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What is an 'auto belt tensioner'?

Took my 09 civic to the dealer due to a whirr sound from the engine. They checked it out and said it sounds normal and that the ‘auto belt tensioner’ moves. What and how does that part work? Thanks.

The tensioner is a spring loaded (sometimes hydraulic) device that maintains the proper belt deflection (tension) on either a timing belt or an accessory drive belt (water pump, alternator, etc.)

Think of it (very broadly) as a spring on a screen door that keeps it closed. In the case of the hydraulic tensioners those are sealed units with fluid under pressure (same principle as a shock absorber).

It would be extremely unusual to have a tensioner pulley bearing making any noise on an '09 Honda. Always possible, but not likely. The fact the tensioner “moves” does not mean anything; it’s supposed to move a little. To determine if this noise is present in any of the belt driven items they should either remove the belt and physcially rotate the items by hand to determine if something (by a fluke) has a rough spot in it) or listen with a mechanic’s stethoscope to try and pinpoint it.