I do not remember if I had to do this or not on my 300DT.

If there is not enough "slack" in the tensioner to wrap the belt around the pulleys by hand, I always use the starter motor to do it. Sometimes it goes the first time, other times a few more tries. On the ones that are really stubborn, I help it along with a ramp made out of wood or plastic and the belt rides up the ramp into the groove once I hit the starter motor.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked