Yup. Brake lights are the easiest to diagnose because you have two sides to look at that are connected together. When one side works, you know the brake light switch, fuse, and wiring up to that point are good.
If you can catch the problem while it's acting up use a stick to hold the brake pedal down a little, (or use a helper to push the pedal), then use a test light or cheap digital voltmeter to measure the voltages on the dead bulb. If you have the newer bulbs with the plastic base, you will have to back-probe the socket through the rubber seal. I really hate it when people pierce the wire insulation to take a reading. That leads to corrosion in the future. There must be 12 volts on one wire and 0 volts on the ground wire. If the ground wire is broken, both the test light and the brake light will be half brightness when you touch the test light to it. The test light will complete the circuit so the brake light will work, but it will also add extra resistance so the brake light will be dim. That won't work the same way if you're using a voltmeter because they don't pass nearly enough current. If the ground wire is broken, the voltmeter will read around 12 volts on both the feed and ground wires.
If the voltages appear ok, check for corroded connections inside the socket.
Friday, May 14th, 2010 AT 4:01 PM