Why is the dash gauge only reading 8 volts instead 12 volts?

earlefelkins.effe  that osund slike you have a bad alternator. Try places like Autozone and others that offer a free scan. Let us know what it shows and what your alternator puts out. Or you can check it with a voltmeter. Set meter to 50V DC. Connect black probe to ground and hold red probe to B+ (usually where the heavy thick cable connects to on your alternator. Now, be very carefulsince you do have to run the engine. Do not stand in the way of the belt etc. Stand aside. You should get around 14~14.5 V on that.