1999 DeVille I have gone through 3 alternators in less than a year and still on the blink. What is causing this problem? Thanks Help.

 

I had same problem but it was due to the fact I was purchasing rebuilt units .... rebuilt in China ! One didn't even work out-of-the-box. Of course the parts store provided a life time warranty so that was the "hook" in the situation.

Next time I'll buy a used/working one from a junk yard or have mine rebuilt by the local electric shop. Outside of a bad shaft bearing/sleeve, the internal regulator is usually the part that fails.