My 90 F250 started having a problem with the glow plugs. I've driven Ford IDI's since 1986 so I'm pretty familiar with all their little problems. But this one has got me stumped. Several weeks ago the wait to start light would only stay on about 2 seconds and then go off and you'd hear the relay clicking real fast. Since the truck is near 25 years old and had it's original glow plug controller, I assumed it had finally bit the dust. I replaced it and the same problem continued. I studied the wiring diagram and the only other thing I could find was the temp sensor attached to the controller, so I replaced that too. Still no change. About all that leaves is the plugs themselves so I had the Ford dealer check them when the truck was in for service. 3 of the 8 tested bad. I've had several bad plug before, but the controller timer would still cycle normally. What have I missed? I'm pretty good at sorting out electrical issues but this one doesn't make sense unless the new controller is bad too.