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VW Beetle Repair: Brake Lights

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My father-in-law pointed out that the brake lights in my beetle were unequally bright. It was a little tough to tell in the day time, but after investigation, it was definitely true. There was something else with the brake lights that was mysterious; when I pushed on the brakes, the front fender lights and the speedometer light came on dimly.

Debugging this problem was really a lesson for me in the first rule of modular electronics--make sure your connections are clean and tight. If I'd gone through first and checked all of the connections in the brake lights themselves, I would have saved myself a couple of evenings of testing and chasing down false leads.

I asked around about this problem, and some suggested that I had a ground problem in the rear lights. You see, the tail/brake light bulbs are dual-filament. One half (5W) is the tail light, and the other half (20W) is the brake light. If the ground of the bulb isn't really connected to the ground of the car, then current can leak from the brake light side of the bulb over to the tail light side. I checked the grounds, however and they were fine. That wasn't the problem. Then I took the light housings apart to try to find the problem, and things changed.

Ultimately I realized that the tail light and brake light circuits were connected, by not because of a bad ground. The wiring was all fine; the problem was that the bulb was installed in the socket wrong. The bulb socket looks like this: The barrel of the socket (and bulb) is ground. There are two paddles at the back of the socket, one is the tail light circuit and the other is the brake light circuit. The buib looks like this: and has one stud for the tail light filament and one for the brake light filament. What had happened was the bulb was jammed in the socket twisted from where it was supposed to be. The bulb stud for the tail light filament was touching both paddles, effectively connecting the two circuits. This meant that when the brake circuit was energized, the tail light circuit was also energized, causing the running lights to come on.

Longer debugging process than it should have been, good practice, good lesson re-learned.

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