How Do You Test a 12V Relay?

A typical relay

A 12 volt relay is an important component in the electrical system of modern cars and trucks. Relays switch the power to high current drawing devices to minimize the distance that large gauge wires must run to cut down on weight and cost. The relay draws very little power to perform the switching, allowing the wires carrying the control voltage to be very small. Using a relay also means a lighter duty switch is used, further cutting cost and weight. Sometimes, but rarely, relays do fail. Testing a 12 volt relay is easy and requires a minimum of tools and time.

Things You'll Need

• 12 volt battery
• Continuity checker
• Test leads

Instructions

1. Grasp the relay firmly with your fingers and pull it straight up out of its socket to remove it from the vehicle.

2. Using the continuity tester, test for continuity between relay terminals 85 and 86, which are marked on the relay. Generally these two terminals will be the smallest terminals on the relay. There should be continuity between these terminals. A continuity tester is a battery operated device that either lights up or buzzes when a circuit is continuous. Touching the probes to each end of a device or circuit automatically tests for continuity.

3. Test for continuity between terminals 30 and 87. There should not be continuity between these terminals.

4. Connect a test lead, which is a wire with an alligator clip on each end, from relay terminal 86 to the negative terminal of a 12 volt battery. Squeeze the clip to open it and release it to attach to the terminal and battery.

5. Connect a test lead from relay terminal 85 to the positive terminal of the 12 volt battery.

6. Check for continuity between terminals 30 and 87. There should now be continuity between these terminals.

7. Replace the relay if any of the continuity tests fail. If the relay passes all the tests it is good and the problem exists elsewhere in the fuses or vehicle wiring.

Tips & Warnings

• Never attempt to test for continuity when the relay is plugged into the vehicle. At best you will get false readings, at worse you will damage or destroy your tester.

References

• bcae: The Famous Bosch Relay