Does the meter work differently when testing a known good system?
Your cheap digital multimeter may be the reason you are seeing the voltage jump all over. The regulator also works digitally so the current applied to the rotor field coil is either maximum or nothing. The output voltage is determined by the amount of time voltage is applied through the bushes verses the amount of time there is no voltage applied. As a result the output voltage from the alternator is also varying constantly. The regulator switched so rapidly that an analog voltmeter cannot react fast enough to show the change in voltage from the alternator output. Your digital meter continually takes a "snapshot" of the voltage and shows it on the display. The capacitor in the alternator is supposed to smooth out the voltage spikes but even with a good capacitor you are able to see the wave pattern in the output voltage when it is displayed on an oscilloscope.
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Today's Featured Article - A Cautionary Tale - by Ian Minshull. In the early 1950s my father bought an Allis Chalmers B and I used it for all the row crop work with the mangolds and potatoes, rolling and the haymaking on our farm. The farm and the Allis were sold and I have spent a lifetime working on farms throughout the country. I promised myself that one day I would own an Allis. That time event
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