This is very common, especially at the mileage you listed. It's usually due to worn brushes inside the alternator. Any testing has to be done while the problem is occurring. Sounds your alternator was tested while it was working, so it is going to appear to be okay.
Worn brushes always start out as an intermittent problem. Mine have gone months like that before the problem becomes permanent. You'll want to address it before it gets that bad.
A common repair is to replace the brush assembly, but that is a job we usually only do for ourselves. It takes longer than just replacing the alternator. The cost of parts is very low, but given the time we have to charge for, and that we have to warranty our work, it is a better deal for both of us to just install a professionally-rebuilt alternator.
Before you just blindly replace an alternator and hope for the best, there are a couple of tests that should be performed. The main one is to measure the continuity between the two small terminals in the back of the alternator. Normal is roughly four to six ohms. Another common problem is a small chip of carbon can get caught under a brush and cause the reading to be "infinite", or an "open circuit", which is indicative of worn brushes. The next step is to tug on the belt a little to "irritate" those brushes. That can make the brushes read correctly when they really aren't worn yet.
We can also use two simple voltage readings to tell if the brushes are worn, but the circuitry changed somewhere around your model year, so that might not work. If you want to pursue that, I'll find the diagram and walk you through it.
Other potential causes of intermittent operation include a defective voltage regulator inside the Engine Computer, a corroded connector terminal, or even a cut wire. Those are very much less common, and a cut wire is usually not intermittent. We want to rule those things out before spending money on an alternator.
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Sunday, March 10th, 2024 AT 7:55 PM
