I have been having a problem with my charging system. The meter will only show 12 volts when I start it, then when my electric fan kicks on it goes to 9 volts. The last time it happened I took the alternator out and put in another one and it did the same thing. I had the first alternator tested at napa and it was fine. I put it back in and the meter was fine, 14 volts. That lasted about 2 weeks and now it is back to the same problem. Any one have an idea what it could be?