Any testing on the charging system has to be done while the problem is occurring, otherwise everything is going to seem to be okay. Given the intermittent nature of the problem, the best suspect by far is worn brushes inside the alternator. Those can be replaced separately as an assembly that costs about ten dollars, but most people just replace the entire alternator.
This has happened to me three or four times over the years. Based on the very high mileage of what I drive, I have replacement brushes on hand, but it would still be prudent to do a verification test to avoid wasting money on the wrong solution. The test involves measuring the voltages on the two smaller terminals on the back of the alternator, but that has to be done while the engine is running. You can use a test light for this, but a digital voltmeter is more informative. Here's a link to an article about using a voltmeter if you want to pursue this yourself:
https://www.2carpros.com/articles/how-to-use-a-voltmeter
I use small jumper wires to connect the voltmeter, then I run them under the back of the hood and under a wiper arm so I can watch the meter while I'm driving. That works best when you have to wait a long time for the problem to act up. If yours acts up most of the time, you can skip that and read the meter under the hood.
One of those small terminals is going to have full system voltage when the engine is running. Normally that will be between 13.75 and 14.75 volts. We are interested in the other terminal. That one will have less, typically between about four and 11 volts. If you find this one goes to 0 volts when the charging problem acts up, the brushes are worn inside the alternator. The additional clue is the charging system will stop working more and more frequently, and for longer periods of time over the next few weeks or months.
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Monday, February 11th, 2019 AT 4:17 PM
