Per FoMoCo, if you get cracks in lands on the rubber side of the belt, this is normal. But, if you get chunks missing, replace the belt. Some will tell you that cracks are a sign to replace the belt. Not necessarily true.
It's not how hard you work, it's how much you get done. Simplificate and add lightness
tom is right. if you get cracks on the other side of the belt, not where the v-grooves are, i would change the belt as well. My factory one lasted all the way to 85K as the previous owner never changed it. I would change it though as it's cheap insurance and if that thing fails...u'll be on the side of the road cursing.
Service interval on quality serpentines is 50K. Be careful because there are cheap ones out there that will fail between 10K to 20K.
I just replaced mine. The original was a solid looking belt with a smooth outer surface (but it was starting to crack underneath). The new one is broken up into several bands and looks more like a snow tire, on both sides.
By the time you can actually visually tell that a belt is shot, then it's already been bad for several thousand miles. They should be replaced every 60K. This is particularly true for the newer "gator" and "poly cog" designs which are fine belts but they make it harder to discern cracks and splits, etc. Sure they can last longer, but as elmo said, why run the risk of being stranded?
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