get a belt holding tool from harbor freight its a bar with an adjustable foot to engage the pulley from behind, that way it presses the belt into the pulley and you undo the bolt, I have an E300D hence no turbo and dont have the sheetmetal partition that the air intake boot goes into, so I use waterpump pliers with an old rough rag to hold the belt and pulley, works a treat.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)