Mercedes-Benz W210 glow plugs and glow relay
Why does the W210 glow light come on, go out and come on again?
The cause can be the glow relay fuse, which looks intact but passes current poorly, or a glow plug that is faulty only when hot even though its resistance is fine when cold. Also check that the plug of the thin control wire on the glow relay is fully seated.
Why does the W210 glow light not come on and the car does not start?
The fault is often in the K40 relay box: its connectors are oxidised or the solder joints have failed. Measure the voltage on the box's wires and check all connectors. If the light does not come on at all, also check the control unit's power supply and the fault codes.
How do you measure the W210 glow plugs and glow relay wiring?
Measure with a multimeter. Start from the relay box connector, so the measurement also covers the wiring. The glow plugs can be removed and tested separately if needed. If one plug is dead, remember that in warm weather the engine can still start.