1994 f-250 intermittent no-start

I have a 1994 f-250 4x4, 7.5L, 4speed with about 170,000 miles. The truck has an intermittent no start problem. It will almost always crank fine and has never quit or stumbled when running. One time it did not crank either but the on the second try it went back to cranking - and not starting. When not starting there will be no spark from the coil wire (as observed when wire is removed from the distributor and shorted to ground through a spark plug). If, during an initial no-spark situation, spark is finally observed and the the wire is replaced on the distributor the engine will start right up. Even when not starting there is always 12V at the ‘+’ side of the coil. The ignition control module, coil, ingition switch, cap and rotor have all been replaced with no change. The problem seems to be associated with the ignition only when starting. I have placed the key in ‘run’ and jumped the starter solenoid with the same no-start problem. I have not done any thing with the EEC relay or clutch interlock switch yet. The interlock switch does have a wire that goes to the PCM but I am not sure of its purpose so don’t know whether or not the switch could cause this situation. Even when not starting, all troubleshooting has, to date, not shown any odd resistance or voltage readings at the ICM or ditributor terminals. Has anyone experienced a situation like this. Thanks for any ideas.

If voltages for the ignition system are ok then perhaps the crank sensor is the trouble. From your description it appears the trouble is with the signal for the ignition pulses.

Thanks for your reply. It seems that way to me too but when the engine does start it never misbehaves. So far, the ignition problem seems to occur only during the ‘start’ phase. I have thought of the crank sensor but I’m not sure this particular engine actually has one. I haven’t spotted one on the engine yet and my Haynes manual doesn’t appear to mention one. Either way, wouldn’t a bad sensor affect the engine all the time and not just while starting? The intermittent issue is making troubleshooting very challenging. In fact I drove the truck yesterday, starting and stopping the engine a number of times without a problem. Mmmm.

Final Note:
I finally broke down and took the truck to a trusted mechanic. After three days they finally encountered the no-start situation and were able to trace it to a problem with the cam sensor (distributor pick-up coil). They replaced it and so far the truck has been starting fine. They said the intermittent problem is not unusual but it usually occurs when the engine heats up and not when it is cold. As a side note - this truck does not have a crank sensor.