Q: crankshaft pulley harmonic dampener fell off after serpentine belt replacement

It could be that the condition of the pulley caused your belt failure in the first place. Deciding that depends on where the balancer separated.The balancer itself consists of a center hub with a pulley ring around it and a bonded rubber ring between the two. Depending on the model of car, additional pulleys may be bolted to the outer ring to accommodate more belts If the balancer separated at the rubber ring which is simply held in by adhesive, the whole thing must be replaced, and may actually have been the cause of your belt failure to begin with. If the pulley separated where it bolts together, I could not help but wonder if the pulley was disassembled and the bolts not properly tightened. Careful inspection of the parts should tell the story. If you try to replace the balancer, you may be able to do it without a gear puller, but removing the bolt in the end of the crankshaft is the big challenge. It is very large and very tight and keeping the engine from turning while you try to loosen the bolt can frustrate the most capable mechanics who don’t have the special tools made for that job. Likewise, if the bolt is not properly torqued when assembled, it could lead to bigger problems than you have now. By contacting Your Mechanic you can have a technician come to your home and take care of this difficult job for you.