Preheater relay for 300D, early version and other vehicles with OM603 engine
The preheater relay is essential for starting the engine, without a properly functioning preheater the engine cannot start. The relays can have both electrical and mechanical defects, e.g. if the relay coil contacts break off.
All diesels are available in early versions with a strip fuse and later versions with an electronic self-adjusting fuse. If you are not sure which version of the preheater relay is installed in your vehicle, remove the protective cap of the preheater relay: there you will see the preheater fuse.
If the preheating fuse is OK and the engine still does not preheat, the relay is defective or all the glow plugs are defective at the same time.