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.