Hit failing alternator with a hammer to confirm diagnosis of failing alternator due to bad brushes

Hitting something with a hammer often feels good but does nothing useful. Here's one case where it actually helps. On my 2004 Ford Explorer the alternator light came on. I brought it to the auto parts store and they put a volt meter on it. The alternator needs to put out a voltage that is higher than the battery's full charged voltage in order to properly function - somewhere in the 14 volt range. Mine came in low - probably below 13.5 volts. So the guy at the auto parts store took a rubber mallet to the alternator and just briefly, for a period of 10 seconds or maybe less the voltage jumped up into the normal range before falling back to where it was before. This confirmed the diagnosis of a bad alternator in general and worn brushes in particular. The brushes are spring loaded, are located in the are a wearable item and must make contact with the fixed part of the alternator. Tapping the alternator with the hammer temporarily changes spatial relationships between the brushes and what they brush against and allow a failing set of brushes to transiently make the contact they need. If the voltage jumps up in response to a hammer tap you have your diagnosis of a failed alternator due to bad brushes.

You can replace the brushes but I opted for a new alternator, put it in, and the car has run fine since.