I've got a 1 year old, less then 1500 miles on Powermaster 140amp alternator, new Ultima Yellow top battery. Std. pulley, not undriven.

What happens is when I start the car the gauge will read normal - just over 14. I'll drive a couple miles and the gauge will start to creep up. Then on acceleration it will peg at 18, which ends up shutting my amp off. At a stop the gauge will come back down to 14 range, and then do the same when I leave.

I thought maybe a bad ground at the engine, but I added a ground strap, and it still does it.

Any ideas? Just a bad alternator?

 

Yes, it sounds like the regulator in the alternator is fried (a shorted transistor).
No matter how the wiring is, the alternator should never be able to give too much voltage. (Exept that a loose connection to the battery may damage it.)
The amplifier shutting off indicates that your guage is not looney.
You better be careful not to fry everything electr(on)ic in your car though.. Keep the revs down until you get this fixed, it's bad for the battery too..

 

check the fusible link in the alternator feedback wire

 

From what you had written there has been a time that the alternator worked properly. If that is the case, see what type of warranty you have coverage is applicable to that unit. If under warranty remove unit and through replacement process. In complete agreement with others that you can do SIGNIFICANT damage to your electrical system, if some damage has not already occurred. Electronic components in your car do not have great voltage plus/minus tolerances. That is most obvious if your alternator breaks and you drive under battery reserve power - watch as your electronic systems such as ABS shut down. Personally I would not even drive you car until this situation is fixed. Even if unit is not under warranty, you must remove and replace/repair unit as soon as possible.