The brief
The B180 came in with the air-con not doing its job: the cold air had gone weak, there was a musty smell every time it was switched on, the cabin felt damp and humid, and the owner had noticed a wet patch on the floor under the car.
The evaporator core is the part of the air-con buried behind the dashboard. Refrigerant runs through it, the cabin air blows across it and comes out cold, and the moisture that condenses on it is supposed to drain away outside. When the core is tired and the seal around it has perished, you lose cold air and you get damp and smell inside instead of dry air. That's the picture here.
The diagnosis
On the gauges the system was undercharged, so it had been losing refrigerant somewhere. The air-con drain line was checked and it was clear, so the damp in the cabin wasn't a simple blocked drain.
What we found was condensate tracking past a degraded gasket around the evaporator instead of draining away cleanly, and the core itself was past its best. Patching the seal alone wasn't the answer with a core in that state.
The call was to replace the evaporator core and, while the dash is out and everything is open, the expansion valve that feeds it, so the whole cold side goes back together fresh.
The work
This is a big job because of where the part lives. The dashboard was stripped right out to get at the heater and air-con housing deep in the cabin.
Out came the old evaporator core and the old expansion valve, and in went a new Mercedes-spec core and a new expansion valve with fresh seals all round. Then the system was put on a long, deep vacuum to pull every trace of moisture out before being recharged to the correct weight of refrigerant.
The dashboard and trim were rebuilt, and the air-con was run and checked for output temperature and a clean drain.
The outcome
Cold air is back, properly cold, the musty smell is gone, and the cabin stays dry with no more wet patch under the car.
The owner got the air-con fixed at the source rather than topped up to limp on, and because the expansion valve was done at the same time, there's no reason to be back inside that dashboard any time soon. On a Singapore commute, an air-con that actually works is not a small thing.