How Can I Check a Headlight Relay?
When you turn on your headlights, you put power to the coil of a relay which closes the electrical contact to your headlights. If your headlights don't work, the relay may be bad. However, the fuse could have also failed; it's important to first check the fuses, as they fail far more often than relays. If the fuse is OK, a few simple steps could help you isolate a bad relay before you need a multimeter or a mechanic.
Listen for the Click
Open the hood and stand close to the headlight relay. Have someone turn on the headlights. If you hear the relay click, you know the coil is working, but you don't know about the contacts.Turn on Your Lights
Turn on your lights. If only one light comes on, the relay is OK and you probably have a bad headlight. The relay operates both headlights.If you don't hear the click, your relay is probably bad.
Replace the Relay
The safest way to test a headlight relay is to replace it with a known good one. If you don't have a spare, your auto supply store may have a return policy, so you could always buy one and then return it if necessary.
Multimeter Tests
You can test the relay with a multimeter, but you have to know use it. Pull the relay out of its socket and check across the coil with the ohmmeter section of the multimeter. It should read low resistance, under 100 ohms. If it reads higher, the coil of the relay is bad. If it is good, replace the relay, turn on your lights and check the voltage at the common (C) terminal. You should read 12 volts. If not, your fuse is probably bad. If you read 12 volts, check the normally open (NO) contact of the relay. It should read 12 volts. If not, your relay contacts are bad. See the references for relay contact layout information.
