no ignition in run, just while cranking

hi guys (and galls) my name is steve and i have a '78 f150 (351m, 4speed, 4x4) and i'm having a problem with the ignition. duraspark (not d.s.2)
it wants to run while cranking it over, however as soon as it catches and i release the key to the run position it stops wanting to run.
the ignition switch is fine (i have a coupe of them lying around) and that is about as far as i have gone so far. would the module be causing this or is there a resistor (or diode, whatever) in the wiring that may be bad?
every so often it will start and run great but as soon as i turn it off it won't start again---only periodicly, except for the firing while cranking.
so, would (hopefully) an ignition module do it for me? or is there something i'm missing?
this truck has alot of sentimental value and i'd hate to junk it for an easy fix.
PLEASE HELP ME!!

Check that the red wire going into the mod has 12v with the key on. The white wire is hot when cranking(it causes a timing retard ckt to activate)and the red wire is used for the full time run ckt.

I have had the ignition module cause this before, but yes, check the red wire first.

The red green wire from the ignition switch is the resistor wire. Mine burnt up and I just put in a new wire and a 1.5 ohm ballast resistor. Put the voltmeter on the high side of the coil and look for 12 volts with the ignition switch in the run position. (about 7.5 volts when running) If no voltage then the ignition switch is bad or the resistor wire is burnt out.

thanx guys. i did look to see if it had 12v at the coil in the run position and it does. where abouts would the resistor be located if i need to check it? under the dash or somewhere under the hood? if it's under the dash i'll have to stuff a midget up there to find it. big hands come in handy but this isn't one of those times.

Originally Posted by stevec

thanx guys. i did look to see if it had 12v at the coil in the run position and it does. where abouts would the resistor be located if i need to check it? under the dash or somewhere under the hood? if it's under the dash i'll have to stuff a midget up there to find it. big hands come in handy but this isn't one of those times.

Its not a resistor, it is a red green wire that has resistance. It is in the loom over the left side valve cover. If you have voltage on the coil then the resistor wire is probably ok. Have someone start it with the voltmeter on the coil. Under normal conditions you will have 12 volts there while starting and it will be about 7.5 volts while running. You said that it runs in the start condition but not in the run condition. If the voltage at the coil doesn't change when you go from start to run then you have a problem in the module or the wiring to the module.

thanks fasthauler, i'll give it a try tonight when i get home. just picked up a new module for 30 bucks from autozone (thought it woud be a heck of alot more than that). i'll let y'all know what pans out

I had the starter solenoid short out the ignition before,disconnect the second small wire from it and se if it still does the same before buying anything. Jeff

Have you checked the fuseable links coming from the starter solinoid to the switch?

Ray

o.k. guys got it going !! it was the ignition switch. i thought i ruled it out trying 3 others but i guess if they sit around for a while not being used they foul up as well. cheap fix. i was just not liking the idea of having to go through the wiring harness looking for a break or bad connection.
i do however have another problem that i'll ask about in another thread.
power steering problems--bleh.