Rick's Free Auto Repair Advice

One headlight doesn’t work

One headlight doesn’t work and you’ve already replaced the bulb

If you have a headlight out and you’re tried replacing the bulb but it still doesn’t work, here’s how to find out what’s going on. Here are the possibilities when one headlight doesn’t work.

Check the headlight socket for signs of melting

For some crazy reason, carmakers have downgraded the wiring to the headlights, causing the electrical connector to overheat and melt. This is now fairly common. This is what it looks like

melted headlight pigtail

Examples of melted headlight connectors

If the pigtail is melted, it’ll have to be replaced. You can purchase a replacement pigtail from any auto parts store or from findpigtails.com

If the headlight pigtail isn’t melted, check for voltage and ground

Using a multimeter, check for power and ground at the headlight electrical connector. Here’s how to do that.

1) Set the meter to 20 DC volts
2) Connect the black lead to the Common (COM) port on the meter. Connect the black probe to a metal grounding point anywhere near the headlight.
3) Connect the red lead to the vΩmA port on the meter.
4) With the headlight switch turned on to low beams, probe each terminal in the electrical connector. You should see battery voltage (12-volts or more) on the display on at least one terminal in the connector.
5) If you don’t see battery voltage, move on to check the headlight fuse and headlight relays. Refer to your owner’s manual or a shop wiring diagram for further testing.

test for power in headlight pigtail

How to connect a multimeter to check for power in headlight connector

Headlights are wired in many ways

On older vehicles, the headlight power is switched by the headlight switch. So the circuit consists of a fuse, the headlight switch and the headlight.

In other vehicles, the headlight switch is used to ground the control coil to a headlight relay and the relay is what switches power to the headlights.

However, in late model vehicles, the headlight switch is just a signal to the body control module (BCM), telling it to turn on the headlights. The BCM can switch power to the headlights, or it can communicate the request digitally to a power module that does the actual power switching.

Headlight wiring diragrams

Headlight wiring diagrams for different vehicles.

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Posted on by Rick Muscoplat

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