Cliff, the tensioner is probably shot and needs to be replaced. If you failed to loosen the center bolt of the tensioner before turning the adjuster nut/rod you inadvertently trashed the tensioner. Take note that the adjuster rod can be installed in the correct orientation or 180 degrees opposite ... check the orientation on your 300E. Also, when reassembling the tensioner mechanism be sure the pointer is positioned properly. Leave the center bolt loose a half-a-turn and turn the adjuster nut clockwise until the pointer moves to the tightened position. Then tighten the center bolt. The tensioner shock absorber dampens shocks to the drivebelt tensioner resulting from sudden changes in the engine RPM's ... sudden downshifts/upshifts of the transmission, A/C compressor switching on, etc.

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Fred Hoelzle