Offroad-Express

Does one glow plug being stuffed prevent them all from glowing? I have pulled all four from a 94 300 tdi Discovery and only 3 glowed. Before pulling them, I had no power at the glow plug rail (wire). Although, with the low plugs pulled, if I put a multimeter to the 12V supply that goes to the plugs and to the -ve on the battery and it does read +-12V with the key in the on position (and the glow light on). So would the stuffed glow plug have been earthing out the connection or something? The stuffed on has no resistance reading on it.
Have tested them by connecting a battery via jumper cables directly to the plug.

Should add that vehicle hasn't been started for 4.5 years.

if you checked the voltage without the glow plugs connected, it could point to bad contacts on the glow plug relay, or bad connection some where causing a large voltage drop. The glow plug draw a huge amount of power 10-15Amps each for a short time, any bad connections result in the voltage on the glow plugs dropping to a low level and they are ineffective.