How to Repair Headlight Wiring

by Matt Scheer

For most vehicles, the headlight circuit features four main components: the headlight bulb, the wiring harness (brings power to the headlight bulb) and the fuse (protects the circuit from power surge damage). If any of these components are damaged, the headlight circuit may not work. You can repair and replace these parts in a straightforward diagnostic procedure to discover the issue with your headlight wiring.

Engine Compartment Wiring

Step 1

Turn on your headlights and check which one is not working. Turn off the headlights. Prop open the hood with the hood support rod.

Step 2

Loosen the negative battery terminal nut with your crescent wrench. Lift the negative battery cable off of the terminal.

Step 3

Disconnect the wiring harness on the rear of the headlight assembly. Check that the contacts are clean on the headlight socket and the wiring harness. Look for discoloration and corroding.

Step 4

Brush the top layer of corrosion away with the pencil eraser. Spread a dollop of dielectric grease on the contacts for the wiring harness to prevent future erosion. Plug the wiring harness back into the headlight socket. Connect the negative cable to the negative battery terminal. Turn the headlights on again. If they still do not work, continue to the next step.

Step 5

Open the fuse box.

Step 6

Pull out the fuse that operates on the headlight circuit with the headlight that is not working. Check whether the fuse is blown. Replace as needed. Test the headlights again. If one still does not work, continue to the next step.

Step 7

Unplug the headlight wiring harness from the non-functioning headlight. Connect the negative lead from the voltmeter to the negative battery terminal. Connect the positive lead from the voltmeter to the positive wire on the headlight wiring harness. The positive wire should have power coming through it. Verify that power is coming through the positive wire. If power is coming through the wiring harness, replace the headlight bulb and turn on the headlights again. If no power is coming through the wiring harness, continue to the next step.

Step 8

Follow the headlight wiring cable back to the fuse box. Look for any tears in the cable insulation. If you find any tears, disconnect the negative battery cable again. Cut the cable at two points, before the tear and after the tear with your wire cutters. Connect each wire in the cable back again using your solder sleeve connectors and the heat gun. Strip 1/2 inch of insulation off the end of each wire. Slide the solder sleeve connector over one of the wires. Braid the exposed leads of the corresponding wire: positive to positive and neutral to neutral. Slide the solder sleeve connector over the braided leads. Heat the connector to melt the solder inside the solder sleeve connector. Turn the headlights on to verify that they now work.

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