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Old 01-25-2017, 05:03 PM   #5
fdryer
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Join Date: Jan 2006
Location: NYC
Posts: 41,156
 

2003 L-Series 3.0L Sedan
Default Re: No brake lights working..all others work..need help!

As you know, running lights are separate from brake lights and are on separate circuits. With ignition OFF, I think every vehicle is wired so anyone pressing the brake pedal will turn on rear brake lights - probably a safety issue when brake lights were designed to work with the brake pedal regardless of ignition switch position. The same can be said of manually turning on running lights, low and high beams without using the ignition switch. With this mind, your brake light circuit is the problem; brake pedal switches nowadays have at least two switches; one for the brake lights and the other for cruise control, the bcm where brake switch signaling goes to and output from the bcm to maybe a relay to power the brake lights. Presuming the bcm is fine leaves the brake switch by brake pedal and wiring at the rear brake lights.

Since brake lights are separate circuits from running lights, someone may have to press the brake pedal down while you probe each brake light socket for 12 volts. Be sure to test brake lights, not running lights. If you're lucky it may be a faulty brake switch and not wiring. The third brake lights are also wired as a separate circuit but run is parallel with brake light wiring so three sets of lights turn on for braking.

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