Brake light fuse blowing – need help with diagnosis

[Gareth Randall]

I lost all the brake lights on my 2002 Jeep Grand Cherokee V8, so I checked the fuse and it was blown. I replaced it, and the brake lights worked for about one second before the fuse blew again.

The Jeep uses dual-filament bulbs which do double duty as both tail and brake lights. The tail lights are working fine, this issue is specific to the brake lights.

I have no idea of the correct workflow for diagnosing the problem – can anybody suggest a plan of attack?

Topic

[Billy]

You have a short circuit – some wiring in the brake light circuit has worn through its insulation and is contacting the frame or some other ground, shorting to ground without going through the bulbs. Without the resistance of the bulbs, the circuit passes too much current and blows the fuse.
Does the fuse blow as soon as you turn on the car, or not till you press the brake pedal? Study the circuit diagram and find the sections of the circuit relevant to your failure pattern. Note that a common point of failure is where wires go through the boot between the body and the tailgate. That may be a good place to look if one or all of your brake lights are on the tailgate.

[Gareth Randall]

The fuse is fine until you press the brake pedal. Then the brake lights come on for a second, and then the fuse blows.

[Author]

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