What’s the difference between a tensioner and an idler pulley?

The main difference between a drive belt tensioner and an idler pulley is its function. Both feature pulleys around which the serpentine drive belt snakes its way in order to power up engine accessories. But, where the tensioner pulley serves to maintain tension on the belt to keep it taught, the idler pulley serves to route the belt and keep it in proper alignment. The idler pulley might run along the flat side or the grooved side of the belt.

What happens if a drive belt idler pulley goes bad?

A loose or seized drive belt idler pulley can cause your drive belt to break or come off of the engine. When that happens, you will likely lose important functions of your vehicle. For instance, if you are driving, you could lose power steering assist, something that can be dangerous if you happen to be turning through an intersection at the time. Also, your alternator would no longer be operational, so your engine would eventually stall out for lack of electrical power. And your battery would die. Because the drive belt powers the water pump on some engines, dangerous overheating could be another result of a failed drive belt idler pulley.

Can you drive without an idler pulley?

If a drive belt idler pulley is going bad, the resulting damage - namely a broken or thrown belt - might not be immediate. But they might be. If you suspect that the idler pulley is going bad, or any other drive belt component, such as the belt tensioner, is failing, make sure to have it inspected and replaced as soon as possible to avoid a potentially dangerous loss of vehicle function.