Automotive, Electrical Diagnosis

Automotive Ground Side Open And Resistance

This lesson focuses on what an open circuit on the automotive ground side looks like and what high resistance on the ground side looks like. After you read this article you will be able totroubleshoot ground side faults and completely understand what your meter is telling you. Please view the video for a complete understanding of this process. I have created an open circuit on the ground side with the trainer circuit board. Place the negative meter lead on the negative battery terminal and the positive meter lead on the positive side of the load. With current flowing, ignition and circuit on, note the reading. It shows higher than normal DC volts at the load. This should throw a red flag. Now place positive lead on the negative side of the load. Note the reading. This also shows higher than normal. Ground side of load should never be more than .10v yet we are showing 13v. This tells you right off the bat that you are missing a ground. What is happening is that the bulb filament is acting as a straight piece of wire because current can’t flow without a ground. The next steps you should take are to trace the ground side back toward the negative battery post until you find the correct reading of .10v or less. Once you find that point, you now know the fault is between the last poor reading and the first good reading.

The next automotive ground side fault I inserted is resistance on the ground side. Using the same testing methods, place the negative meter lead on the negative battery post and the positive meter lead on the positive side of the load. With current flowing, ignition and circuit on, note the reading. The lamp is dim yet the positive side shows good near battery voltage of 12.62 V DC. This is a good circuit. Next place the positive meter lead on the negative side of the load and note the reading. This reading shows 8.51 V DC. We know this is not right because we can only have .10V DC on the ground side. Now you will have to trace back toward the negative battery post until you pick up a good reading again of less than .10V DC. That’s all there is to it! Now you now how to solve ground side opens and ground side resistance faults.

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