The brief
Mr Pan's A-Class had picked up a heavy vibration, and he brought it in to get it sorted. A vibration that builds like that usually means the mounts holding the engine and gearbox have given up. The engine and gearbox sit on rubber mounts that carry their weight and soak up the shake, keeping it out of the cabin. As the rubber ages it sags and cracks, and once it does the drivetrain moves around more than it should: the vibration comes straight through the floor and the wheel, it gets worse under load and at certain revs, and the slack lets it knock against its stops. Mounts that have gone produce exactly that, and left long enough the loose drivetrain stresses other parts, so they get changed as a set.
The diagnosis
On the lift each mount got a pry-test. The engine mount and the gearbox mount had sunk visibly with cracked, separated rubber, the play obvious under load. All the drivetrain mounts were tired at the same time. When they go together like that you do them as a set, fitting one fresh mount next to a sagging one just loads the new one harder, so the call was the full set.
The work
The engine and gearbox were taken onto transmission jacks to carry their weight, and each tired mount came off in turn. New genuine Mercedes-spec mounts went in across the set, every fastener torqued to the manual figures. With everything bolted up, the drivetrain was checked to be sitting square in the bay before the engine was let down to load onto the fresh mounts. A road test confirmed the heavy vibration was gone, smooth at idle and on the move.
The outcome
Smooth at idle, no vibration through the floor or the wheel, no shudder under load, no clunk on shifts or over bumps, and the engine note back to normal. The A-Class went home with the drivetrain held properly again. Engine and gearbox mounts wear as a set and they take the car's refinement down with them, so doing the whole set together reset the lot rather than chasing the next sagging one in a few months.
