Glow plugs wearing out?

[GREASE FIRE]

i have Beru glow plugs, they are about 4 years old and i used a 12 volt continuity tester (the kind that lights up) on them. According to the tester, they all work. The cabin light dims when i cycle them, which also makes me think they work - and i have a manual overide button and the lights also dim when i use that.
But, the truck will not start when "cold" (even on warm days). The engine needs to have been at least slightly warmed up, or else the block heater needs to have been plugged in overnight to get it to start.

is there any chance the glow plugs could be "working" but not heating up enough? i always thought the gp's either worked or they did not - but this situation is making me wonder if there could be an "in between" state for them.

does anyone know this?
thanks,
Paul

 

[Diesel JD]

Paul, it's an 89 so you have the solid state system. If they all work, light up with the test light you could then take the time to ohm them out. Usually you can assume all the glow plugs are working if you can get a 7-10 second glow out of them in warmer weather and closer to 15 in very cold weather. They usually either work or don't but I would imagine one with high resistance but not high enough to be "open" would be enough to mess around with the SS controller as it measure the resistance drop in the glow plugs and determines how long they should stay on that way.