You didn't bother to list the mileage so I can't make any generalizations to that, but brushes don't wear out very often. When they do, the symptom always starts out as intermittent failure to charge the battery, and it gets progressively worse over weeks and months. Chrysler alternators and some older GM and Ford generators are the only ones where you can test for worn brushes. On all others you can only go by the symptom.
Start by measuring the battery voltage when the no-charge is occurring. It should be between 13.75 and 14.75 volts. If that is okay, there are four computers in your charging system and any one of them could cause the "Battery" light to turn on.
More likely you'll find battery voltage staying near 12.6 volts or less. First, measure the voltage on the large white wire at the generator. That should always have the same voltage as you find at the battery. If it is different, check the 140 amp fuse and be sure the connections are clean and tight. You can find bad connections there by measuring the voltages on the studs while the engine is running. Look for anyplace the voltage is different.
Next, at the generator, check that you have full battery voltage on the white / blue wire all the time, and on the black / red wire when the ignition switch is on. If either of those are missing, suspect a blown fuse.
Wednesday, December 2nd, 2015 AT 4:01 PM