How to Check if a Relay is Bad

A relay is an electrically operated switch used to isolate one electrical circuit from another. In its simplest form, a relay consists of a coil used as an electromagnet to open and close switch contacts.

Instructions

Testing The Coil And Switch Contacts

1




Place the test leads from an ohm meter across the coil terminals (shown as points A and B in the schematic diagram). This will check for continuity of the coil. The ohm meter should show a very low resistance if the coil is good. It is possible for a coil to have shorted windings, but this is rare. Use this procedure to check the coil in any type of relay, regardless of the number of "throws" and "poles", or the voltage needed to trip the coil.

2




Test a relay's switch contacts with an ohm meter. In the case of a relay that has a single pole single throw contact, as shown in the schematic diagram, place the test leads across points C and D.

3

Measure the resistance, which will either be infinity or a dead short, depending on whether the relay configuration is normally open or normally closed. A reading of infinity indicates an "open" circuit, and a reading of zero ohms (short) indicates a "closed" circuit. Next, apply the proper voltage to the coil (points A and B) to energize it.

4

Apply the voltage either with a battery or power supply set to the proper voltage specified by the relay, such as 12 volts DC (direct current). With the test leads still attached to points C and D, once the coil is energized, the ohm meter should give the opposite reading as it did when the coil was at rest and no voltage was applied. If the ohm meter does not show a change between the on and off states of the coil, then the switch contacts in the relay are bad. Typically dirt, dust and carbon buildup will cause relay switches to fail to make good contact.

5




Measure the contacts on a double pole single throw relay by placing one test from an ohm meter on the center switch contact (point E on the schematic diagram), and the other lead on point C. Note the ohm meter reading, either open or closed.

6

Apply the proper voltage with either a battery or a DC power supply to energize the coil. Again, note the ohm meter reading. It should be opposite the reading when the coil was at rest. Continue by checking the switch contacts between E and D.

7

Use the same procedure, checking the open/closed status during the two states of the coil, energized and at rest. If any of these readings to not show a difference between the times when the coil is energized and at rest, then the switch contacts are defective. Note also that the at rest/energized coil state should give opposite readings for switch C E as it does for switch D E. For example, if the contact between C and E is normally open when the coil is at rest, then D and E should be normally closed, and vice versa.

8




Attach an ohm meter test lead to point D (as shown) and one on point C of a double pole double throw relay, and check for open and closed conditions with and without voltage is applied to the coil.

9

Move the one test lead from C to E and repeat the test. Note that this time the results should be opposite what they were for D and C. For example, if the contact between C and D is normally open when the coil is at rest, then D and E should be normally closed. When the coil is energized, the opposite readings should be present. A double pole double throw relay has another set of contacts, shown here as F, G and H. Use the same procedure to test this section of the switch.