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I have a volt meter gadget plugged into the cigarette lighter socket. It used to read at 14.3 volts when the car is running. Suddenly it dropped to 11.2 volts without any accessories or lights on. I decided to disconnect the negative cable from the battery and the engine stopped running. Is it the alternator that's bad or just the carbon brush?

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  • The brushes are usually very difficult to get to so it will be difficult to check the brushes unless you remove the alternator. Brushes generally last over 100,000 miles. Do you know how many miles your alternator has done? – Mike Poole Aug 13 '19 at 7:00
  • Disconnecting the battery with the engine running can sometimes destroy the alternator. I used to like customers like that - a quick and easy job : fit new alternator... Don't believe the people who say "it is how to test an alternator", wrong although it is fine for a dynamo... – Solar Mike Aug 13 '19 at 7:59
  • Disconnecting the battery with the engine running can also destroy the car electronics, since the battery smooths the rectified AC from the alternator. – HandyHowie Aug 13 '19 at 9:18
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    @SolarMike The rectifier just moves the negative cycle of the AC into a positive cycle. You end up with pulsed 'DC' waveform. See for example - electronics-tutorials.ws/power/single-phase-rectification.html The battery acts like a capacitor to smooth out the pulses. Because the alternator has 3 phases, the voltage doesn't go down to zero volts though. – HandyHowie Aug 13 '19 at 9:56
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    @HandyHowie This is much more relevant especially further down as it shows the ripple electronics-tutorials.ws/power/three-phase-rectification.html Have you measured the pulses from an alternator with an oscilloscope? If not, it is worth doing just so you know... – Solar Mike Aug 13 '19 at 10:21
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There could be a number of reasons for the failure -

  • Snapped belt.
  • Disconnected cables to the alternator
  • Broken fuseable link in the fusebox
  • Faulty rectifier in the alternator
  • Worn brushes in the alternator
  • Broken coil in the alternator
  • Faulty regulator in the alternator

After checking the cables and belt, you would need to dismantle the alternator to diagnose further.

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