Serpentine belt

[James Morse]

How do I know if it's bad, if it's not obviously broken or torn? Should it be replaced time to time?

 

[dvdswan]

Dry and cracking, broken ribs.

 

[Curious Hound]

It's a judgement call. 100k miles would be a good time to replace it. I normally keep the old one under the seat as a spare. Some cracking of the ribs is normal as they age. But, eventually, chunks will start to break off. You don't want to wait that long.

The ribs help it grip on the pulleys. But there are nylon cords in the flat side that give it strength. If the ribs get glazed over and real shiny, it will slip and not work efficiently. That's a good time to replace it also. If the flat side is shiny, don't worry about that. The flat side doesn't grip any pulleys that do work. It just contacts the tensioner pulley and idler pulley.