My alternator is overcharging.?

So I have a 96 Plymouth Voyager with a 3.3L. My alternator worked just fine last week when I took the engine apart to change the rear three spark plugs (requiring the removal of the intake plenum and the wires to the alternator). When I reassembled it yesterday, the battery dash light came on, and so I checked the voltage across the battery and it was putting out more than 17 volts. So I pulled the alternator back out and checked the pins and connections which were all fine. I then took it to the local auto parts store and had it bench tested, which they said it passed with flying colors. I took it home and reinstalled it and it was still over-charging. When I asked what could cause overcharging, the guys at the auto store said the only thing that could cause that was a faulty regulator, but when I asked they said that alternator was internally regulated and it had just passed the tests.

So I ask all of you... what could possibly be causing my alternator to overcharge so radically?

Thanks in advance for your answers.

Mort

P.S. the one upside is that my electric windows really move fast.