I have changed out my Alternator with a brand new one, and the battery is reading as good. but my battery gauge is reading way off the normal readings. My car starts and run but I cannot figure out what is happening.
could you elaborate on this? what kind of gauge? And what exactly is it reading?
The standard Battery gauge inside the dash. it marks way over the 19 as soon as i turn on the car.
That is probably volts, and 19 volts certainly indicates something is wrong. It reads 19 volts when you turn the key on but before you start the car? That would indicate something wrong with the meter, as there is no way a car battery can read 19 volts with the engine not running.
If it reads 19 volts with the engine running, that is a different issue. Could still be a faulty meter, or possibly a bad battery with very high internal resistance, although I doubt it would start the car if that were the case.
I’d get or borrow a meter and clip it across the battery to verify the voltage. You can get a voltmeter that plugs into your cig lighter socket for about $12 at Amazon. But any garage or auto parts place can check the battery voltage for you.
PS: I once thought a meter reading 50% high was unlikely, but recently I had a cheap DVM do exactly that, suddenly read 9 volts on a 6 volt battery.
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Depending on the year of your vehicle, the computer could be what regulates the voltage from the alternator.
Tester
Don’t rely on the dash gauge, use a voltmeter to verify. If the independent voltmeter reads correctly, then you have a gauge problem-
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It sounds like the voltage regulator is not working so it is dumping the full voltage from the alternator into the battery. The regulator is either internal to the alternator or in the computer as suggested.
You may have some AC ripple voltage causing the trouble. To see if that is the case simply turn the ignition switch to the RUN position without starting the engine. If the meter now reads correctly then your alternator most likely has some bad diodes inside it or the regulator is bad, as others have already suggested. It meter is still showing high voltage then you will most likely have to replace the meter.
I am not sure what year your envoy is, my guess a bad stepper motor in the cluster, Inexpensive diy if you don’t mind pulling the cluster and doing some soldering, If everything checks out ok but the gauge, suspect the gauge 
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If you dash meter shows 19 volts with the engine running, but around 12 volts with the engine not running, this requires immediate attention b/c that high of voltage could damage the car’s electronics, including the engine computer. Not a problem you want. If I had that problem I’d double check with my own volt meter, just very briefly running the engine. If its actually 19 volts I wouldn’t run the engine any more until the problem was fixed.