Mercedes-Benz W204 glow plug relay and glow plug control unit
W204 CDI: the glow light stays on or the engine starts poorly when cold, is the glow plug relay broken?
Possibly. The glow plug relay, or glow plug control unit, can fail, and new glow plugs alone then do not fix the fault. A faulty relay can also burn out a plug. The plugs and the relay can be checked with an ordinary multimeter.
Where is the glow plug relay on the W204 CDI?
On the OM646 engine the relay is low on the left side of the engine, under the intake manifold above the starter motor. It is reached by removing the underbody covers, and the wires running under the air cleaner may have to be moved. On the C320 CDI (V6) the unit is at the end of the cylinder head on the right under the plastic engine cover.
Why can a new glow plug control unit fail straight away?
Wrong or faulty glow plugs can break a new control unit too. That is why suspect plugs should be replaced at the same time. After the correct plugs and a new control unit, the fault and the engine warning light went away.