The brief
The A3 was squealing on every cold start. A high-pitched chirp from the front of the engine the moment it fired up.
It would quiet down once the engine warmed through, but then come back whenever a big load came on, like switching the aircon on or when the alternator was working hard to recharge the battery.
That pattern is the classic signature of an ageing auxiliary belt. When the belt is cold, or when a load suddenly demands more grip than a worn belt can provide, it slips against the pulleys, and slipping rubber squeals. The fact that it changed with temperature and with load, rather than being constant, is what told us it was the belt rather than a seized pulley or a failing bearing.
The diagnosis
We took the belt off the engine for a proper look.
The ribbed face was glazed, with a shiny hard surface where it should have a slight matte grip, and there were surface cracks across the ribs. A belt in that condition has lost the friction it relies on, which is why it was slipping.
We tested the tensioner too. Its spring had gone weak, which means it was no longer pressing the belt against the pulleys with the force it should, making the slipping worse. And the idler pulley, while it still spun smoothly, was on the same age clock as everything else.
The right call on an accessory belt drive is to do the whole set: belt, tensioner, idler pulley. Replacing just the belt onto a weak tensioner gets you a fresh belt that still slips, and you are back in soon enough.
The work
Released the belt tension at the tensioner, then removed the old belt and the failed tensioner.
In went new VAG-spec parts: a new tensioner, a new idler pulley, and a fresh belt routed correctly through the pulley path. Belt routing matters, since a belt run the wrong way around an idler wears out fast and can throw itself off.
We re-tensioned to the spec figure, not by feel. Too loose and it slips; too tight and it overloads the bearings in the alternator and the other accessories.
Then a warm-up cycle with the major loads applied, aircon on, electrical load up, to confirm the belt ran silently across the rev range under real demand.
The outcome
No squeal on cold start. No slip under load, with the aircon and the alternator both pulling hard. A silent belt drive across the whole rev range.
For the owner, the immediate win is the noise gone: no more chirp announcing the car to the whole carpark on a cold morning. The longer-term win is reliability. A worn accessory belt that finally lets go can take out the charging and the cooling in one go and strand the car. Doing the full set, belt, tensioner, idler, while the symptoms were still just a squeal means that whole failure mode is off the table for a long time.
