My car has a persistent plug fault light, please advise

Hi Paul, I have a Mercedes Benz ML 320CDI 4matic. The car has a persistent glow plug fault on the dash board yet the glow plugs have been replaced twice after a short period. Please guide me on how this system works and what could be the problem. Isaac Musoke

Hello Musoke, your W164 Mercedes ML 320 CDi glow plug system fault requires a computer diagnostic check and investigation to check whether there is a faulty circuit or output stage problem with the control unit.
This will rule out the possibility that they fitted incorrect glow plugs or the control unit which regulates and monitors current to the glow plugs is faulty.

Glow plugs are devices which are heated via an electric circuit to increase temperature in a diesel engine cylinder’s combustion chamber prior to a cold start.
Diesel engines use compression ignition where highly compressed diesel air mixture produces enough heat to ignite itself. At cold temperatures the air diesel mixture will not ignite easily.

To ensure combustion when the engine is cold glow plugs are fitted to each cylinder to act as ignition aids.
Modern faster glow plugs like the ones in your Mercedes preheat over 3 seconds while those in older cars may take 15 seconds.
The glow time control units or relays switch the current of the glow plugs on and off while controlling timing.

Regulation of the glow plugs by the control unit is important because the amount of current needed is higher when they start heating and varies between four, six and eight cylinder engines.
Glow plugs operate in the preheating and standby phases depending on the type of engine and ambient temperature measured by sensors.

Preheating is confirmed by the lamp on your dashboard which is switched off during standby phase (light on dash stays on permanently if the glow plug system is faulty).
More modern diesel engines like yours have the post heating phase useful for maintaining exhaust gas standards and optimum combustion.

Your Mercedes also has a fully electric glow time relay system which are control units and communicate with the engine control unit and on board diagnostics.
These control units power the glow plugs and measure the current flowing through them. Here they promptly detect a short circuit in the harness or glow plugs themselves if they are burnt out.

When these control units fail, the OBD system will switch on the glow plug light permanently or delay to switch it off when you start the car.