Mercedes-Benz W211 CDI glow plug light stays on or comes on again
Why does the glow plug light on a W211 CDI stay on or come on again after starting?
The most common cause is the glow plug relay, whose internal conductor has burned or whose output stage is faulty. The relay is on the left side of the engine bay behind the wing. After the relay was replaced, the light has stopped coming on.
How is a faulty glow plug found?
The glow plugs are measured with a resistance meter; direct current can short-circuit a glow plug. Cold, the resistance of all glow plugs should be of the same order, about 0.9 ohms. A broken glow plug can also burn out a new relay, so check the glow plugs when replacing the relay.
What else can prevent glowing and starting?
The earth cable between the engine and the body may be oxidised. The lowest fuses in the interior fuse box may be dirty, and the wiring behind them may have loosened. Interruptions in the voltage supply can make other modules fail too, so the fault is not always in the engine control.