The crankshaft pulley turns the serpentine belt. That belt then powers multiple engine accessories by transferring rotation from the crankshaft to each driven pulley.
In most modern cars, a single serpentine belt loops around the crankshaft pulley and a series of accessory pulleys (alternator, power steering pump, water pump, A/C compressor), plus tensioner and idler pulleys that guide and maintain tension. If any of these components fail, belt failure can affect charging, steering, cooling, and climate control.
How the serpentine belt drives engine accessories
When the engine runs, the crankshaft turns the crankshaft pulley, and the belt transmits that rotation to the various accessory pulleys. This arrangement keeps several systems working from a single mechanical drive, promoting efficiency and a simplified design compared with multiple belts.
Pulleys turned by the serpentine belt
Before listing the pulleys, note that the belt is driven by the crankshaft pulley; the following pulleys are turned by the belt as it loops around them. There are also tensioners and idler pulleys that guide the belt but do not generate power.
- Alternator pulley — powers the electrical system and charges the battery.
- Power steering pump pulley — provides hydraulic assist for steering.
- Water pump pulley — circulates coolant through the engine for temperature control (in engines where the water pump is belt-driven).
- A/C compressor pulley — drives the air conditioning compressor to cool the cabin.
- Idler pulleys — guide the belt's path and help maintain proper tension (not power sources).
In short, the crankshaft pulley is the drive pulley that turns the serpentine belt, while the other pulleys are driven by the belt to perform their respective functions. Tensioners help keep the belt tight to prevent slippage.
Maintenance and signs of wear
Regular inspection of the belt, tensioner, and pulleys is important. Look for cracks, fraying, glazing, or missing ribs on the belt, as well as unusual noises or belt tracking issues. A failed belt can disable several systems at once, making timely maintenance essential.
Summary
The engine’s crankshaft pulley is the source of the serpentine belt’s rotation. The belt then drives multiple accessories—such as the alternator, power steering pump, water pump, and A/C compressor—via individual pulleys, with tensioners and idler pulleys ensuring proper tension and routing.