How Can I Test a Headlight Relay?by Richard Asmus
As complicated as they may seem automobile headlight systems are little more complex than the light switches in your home. Turning on a light switch in your house closes the electrical circuit between the house power and the light, causing the light to turn on. Automotive headlight systems are a bit more complicated, because you need a smaller "switch" to flip the larger one. A relay uses a tiny bit of power from the headlight switch on your dashboard to "flip" another, bigger switch, which turns the headlights on. They're generally pretty reliable, but will eventually fail with time and use.
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.
Listen for the Click
Open the hood, locate the fuse box with the headlight relay or relays, and open it. Put your ear close to the relay and listen. Alternately, you can wedge the handle of a long screwdriver into your ear, and touch the screwdriver point to the relay. You can hear the relay click though the screwdriver like a stethoscope.
Replace the Relay
Have an assistant turn on the headlights. If you hear the relay click, you know the coil is working, but you don’t know about the contacts. If you don’t hear the click, your relay is probably bad. 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. Alternately, you can remove a working relay from elsewhere in the box and plug it into the headlight relay socket. Most vehicles use several interchangeable relays in the same box; check the number stamped on top to make sure they the same before you start pulling them.
You can test the relay with a multimeter, but you have to know how to use it. Pull the relay out of its socket and check across the coil with the ohmmeter section of the multimeter. Orient the relay so the middle -- noted "87a" -- terminal post is vertical to you, and the "sideways" 30/51 post is on the right. Test the center 87a post and the 30/51 post on the right; you should see under 100 ohms of resistance. If it's over that, replace the relay. Identify the "85" ground terminal and "86" power terminals on the relay. Look in the fuse box and find the corresponding holes. Set your meter to read in "Volts DC," and touch the probes to the corresponding ground and power terminals in the box. You should see about 12 volts with the headlights turned on, and nothing with them off. If you see this voltage, but the headlights still aren't working, replace the relay. If you don't get a voltage here, check to see if you have a blown fuse or bad power connection elsewhere.
Richard Asmus was a writer and producer of television commercials in Phoenix, Arizona, and now is retired in Peru. After founding a small telecommunications engineering corporation and visiting 37 countries, Asmus studied broadcasting at Arizona State University and earned his Master of Fine Arts at Brooklyn College in New York.