1999 Dodge Intrepid Chargins system Light

I had the charging system light turn on during a trip about a month ago. The light would come on intermittently until I noticed that the wipers would move very slow. I had the alternator checked and it was confirmed that it was bad. I replaced the alternator with a used one and everything seemed to be fine. I am still getting the charging system light coming on every once in awhile. I took it to my mechanic and he checked the charging system and everything looked good. I cleaned the terminals to the B+ post and wire prior to installing the new alternator. When the light comes on, there appears to be no loss of power. I hace tried to test the alternator by turning on everything to make the light come on and it won't. My question is what the field sense connector wore do? I did not clean those terminals prior to putting the new alternator in. Could a bad connection there cause this problem?