Don't wait till it breaks, go ahead and replace it anytime after 200k miles. I replaced the one in my DD today and there was a substantial difference between the old, original one and the new one in regards to tension. I had noticed that anytime after about 3000 rpm (acceleration) I would get some belt squeal. I did not find anything that seemed to be causing any additional drag on the system and after reading about these springs breaking, went ahead and replaced it. I'll know tomorrow if it got rid of the squeal.

P/N is 606-993-03-10

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Jim