The brief
The S400L is a big car with a lot of engine to keep cool, so a small leak shows up fast. The owner had been topping the coolant up roughly every week, the temperature gauge had started creeping up in slow Singapore traffic, and there was a clear puddle of coolant under the front of the car after it had been parked. He brought it in before it boiled over rather than after.
The radiator does the actual cooling, passing the hot coolant through a fine core while air flows over it. The end tanks are plastic and the core is aluminium, and after years of heating and cooling the seam between them can crack. Once that happens the system can't hold a full charge, and the engine starts running hotter than it should.
The diagnosis
We put the cooling system under pressure and watched where it lost it. The leak came from a hairline crack along the seam of the radiator's plastic end tank, with the tell-tale crusty residue built up around it. The hoses, the water pump and the expansion tank all held and checked clean.
With the leak isolated to the radiator, and a cracked end-tank seam being something you replace rather than patch, the diagnosis was clear: the radiator had to come out.
The work
The system was drained, the radiator hoses came off, and the cracked unit was lifted out of the front of the car. A new Mercedes-spec radiator went in, with the hoses reseated on fresh clamps rather than the tired old ones.
Then it was refilled with the correct coolant, bled properly so no air was left to cause hot spots, and held under pressure to confirm the new radiator and every joint were sealed.
A run afterwards, including a stretch of slow traffic, confirmed the gauge stayed put and nothing was leaking underneath.
The outcome
No more puddle, the coolant level holds without weekly top-ups, and the gauge sits steady even crawling through traffic.
The S400L went home with the cooling system back to spec. On a car this size, catching the radiator while it's still just a leak is the cheap version of the story. Letting it run hot is how you end up talking about a head gasket instead.