There are two common suspects. First, follow the smaller positive battery wire to the under-hood fuse box and be sure that connection is clean and tight. Follow the smaller negative battery wire to the body and be sure that one is tight and not rusty.
Second, you can start the alternator testing by measuring the battery voltage. With the engine off it should be 12.6 volts. With the engine running it must be between 13.75 and 14.75 volts. This one can intermittently drop to 12.6 volts. That's an indication of worn brushes inside the alternator. That is a fairly common problem with an inexpensive fix.
If that voltage stays within the acceptable range with the engine running, the more important part of the test requires a professional load tester. Of interest is the maximum output current. A typical value is 90 amps. If you find the most you can get is exactly one third of the alternator's rated output current, it has one bad diode of the six. That will cause voltage fluctuations and it can confuse the computers.
Saturday, October 11th, 2014 AT 9:16 PM