Truck Repair: 7.3 diesel starting problem, test light, glow plugs

Question
I did not start my truck for about 6 weeks now it turns over fast enough but will not start.this is 1992 f-25o 7.3 non turbo

Answer
Check your glow plug relay.  It is located to the front left of the fuel filter housing (Or close).  There is two main terminals one is battery + and the other goes to the glow plugs.  Using a test light, have someone turn the key to ON (the wait to start light should come on in the dash)  and test the terminal that did not have battery +.  If your test light comes on, then the relay is good, if not then the relay needs replaced (very common prob)  Let me know if you need further help.