This is true, they are one unit.But if you bought a rebuilt altenator, especially from O'reilly's, it may have to be replaced again.My suggestion to you is to have it tested to find out it's condition. If it is bad, either buy a brand new unit, or have the altenator rebuilt by a specialist in your area.I have found that having an altenator shop rebuild your altenator is probably better than buying a new one. I'm suprised those folks haven't repaired themselves out of business.Most rebuilt altenators at auto parts retailers are rebuilt buy folks who aren't that experienced and sometimes can't even spell altenator.Get your altenator diagnosed and I hope this info helps.