`94 6.5 turbo diesel - glow plugs cycle off too fast.

94 6.5 diesel 4x4 : we installed new glow plug relay not long ago.you can hear it click on & off so i know the relay itsself is working, but is there a timer on this thing that tells it how long to stay on? very seldom does the glow plug light stay on for 5 seconds. usually only about a half second then its off, or just a flash. this is not long enough to make it start up. you have to turn the key off & on hundreds of times & once or twice it might stay on for 5 seconds then it may start.
i read on a post about a ford diesel, that a member claimed if you have a bad glowplug, the more bad glow plugs you have, the shorter the cycle time. is this true, & is it true for GM diesels?
any informed help is appreciated!