Glow plug problem

[jim49]

Hi there,I'm new to this but I'll give it a shot. I've got a 96 f450 with a 7.3. Glow plugs were replaced and working great and have gradually got worse. I tested resistance and all are at .8 ohms. The relay is shot which I thought was the problem, so I took both the large leads off the relay and tried holding them together for 15-20 seconds then cranking, but no start. The batteries show 13 volts on my meter when fully charged, and there is 13 volts at the large wire on the relay. If I unplug the plugs @ the valve covers, there is good voltage at each of the outer pins leading to the glow plugs. I don't know what to do from here.
Hoping someone can help me!
Jim