Not an expert but have read about this
Seeing as I am not only new to the forum but also to diesels, but have done some reading on this. Where is it you live first off? The glow plug timing relay takes information from the coolant temp sensor of the engine that determines how long the glow cycle should last. Let's say the engine is warm to hot, it simply glows the light for a second or so telling you that the engine has suffient heat to cause combustion explosions to take place. If the engine is cold as well as outside ambient temps. and you are getting the same results, a glow light for only a second or so would indicate to me that the coolant temp sensor is relaying the wrong info back to the glow timer. Opening myself up to be beaten down by the techs, hell live dangerously. I would also check cyl. 1 glow plug to ensure it's not shorting out. It could be telling the timer that all glow plugs are hot and ready to start when in fact the others aren't even hot yet.
Just my thoughts on how the system works.