Alternator/Water Pump/AC Compressor serpentine belt. 6PK-1388 size. Sometimes known as a serpentine belt, power steering belt, alternator belt, or A/C belt, the accessory drive belt, simply put, is a belt that drives the accessories bolted onto the outside of your engine. These are often critical to the running condition of your vehicle, so it is important to keep an eye on the condition of your belts and replace them when they show signs of cracking or delamination. If you wait for your belt to break, the alternator will stop charging, the power steering will fail, as well as the vital water pump in many cases, leaving you on the side of the road. Occasionally multiple belts will be offered for your car depending on accessories installed, so be aware shorter belts are often for cars without A/C.