The following applies to the DW10 (2.0HDi) although others such as the 1.4 and 1.6 will be similar.
In my experience, its difficult to see the 0.5 second illumination, usually the supply to the circuit has been terminated before the bulb has come on possibly due to the slight delay caused by multiplexing, so you will only actually see it in very cold conditions.
As soon as ignition is switched on: The engine management control unit supplies the glow plugs and controls the lighting of the warning light on the instrument panel, depending on the coolant temperature.
NOTE : The warning light on the instrument panel only comes on during the pre-heating phase.
Coolant Temperature ... Approx Pre-Heating Time
-30°C ... 20 seconds
-10°C ... 5 seconds
0°C ... 0.5 seconds
+18°C ... 0 seconds (inoperative, in effect)
If the starter is not operated, when the warning light goes out, the glow plugs remain supplied for a maximum of 10 seconds.
During the starting phase, the glow plugs are supplied under the following conditions :
* The coolant temperature is below 20 degrees C
* The engine runs at more than 70 rpm for 0.2 second(s)
Post-heating consists of prolonging the operation of the glow plugs for a maximum of 60 seconds from the end of the starting phase .
Parameters which will interrupt post-heating :
* Coolant temperature greater than 20 degrees C
* Flow injected greater than 35 mm3
* Engine speed above 2000 rpm