my voltage guage on my 96 gt is jumping all over the place, it rises then is stable then drops its very irratic, when it dips down it makes all the lights dim and the idle drops also, any ideas guys
The gauge is simply telling you that there's a problem somewhere in the charging system. It doesn't "make" all the lights dim, the lights are dimming -- and the gauge is dropping/jumping -- because the alternator output is unstable.
Check your battery cable connections. Make sure they're not loose or corroded. Chances are good though that the alternator is on the way out.
Some sets of underdrive pullies can really tax your electrical system when the car is idling or stationary, causing your lights to dim and your voltage to become unstable.
Honestly from the symptoms you describe, I think if anything is needing replacing it will be the alternator due to a blown diode. But before you rush out and spend your hard earned money to replace something, take the following steps to minimize the possibilities.
Check your battery terminals and all the major ground straps, they should be clean. If not, take care of that. If the problem continues, check to see the alternator is placing around 14.5 volts to the system when the car is running. To do this just use a voltmeter accross the battery terminals. When the car is off, and no electrical loads are running, the voltage should stay between 11.8 and 13.1 volts.
If your contacts are all clean and the problem is still there, you either have a bad alternator, a bad battery, or an intermittent but serious ground. Ask yourself, when was the battery last replaced (typically every 5-8 years), is your alternator making any odd noises, have you made ANY modifications to the electrical system (audio/video equipment, power hydraulics, etc), or has your vehicle sustained any damage. Answering these questions for yourself will probably point you in the right direction. If nothing comes of these actions, it's time to seek the assistance of a professional.
I hope this helps, keep us updated so we can give any additional help if more information becomes available. Good luck.
Either you have a bad voltage regulator in your alternator or a blown diode or more. If you do a battery test, your voltage with no load should be at least 12.2vdc (50%) or more. 12.6vdc is 100% charged. If it is at 11.9 or less, it's toast.
Charging voltage should be between 13.5 to 15.0vdc. Anymore or any less is not good.
Checking for bad diodes, do an AC Ripple test. + to generator battery output. - to mounting bolt for ground. Run engine, add load, headlights. Test AC volts and run engine at 2000 rpm. Good test is less than 0.4 vac.