The serpentine belt
The serpentine belt, also called the drive belt or accessory belt, is a long ribbed rubber belt that snakes around several pulleys at the front of the engine. The crankshaft turns it, and it in turn drives the alternator, which charges the battery. On many engines it also drives the water pump, the air conditioning compressor and, on cars with hydraulic steering, the power steering pump.
Older cars often had several separate V-belts, one for each accessory. Most modern engines use a single serpentine belt with an automatic tensioner that keeps it tight.
Signs of a worn serpentine belt
- Cracks across the ribs, missing chunks, or frayed edges.
- A shiny, glazed surface.
- A squeal at startup or when you turn the wheel or switch on the air conditioning.
- A battery warning light, which can appear if the belt slips or breaks and the alternator stops charging.
If the belt breaks, the engine may overheat quickly when it also drives the water pump, and steering can become very heavy on cars with hydraulic power steering. Pull over safely and turn the engine off.
Replacing a serpentine belt is within reach of some owners. It needs the correct belt length and rib count, a diagram of the belt route (often on a sticker under the hood), and a tool to release the tensioner. Many people have a shop do it, often along with the tensioner and idler pulleys if they are worn.
Timing belts and timing chains
Inside the engine, the crankshaft turns as the pistons move up and down, and the camshafts open and close the valves. The timing belt or chain links them so the valves open at exactly the right moment. Some engines use a toothed rubber timing belt. Others use a metal timing chain, similar to a bicycle chain.
| Timing belt | Timing chain | |
|---|---|---|
| Material | Toothed rubber with internal cords | Metal links |
| Where it runs | Behind a cover at the front of the engine | Inside the engine, bathed in oil |
| Replacement | At an interval set by the vehicle maker | Usually meant to last much longer; replaced if worn or noisy |
| Who replaces it | A shop | A shop |
Why the timing belt schedule matters
Many engines are built so the valves and pistons share space at different moments. These are called interference engines. If the timing belt breaks while the engine runs, the pistons can strike the open valves, which can cause severe engine damage. A timing belt usually gives little warning before it fails.
That is why the replacement interval in the maintenance schedule matters so much. The manual or a dealer service department can tell you whether your engine has a belt or a chain, and when a belt is due. If you bought the car used and have no record of a timing belt change, ask a mechanic to advise you.
Timing belt jobs are complex. They often involve removing other parts and lining up timing marks precisely, and shops commonly replace the water pump and tensioner at the same time when they are driven by the belt. This is a job for a qualified mechanic.
Your owner's manual lists the belt service items for your engine. Our DIY or shop page has more on which belt jobs suit owners.