What it Means

Left to Right: After cleaning the threads, the bulb was again bright and no measurable voltage drop was present.; A DVOM can be used for more accurate measurements.; Resistance was only 3.1 ohms...not as dramatic a reading as the voltage drop test, and it may have gone unnoticed.

For your voltage drop test to be accurate, you must test the entire groundside from load all the way back to the battery's negative post. To test with a DVOM (digital volt ohmmeter), make a lead extension long enough to reach from the battery to anywhere on the car. Attach your meter's ground lead to this extension if the battery is out of reach. Then use your positive meter lead to check voltage drop as close to the load's ground connection as possible. If your reading is high, like mine, start working your way back along the ground path to the battery until your reading returns to normal. The fault then is isolated between that point and the point where your reading was last high.

I started working back by following the ground path from the bulb socket to its point of contact with the body. The mounting plate is held in place with three bolts, all with plastic insulating washers under their heads. As soon as I began removing the first bolt, the bulb began to flicker from dim to bright, and I knew I was close. Leaving the other two bolts tight, I placed the probe into the body's mating threads and measured 0.0 volts. That told me the ground path through the body and back to the battery was OK.

Corrosion on the threads of all three mounting bolts, right where they met the body, was evident upon a close inspection. That was the "unwanted" resistance I was looking for. A chase of the bolt threads and a quick tap through the mating body threads was all that was needed to restore the electrical contact I had been missing. But even so, I figured a little dielectric grease on the threads couldn't hurt to keep it that way. A quick verification after everything was reassembled, and this Jeep's brake lights once again were nice and bright.

Pete Meier is an ASE certified Master Technician, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Pete directly at http://www.autoservicetech.com/.