What wire should I check if the headlights fail but the parking lights work on a 1979 Ford truck?

Check for power at the dimmer switch connector terminal of wire 15 (y/bk). If no power there, check for power at the light switch terminal of wire 285 (br); if that also has no power, replace the light switch.

Where is splice 801 located?

Splice 801 is located behind the instrument cluster.

What should I check if headlights work but parking, side marker, and license plate lights fail?

If power is not present at terminal of wire 25 (bk/o), check the black and orange wire to the fuse link, inspect the fuse link, and check the black and orange wire to the battery. If power is present, check the terminal for damage and repair/replace as necessary, or replace the light switch if the terminal is OK.

How do I test a dimmer switch that only gives high or low beam?

Position the dimmer switch so you can operate it by hand, insert a test light on terminal of wire 12 (gr/bk) and operate the switch several times, then repeat on wire 13 (r/bk). If the test light does not flash off and on as the dimmer button is pushed, replace the dimmer switch.

Does the headlight switch have a fuse for the headlights?

The switch has an internal circuit breaker for the headlights circuit and one for the parking/marker lamp circuit; these circuit breakers are automatic resetting.

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1979 Ford Bronco F100-F350 Exterior Lighting Troubleshooting & Wiring Guide

Exterior Lighting System troubleshooting page: headlight switch, dimmer switch connector diagrams

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Ford manual Exterior Lighting System page: headlight switch internal circuit breakers and single-lamp/headlight troubleshooting with dimmer-switch connector terminal diagrams (wire 285, 13, 12b, 25).

AI caption August 2014 · Photo 13 of 48

This page, from a 1979 Ford Bronco/F100–F350 service reference set, covers the Exterior Lighting System. The 'How It Works' section explains headlight switch operation: constantly powered, first detent operates parking/marker lights, full knob extension operates headlights, knob rotation dims cluster lighting and operates the dome light, with automatic-resetting internal circuit breakers for headlight and parking/marker circuits. The Trouble-Shooting section provides step-by-step diagnostic procedures for common failures: a single lamp failure (burned-out lamp, socket, corrosion, ground issues); headlights fail while parking lights operate (checking power at dimmer switch wire 15 y/bk and light switch wire 285 br, splice 801 behind the instrument cluster); headlights operating in only high or low beam (test light checks on dimmer switch wires 12 gr/bk and 13 r/bk); one headlight failing on both beams (ground and filament checks); headlights working but parking/side marker/license plate lights failing (checking wire 25 bk/o, fuse link, and black/orange wire to battery); and all exterior lights failing (switch, connector, and brown wire checks). Photo captions show test-light checks, and pinout diagrams identify the dimmer switch connector (13 r/bk, 15 r/y, 12 gr/bk) and headlight switch connector (25 bk/o, 285 br, 15 r/y).

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Frequently asked questions

What wire should I check if the headlights fail but the parking lights work on a 1979 Ford truck?: Check for power at the dimmer switch connector terminal of wire 15 (y/bk). If no power there, check for power at the light switch terminal of wire 285 (br); if that also has no power, replace the light switch.
Where is splice 801 located?: Splice 801 is located behind the instrument cluster.
What should I check if headlights work but parking, side marker, and license plate lights fail?: If power is not present at terminal of wire 25 (bk/o), check the black and orange wire to the fuse link, inspect the fuse link, and check the black and orange wire to the battery. If power is present, check the terminal for damage and repair/replace as necessary, or replace the light switch if the terminal is OK.
How do I test a dimmer switch that only gives high or low beam?: Position the dimmer switch so you can operate it by hand, insert a test light on terminal of wire 12 (gr/bk) and operate the switch several times, then repeat on wire 13 (r/bk). If the test light does not flash off and on as the dimmer button is pushed, replace the dimmer switch.
Does the headlight switch have a fuse for the headlights?: The switch has an internal circuit breaker for the headlights circuit and one for the parking/marker lamp circuit; these circuit breakers are automatic resetting.