How to Test an Electrical Relay
An electrical relay is a remote switch adapted from an electromagnet which uses an electrical current to operate. When the relay is operating correctly the electrical current through the pulldown coil produces a magnetic field that either opens or closes the contact point within the relay depending on it's design. If a relay-operated item begins to malfunction or ceases working entirely it is recommended that you test the electrical relay. This is a straightforward process that is detailed in the steps below.
Step 1 – Test Electrical Item
You need to check whether the item powered by the relay switch is malfunctioning first. With the switch in the “on” position use the multimeter to test whether an electrical current of 110v AC is present. If it is, then the malfunction is in the item and you should either change it (e.g. in the case of a light bulb) or take it to be repaired.
Step 2 – Measure Power Supply
Check both power supplies to the relay to confirm the presence of 110v AC and 12v DC. If these currents are present then the relay is definitely the problem. If one or both of the voltages are not present then you need to check your electrical circuit because it is likely the problem lies there.
Step 3 – Test Relay Contacts
If the relay is malfunctioning, then it is also possible to narrow down the actual component causing the problem. Test the 110v AC current where it enters and exits the relay with the multimeter. If a reading of 0v is received, then the contacts are functioning correctly. A high voltage reading for this test indicates that the contacts are either burned or damaged and you should replace the relay.
Step 4 – Check Ground
Disconnect the power supplies from the relay and remove it from it's position. Test the ground wire with the multimeter to ensure that it shows zero ohms to confirm that the relay does need replaced.
Step 5 – Measure Coil Resistance
You can also measure the resistance across the pulldown coil within the relay with the multimeter. If it measures zero ohms then the coil is functioning correctly, an unstable reading indicates a problem with the coil and you should go ahead and replace the the relay to allow your item to function correctly.
Step 6 – Replace Relay
Place a new relay into position and switch the circuit on. If you are using a relay recovered from another piece of equipment then you should use the procedures above to check that all components of the relay are functioning correctly with the multimeter before installing it.