How to Check a Standard Automotive Relay

Typical automotive relay

Automotive relays are used to activate a high-current electrical circuit by using a low-current switch. If a relay is improperly installed, the relay will fail to function correctly.



If a relay switch does not appear to work, check it using a 12-volt power source and a digital multimeter.

Things You'll Need

• Single-pole, single-throw automotive relay
• 12V DC power supply
• Alligator-clip voltage test leads (2)
• Digital multimeter with installed probe leads

Instructions

1. Attach one end of the first alligator clip lead to the relay terminal labeled “85”. Attach the other end of this alligator clip lead to the positive power supply terminal. Attach one end of the second alligator clip lead to the negative power supply terminal.

2. Turn on the multimeter and set the measurement scale to “Resistance” or “Ohms.” Attach one of the multimeter probes to the relay terminal labeled “30.” Attach the other multimeter probe to the relay terminal labeled “87.” View the multimeter display. If the relay is working properly, the multimeter will display an error message demonstrating that the resistance is beyond the multimeter’s measurement capability (such as by displaying an infinity symbol).

3. Attach the free end of the second alligator clip lead to the relay terminal labeled “86.” View the multimeter display. If the relay is working properly, the display will show a resistance value between zero and two ohms.

Tips & Warnings

• If the multimeter does not display the above readings while performing these tests, discard the relay and replace with a different relay.

References

• The 12 Volt: SPDT and SPST Automotive Relays
• AirBorn Electronics: Using a Multimeter