Like Poly said, the Charging System Warning Light (the little battery on the dash) should go off if the electric systems voltage is too low or too high or the PCM senses a fault in the field control of the alternator.
First, are you're sure the tools you're using to measure the voltage is accurate? Have you tried another tool to confirm they're both reading the same voltage and one might NOT be off?
If the voltage is truly that high, and its NOT lighting a Warning Light on the Dash, then its most likely because the PCM doesn't see the voltage being as high as it is. And since the PCM is controlling the voltage, I suspect that is your problem right there. NOT a bad alternator or a bad PCM.
Like suggested, corrosion on the connectors for the field control or voltage sampling might be at fault. Bad or corroded grounds might be at fault. They create resistance, which will create a voltage drop or in the case of grounds, a mis-reference for the voltage, the PCM might think its at 14.5VDC when in reality it is holding 16.5VDC.
First confirm your measuring tool is accurate, try another one.
Then I'd try cleaning the connectors with spray electrical cleaner, use a wire brush to remove any built up dirt or corrosion, treat with dielectric grease and reconnect. The would be the control connectors to the alternator and the connectors to the PCM.
Then start checking grounds, disconnect them and clean them and reconnect.
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