The brief
The R300L is a long-wheelbase MPV with both front and rear air-con zones. The owner had the front AC working fine, but the rear vents were blowing warm air, and the rear windows had started fogging on humid mornings. That points specifically at the rear cooling coil.
The rear cooling coil is the cold evaporator that the rear cabin air blows through. Refrigerant flashes to gas inside it and pulls heat out of the air. When it leaks, the rear loses cooling; the moisture it normally drains away outside the car instead finds its way past a worn seal and onto the interior, which fogs the rear glass.
The diagnosis
A pressure check showed the AC system undercharged, refrigerant had been escaping somewhere. A leak test on the rear cooling coil found a small leak at one tube end, and the rear windows were fogging because condensate from the failing coil was routing past a degraded gasket at the box.
There is no recharging your way out of a leaking evaporator, the gas just escapes again. The fix is a new rear core, and with the box open it makes sense to renew the rear expansion valve at the same time rather than refit an old one.
The work
The remaining refrigerant was recovered, the rear cargo trim taken off to open the rear HVAC compartment, and the failed rear cooling coil removed. A new Mercedes-spec coil went in with fresh seals and a new expansion valve, then the system was vacuumed to a deep pull to clear any moisture and recharged with the correct refrigerant volume.
The rear vent temperature and the system pressure were checked before the car went back to the owner.
The outcome
Cold air at the rear vents at idle, no fogging on the rear windows, and the system holding pressure.
The R300L went home with the rear air-con working the way it should. For a long MPV in Singapore's heat, the rear zone is not optional, and replacing the core properly, with a new expansion valve and the system vacuumed and recharged, means it is sealed and cooling again rather than limping along on a top-up that would have leaked back out.