Charging voltage? 89 Calai

[cujotom]

Hi All,
On my 89 calai with the quad 4 motor what should the approx charging voltage be with a full load? The heater blower motor, headlights, wipers, radio, etc are all running? It seems strange when the blower motor runs the voltage drops into the yellow range on the voltage gauge (below 13 volts). Or when the turn signal is on the gauge tic tocks with the signal. The needle never goes into the red although, and the lights do dim at night when you put the headlights on. Is it time for an alternator or am I being quick on the draw?