I have a C172S on which the voltmeter has been, for years, displayed a rock solid 28.1-28.2V. On the last couple of flights, though, it started jumping around, between 28.3-28.6V and higher. At one point it got to 29.6V. My shop said it was either the voltage regulator or a relay. They replaced the relay, now instead of the voltage regulator they want to look at the battery. I'm trying to figure out if it's time to look for a new shop and/or if it's safe to keep flying in its present state (that I won't fry the electronics). Any thoughts would be appreciated.
It might be that the brushes on the alternator are worn and chattering on the slip rings. As the brushes wear down over time, the springs in back of them exert less pressure on the slip rings and vibration can cause them to jump and momentarily break contact. The brushes on many alternators can be inspected and replaced without disassembly but it usually requires that the alternator be removed since it's too hard to get to them otherwise. Once the cowling is off you can tell if they are accessible without removing the alternator. If the brushes are chattering, the voltage reading would usually fluctuate below normal rather than above but that depends on the voltmeter. If the alternator turns out to be okay, then the voltmeter itself might be the problem or there may be a loose wire connection. Attach another voltmeter temporarily to check that out. If you have an auxiliary power receptacle, there are plug-in voltmeters available. If not, then your A&P will have to do it.