What is a headlight relay and how does it work? What are the symptoms for a headlight relay failure? How do you test the headlight relay ?
What is Headlight Relay?
Most headlight systems include only a few components like bulbs, relay, fuse, switch, and fuse. While some vehicles may have minor modifications such as adaptive headlights, daytime running lights or fog lights the overall design is the same. The switch that turns on your headlights activates a relay. The relay provides the electrical connection between your bulbs and the batteries. Fuses protect the wiring and the system circuitry.
Relays are used to control low beams or high beams in vehicles equipped with computer-controlled headlight system. Some vehicles have an additional relay for activating the daytime running lamps. A headlight relay converts high voltage to low voltage by using an electromagnetic switch. A coil and a switch mechanism are contained within the relay. They also have a number of connections. Most automotive relays are open so that no electricity flows between the connections until the relay activates.
A signal is sent from the computer to the body control module (BCM), when the driver turns on the low beam lights by turning the headlight switch. The BCM responds by grounding and allowing current flow through the coil. The coil’s current creates a magnetic field that closes the normally closed relay, turning on the low beam lights.
It is important to note that not all computer controlled headlight systems use relays. One example is that the BCM can send a request for activation of headlights to a smart fuse box (often called an integrated power modules (IPM)). After receiving the request, IPM activates the low beams by using the integrated drivers.
Older headlight systems aren’t often computer controlled and don’t have headlight relays. Instead, current is usually passed directly from the headlight switch via the dimmer handle to the lights.
What are the Signs of a Bad Headlight Relay?
Headlight relays, just like all relays, can fail. Headlights that fail will stop working as intended. You will notice the following symptoms.
Headlights may not light
Headlights that won’t turn on are the most obvious sign of a bad headlight relay. If the relay fails to turn on, the electric current can’t reach the headlights. The low beams won’t work if the low beam relay is damaged. The same applies to the daytime running light relay and high beam relay. If either fails, the headlights that are correspondingly affected will not work.
The headlights may not turn off
The off position will cause the headlights to stay on and not turn out if the headlight relay is damaged. It could be the relay itself or the circuits powering the headlight relay that are the problem.
Hidden Headlights (Pop-up Headlight) May Not Work Properly
Some cars, especially sports cars, have hidden headlights which can turn on or off. These headlights can be activated using a computer-controlled relay. The relay will fail to turn the hidden headlights on if it is not in the on position. You may need to manually open and close the relay in such cases.
The fault may be electrical or bulb-related. The problem is usually caused by the bulb.
How do I test a headlight switch?
The engine compartment fuse box usually houses the headlight relay. Relays used in automotive are usually compatible. It is easiest to test a relay for headlights by using another relay in the fuse box. If your headlights go out during the night and you are unable to reach a mechanic, the most important relay can be temporarily used in the fuse box. Pay attention to the number on the relay.
You can also listen to the relay to check if it is working. As a stethoscope, you can use the handle on a long screwdriver. The relay will be activated when you touch the tip of your screwdriver to it. The relay will click if it is functioning. If the relay doesn’t make any sound, it means that the coil is working properly and should be replaced.
