It sounds like the voltage regulator section of your alternator is bad or well on the way. That is what controls the output voltage of your alternator. Also, the rectifier diodes might need to be replaced if they are a separate component and not integrated into the alternator. Any good garage or even a reputable tire center can usually tell by connecting a nifty little gizmo to your battery that checks your alternator output,
Also, if you get a new alternator, be ready for a new battery as well, 'cause 17 volts will fry most lead-acid types.
Solar Tech Energy Systems, Inc.
Victron Solar Components and Inverters, Zamp Solar Panels, LiFeBlue and Battle Born Lithium Batteries, Lifeline AGM Batteries