As the relay turns on or off, you should see the change in the resistance accordingly.

Control Unit Leads


If after testing the resistance between the leads of each relay you find the resistance value is not changing, it is possible the relay circuit is damaged. If this is the case, contact support@monnit.com for assistance.