One Side Slow, One Side Fast

As I do on every car when I first bring it in, I checked the operation of the wiper/washer system and started checking the lighting system. The left turn signal indicator blinked at a normal speed, but the right side blinked much faster. Typically, this is a sign that a bulb was blown on the "fast" side.

That wasn't the case. I found the right rear bulb on, but dim. Since this bulb is also used by the brake lights, I checked that next. No doubt about it, it was trying to work but couldn't. The question now was "why not?"

Left to Right: A probe on the socket confirmed what I suspected.; Using the toggle switch to supply a known good ground brought the bulb back to life.; Look closely...see the corrosion on the four to five threads near the bolt head?

With the lens removed for access and the brake applied, it was easy to see that the backup lamp also was dimly lit, one more indication of a bad circuit ground. The voltage reaching the brake light bulb was trying to find its way back to the battery through the other bulb instead of a more direct route to ground.

Where is Home?

This was a simple circuit, according to the schematic. Power applied through the brake light switch then passes through the multifunction switch on the steering column. From there, it travels to the bulb, then back to the battery by grounding to the body at the socket and through the taillight mount. A quick probe of the socket terminals told me system voltage was present, and I had not expected to see anything less, since all the other rear lights were lit.

The bulb itself is grounded to the bulb socket, and this spot was the one closest to the circuit's load. To test the groundside for any voltage drop, this is the place I wanted to be. Touching the probe tip to the socket shell with the bulb "on" gave me a reading of 6.4 volts, definite proof that I had a ground problem. A quick toggle to supply a good ground to the socket made the bulb nice and bright and just verified what I already knew.