If you tightened the tensioner after replacing the belt, you most likely have over tensioned the belt. When that happens, one of the other bearings fail - most likely the alternator bearing. I destroyed two before I realized that tightening a failed tensioner was causing it. Best to just replace the tensioner. Check the service manual under belt tensioner replacement and it gives instructions to check if a tensioner is good or bad.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold