The idler pulley is a plastic pulley that puts tension on the belt. It is up towards the top, look for a pic on Fastlane. The shock works with the spring to take up the slack in the belt, it is a self adjusting system. Is the belt wearing funny? Is it tight?

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2006 CL500
1969 280SE
2023 Ram 1500
2026 Genesis GV70
2007 Tiara 3200