i have a 2003 monte carlo ss with the 3.8. the voltage gauge is wanting to go from 8 volts to 18 volts. i took the postive post off the battery while running and the car did not die. also replaced with a new alternator and i'm still having the same problem. now, the voltage gauge does register correctly on occasion. but i find this problem keeping my car from starting, dash lights are lit up and several guages are not functioning as well! anyone with any suggestions here. seems this problem started all of a sudden, i have even checked all of my fuses on the side of dash as well as under the hood. thanks; gary

Surprised you haven't gotten a MIL light (Check Engine) for under- and over-voltage. Too much over voltage and every light bulb will burn out eventually and you're not doing the electrical system any good disconnecting the battery while running, PCM's don't like it.

Also, the same reason the gauges act strange, too much varying voltage. You might disconnect the battery and clean the posts and the cable ends. Make sure both battery cables are in good shape, no broken strands, no corrosion at both ends. This problem happen overnight, done some work to the car, or progressively gotten worst?

You should observe the battery charging voltage, the same voltage throughout the electrical system to see the variance with a DMM. If it reads normal, 14- 14.5 VDC, temporary mount the DMM inside the car interior so you can keep an eye on it.

Definitely a regulator problem and its inside the alternator if all power distribution connections are made with no corrsion and same goes for the return side.