How to Test Generator Brushes


Generator brushes are essential for enabling the electricity produced by the generator to pass from the central armature to the generator terminals. The brushes are usually made of carbon fiber, but because they constantly touch the rotating armature, they only last a certain amount of time before you will need to replace them. Fortunately, they are inexpensive and reasonably easy to replace. If your generator warning light doesn't go out once the generator is operating, you may have worn brushes, but before you to decide to replace them, it's sensible to test them.

Things You'll Need

  • Needle-nose pliers
  • Ruler or tape measure
  • Turn off the generator before testing the brushes. Locate the two brush holders on opposite sides of the armature on the front or rear of the generator. Each brush has a copper wire extending from it that connects to the generator terminals.

  • Pull the copper wire gently using your fingers or a pair of needle-nose pliers as far as you can until you can't move the wire out any more. Don't pull too hard or you might break the wire. The brushes have springs behind them that apply pressure, so as the brushes wear they continue to touch the armature. Sometimes the springs or brushes get stuck, so the brushes don't move inward as they wear.

  • Stop pulling the wire so that you release the spring. The pressure from the spring moves the brush toward the armature. Repeat the process on the other brush.

  • Turn on your generator. Check if the generator light goes out. If it does, the problem is solved; if it doesn't, turn off the generator.

  • Use a ruler or tape measure to measure the distance you can pull the copper wire attached to each brush using your fingers or the pliers. If the brushes aren't worn, you can only pull the wire about 1/4 to 1/2 an inch, depending on the size of the generator and the brushes. This means that the brushes are fine, but you have a problem with the generator. If the wire moves more than 1/4 to 1/2 an inch, you need to replace the brushes.

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