Mercedes-Benz M256 (2.5 and 3.0)

Mercedes' first petrol straight-six since the M104, and the engine that replaced the M276 V6. 230 to 320 kW, and mechanically the most unusual thing in this catalogue.

It has no drive belts. A 48-volt integrated starter-generator sits between the engine and the gearbox and runs the water pump and the air conditioning electrically, which is why there is no accessory belt to inspect, tension or replace — and why a diagnosis that begins "check the belt" does not apply here at all. The same unit adds up to 16 kW and 250 N·m of its own.

Beside the turbocharger there is an electric supercharger, a BorgWarner unit spinning to 70,000 rpm off the 48-volt system, whose whole job is to fill the moment before the turbo has exhaust to work with. A hesitation off idle on this engine is therefore an electrical symptom as readily as a mechanical one.

Anything on this engine can involve the 48-volt system, and 48 volts is not the 12 volts anyone is used to. It is below the threshold that makes high-voltage training mandatory and well above the level at which a casual short does damage. Its published fault record is still short — the engine is recent — and this page will say so until that changes rather than borrow the V6's problems.

⚑ Two sizes, both with the 48-volt system. Our rows carry 2.5 and 3.0 litres across five models. The integrated starter-generator and the electric compressor are what a reader is usually asking about, and they are common to both.