Relay testing

We’ve recently covered how a relay works. With that knowledge, it becomes very simple process to bench test a suspicious part.

Relays are one of the few things in the automotive world that come with built in instructions. Most standard relays have a schematic identifying the internal configuration on the side. As a bonus, the base is usually labeled as well:

If you apply power and ground (via a 9 volt battery, or your car battery) to pins 85 and 86, you should energize the coil, at which point you’ll hear the relay “click”. You can further test operation by using an ohmmeter to check the 30 and 87 pins while the relay is energized.

A functional relay should show no resistance between the 30 and 87 pins while the coil has power. If your meter shows a reading when the coil has voltage, it’s probably time for a replacement. Conversely, pins 30 and 87 should show as an “open” circuit when 85 and 86 are not energized.

Another (even faster) check is to swap your suspected faulty part for another relay. Since all ISO standard parts are the same, it’s as easy as pulling the suspected faulty part, and swapping a known good temporary replacement.