Is there a test to tell if it's the alternator or the voltage regulator failing. Right now it's an intermint problem. Normally the needle hovers right at the upper edge of the center mark and everything works fine. A few time now it goes just to the bottom of the center mark. When that happens the wipers are a little slower, blower fan a little slower. Both the alternator and voltage regulator are the original at 36 years old, so should I just replace them both? Any thoughts?