The brief
Mr Bala's 318i had a problem with the rear driver-side window, it would wind down and then refuse to come back up. He brought it in. A window stuck down isn't just annoying, it's the car open to the weather and to anyone walking past, so it needed sorting properly. When a powered window winds down but won't come up, the regulator inside the door is the usual cause. The regulator is the mechanism that the motor drives to raise and lower the glass, and on these it's built with plastic guides and a cable, and over years of use the plastic cracks or the cable frays and jumps its track. Once that happens the glass drops and there's nothing left to lift it. A broken regulator doesn't go back together, so it gets replaced as a unit.
The diagnosis
The door card came off for a look inside. The window regulator had failed, a plastic guide cracked and the lift cable off its track, which is exactly why the glass dropped and stayed down. The window motor itself still ran, and the switch and wiring were fine. That's a regulator replacement, the whole assembly, rather than trying to patch cracked plastic that's only going to let go again.
The work
The door was opened up, the glass supported and freed, and the broken regulator removed. A new genuine BMW-spec window regulator went in, the motor transferred across and checked to be running smooth, and the glass refitted and set into its runs so it tracks square. The door's vapour barrier and card were put back properly so there are no rattles or leaks. A test of the switch confirmed the window winds up and down smoothly, seats fully top and bottom, and the one-touch and anti-pinch work as they should.
The outcome
The rear window winding up and down smoothly, sealing fully at the top, no grinding, the one-touch working, and the door back together tight. The 318i went home with the window sorted. A failed regulator leaves the car open to the weather, so replacing the whole assembly fixed it properly with no cracked plastic waiting to fail again.
