Headlights don’t work

Headlights don’t work

“Headlights don’t work” is a common question is auto forums. Or, people wonder why one headlight works but the other doesn’t.

How headlight systems are designed

Headlight switch switches power

In older vehicles, the headlight switch is what switches power to the headlights. Depending on the year, make, and model, the headlight fuses can be located before the headlight switch or between the headlight switch and the headlights.

Headlight switch operates headlight relay

In other vehicles, the headlight switch is used to operate a headlight relay. It’s the relay that actually switches power to the headlights. And, there’s always a fuse somewhere in the circuit.

Headlight switch is just a request to the body control module

In late model vehicles, it’s the body control module that controls power to the headlight. The headlight switch is just an input to the BCM, telling it that the driver has requested headlights.

In some vehicles, the BCM has relays that switch power to the headlights. But in other vehicles, the BCM sends a digital signal to a smart junction box, telling it to turn on power to the headlights using a relays.

If you headlights don’t work, you MUST get a wiring diagram to diagnose the problem

It is simply IMPOSSIBLE to answer your question without that information. Anybody who does, it just throwing outdated information your way that may cause you to chase your tail in a fruitless effort to solve the problem.

©, 2015 Rick Muscoplat

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