The headlight relay is a remote electrically operated switch and is used in some vehicles wiring to minimise the voltage loss from the battery to the headlights. In basic headlight wiring systems, the power will travel from the battery to the alternator, to the headlight switch and then finally to the headlights. However, when a model> headlight relay is fitted, the voltage goes from the battery to the relay and then directly to the headlights. Because the power has a shorter distance to go, the voltage loss is much smaller meaning that the headlights perform better and put out more light. It’s also possible to successfully add higher output bulbs without overloading your headlight wiring.
The headlight relay is also used to protect the switch from overheating from high amperage. Many headlight wiring systems use two relays to control the flow of power successfully, one for high beam wiring and one for the low beam wiring.
Relays can fail because of oxidisation, overheating or due to a faulty connection in the relay socket. If the headlight relay shows signs of failure your headlights may become dim, which can eventually lead to headlight failure.