With the cold weather, I see a lot of people here are having glow plug system problems. Take a look at this video.
It shows a starter/ generator ammeter placed on the battery cable negative (breaking the connection to put this ammeter in series is not required) and a glow cycle without starting engine. The black pointer (piece of tape) on the ammeter was placed there when 5 known working glow plugs was installed. Per the ammeter (generator) scale. it is approx 35 amps at the marker. When I experience a hard start condition, I just place the ammeter on the negative terminal, turn the key on to the glow position, and watch the meter. If it reads up to the marker, all is fine. If it reads 28 amps, 1 glow plug is bad, 21 amps 2 glow plugs are bad and so on. If it does not register at all, it is likely the glow plug controller's fault. To check which glow plug is bad, place the ammeter on the wire to each glow plug.
What do you think? Easy?