[rdhzrd]

Most likely your glow plug controller is defective. Everything will appear to work but the solinoid is not switching. Check it out because its not that common for glow plugs to fail in the powerstrokes however they do fail sometime. Check all of them through the valve cover plug in. Hook up your test light in reverse and touch each large pins ( 8 ) on the engine side with the key off,you are just checking for a good circuit,if your light comes on that plug is good etc. The other little pins in the valve cover gasket are for the injectors. If you have a bad glow plug on each side ,change them all because its a lot of work to get to them.