The brief
The A 45 AMG had an unusual squeaking noise coming from under the bonnet, one of the common signs of a failing fan belt tensioner. On a performance car like this it's the kind of thing you want sorted before it lets go. He brought it in. The drive belt, the long serpentine belt at the front of the engine, runs the alternator, the power steering and the air conditioning compressor off the crankshaft, over a tensioner and idler pulleys. The tensioner holds the belt at the right tension with a spring, and over the miles it loses that spring and its pulley bearing gets rough, so the belt slips and squeals. The belt itself hardens and glazes too. A proper job does the belt plus the tensioner and the idlers as a set, because a fresh belt on a tired tensioner soon squeals again.
The diagnosis
A check confirmed it, the tensioner had lost some of its spring with a rough pulley bearing, and the belt was glazed and cracked from its miles, slipping under load, which is the squeal. The idler was on its way too. So it was a full drive belt set: the belt, the tensioner and the idler pulleys together, not a belt alone that would soon be chirping on the old hardware.
The work
The old drive belt was removed, then the tensioner and idler pulleys, and a new genuine Mercedes-spec belt fitted along with a new tensioner and idlers, the routing checked against the diagram and the belt seated properly across every pulley. With it all back together the engine was run to confirm the belt tracked true and ran quiet. A road test confirmed no squeal, no chirp, and everything the belt drives working as it should.
The outcome
A fresh drive belt running quiet and true, a new tensioner holding it properly, no squeal, and the accessories all driven cleanly. The A 45 AMG went home with the belt and tensioner sorted. A failing tensioner lets the belt slip and squeal, and a slipping belt that gives up can damage other parts, so changing the set put the running quiet and headed that off.
