My 82 GMC Jimmy began showing discharge whenever heater fan is on high. also begins to show discharge when lights are turned on. Now it does not do this all the time and will sometimes fluctuate from discharge back to charge. replaced the alternator and tested with my multimeter and it shows it charging at 14-14.5 volts until i turn on either the lights or the heater with fan on high or both on and read 11.5-12 volts. guage in dash shows discharge at this time . Any ideas would be appreciated.