Testing an Alternator With an Oscilloscope

Troubleshooting an alternator issue is much simpler with an accurate device like the oscilloscope. Although using an oscilloscope will make the task easier, it can become difficult troubleshooting the issues if your knowledge of alternators is limited. Learning the basics of an alternator should be done before taking on the task. This will make understanding the digital storage oscilloscope much easier.

To begin testing the alternator the digital storage oscilloscope must be connected to the automobile. Start the engine of the automobile, and carefully unscrew the battery from the clamps. Next select the AC or DC positions over the o-scope, it is preferable to keep it in parade position while the various waveforms are testing. Once this is done it will be necessary to connect the oscilloscope’s negative croc clip to the ground connection. Now connect a prod to the positive clip, this is done so prod point can be connected to the terminal. This was just attached to the battery positive.

The first image shows a typical oscilloscope waveform where the alternator is running normal. Judging by the curve of the waveform the alternator may be running at a fast idle. The second image shows a normal alternator when connected through the DC position. Notice it is straight across with a proportionate increase in voltage. In the third image we see a correct waveform for an alternator running at a slow idle speed. Image number four shows a waveform where the diode has failed. The image also shows there is an open stator winding, this is typical during slow idle speed. The fifth and final image shows an alternator with a fast idle where the waveform appears to jump rapidly.

By comparing the images one can troubleshoot to determine how the alternator needs to be adjusted. This can save mechanics much time when it comes to repairs and alterations. By using a digital storage oscilloscope fixing major problems like this can be done quickly and efficiently. This is just one of the many examples of how a digital storage oscilloscope can be used in everyday applications.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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