A cracked balancer would not be noticed unless you looked at it. IT WOULD NOT cause noise or lack of belt tention. It would have to come apart to do that. If it is cracked it will probably come apart soon and reek any manner of havoc, but it has to come apart to do that.

The tentioner is a drum on a bearing the belt rides over. It also is spring tentioned into the belt. It does come apart and make a bunch of noise and the belt just hangs there.

I wasn't there so I can't tell you what happened. The story you heard is entirely plausible. Without any evidence to the contrary, I'd believe them and go on with your life.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician