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Hi,
I have a 99 with low miles and an Optima Yellow Top a few months old. I am getting 13.6v on cluster at idle and doesn’t go up when driving. I am pretty sure it used to go up to 14.1v while driving. Is this a sign of bad alternator?
You can just remove the alternator and get it tested at a auto parts store, the voltage you are seeing maybe correct, it all depends on the state of charge of your battery, it may be fully charged hence the lower voltage reading.
You need to check the voltage at the battery with a voltmeter which could be 14.5 volts when the dash reads 13.6 volts. The dash voltmeter connects to the battery through the ignition switch contacts which if oxidized from use could add resistance resulting in a low reading.