I believe it measures the total voltage available, from the battery and what is supplied by the alternator. If your reading 8 volts then either the alternator is not putting anything out or the battery is shot. Try disconnecting the alternator wiring harness and check the guage again. Also, watch the gauge when turning on all electrical accessories.
89' YJ - 4.2 - NV4500 - NP231w/sye - 1" Body Lift - 2 1/2" Rancho Suspension Lift - 31" Firestone MTR - AR Outlaw II - Tomken Bumpers F & R - Warn 8000 front - Warn 6000 rec. mount rear - FlowFit Seats - Tom Woods Rear & Custom 2" front driveshafts