replaced my glow plugs about two weeks ago, and the truck would start with no issues at all. now they have stopped working again, i'd say i have two still working. the voltage going to the plugs is battery voltage (bout 13V or so) someone said it could be the dropper resistor but the plugs say 12V on them so should be fine right? When starting the plugs stay on for the same time as they have always stayed on about 5 seconds or so. I was having trouble with the plug control box a while ago but that was the plugs and light not coming on, i'd just have to pull the box out and plug it in and it would work again. I have also just changed the head over and it was about 1500 miles (1 week) sfter the head change the first set went wrong.

any help would be gratefully received

is the relay sticking on?

I'm pretty much certain the relay is stuck on, because that is the only thing that can burn out glow plugs that fast I've ever encountered.

the relay between the big fuse and the control box thingy? i'll check again but the light goes out on the dash. that would mean the plugs are on all the while when driving?

what colour are the glow plugs when you pull them out?? it could be that the engine is burning a lot of oil which is caking up the plugs

Get someone to turn the ignition on while you check the power at the plugs, The power should go after about 30 seconds after the light goes out. If not the timer or relay has failed

And yes, when the timer relay fails it can leave the plugs on, whilst the dashboard light still works correctly.

correct me if i'm wrong but i think the plugs can remain on for quite a while (a few minutes) after the light goes off - the light is a "wait for me" warning but does not indicate whether the plugs are off, the ecu may choose to keep them on for a while.

apparrently you can check how many plugs are working by the current in the cable - after an initial peak it settles to 8 amps for each functioning plug, should be 32A.