on my 87 diesel e350, i went to start it today and nothing happened. the lights on the dash all lit up like normal when you turn it to start, but the starter didn't engage.
when i jumped the small wire on the starter solenoid (the one on the fender), it cranked right over and fired up.
then i went under the truck and found the wire harness for the NSS, and started probing around in it. when i applied batter power to the pin that goes from the NSS to the starter solenoid, the starter kicked in, so i know everything from there down is ok.
then i tried to find the wire in the NSS group that recieves power when the key is in start, and no wire there got power when a buddy held the key to start.

so for all that, i'm down to believing that either the ignition switch is failing and somehow is getting all the lights to respond normally but not sending power toward the starter, or the wiring between the ignition switch and the NSS has a problem. what do y'all think?

 

Your probably right but first check the adjustment on the switch. The switch tends to slide down the column and thus the key not engage it all the way.

 

well i did a little more digging right after i posted that, remembered that the harness where i had been testing runs forward to another 4-pin connector up by the front of the engine, just above the power steering pump. so i used that as my center for further playing around. got the same results as i did under the truck.
the wiring diagram showed a yellow wire from there to the ignition switch, which didn't have power at this end, so i dropped the steering column to get into the ignition switch. probing the wire there got juice while starting, so i ran a new wire from near the ignition switch to near that connector, and it worked fine once or twice. then the NSS refused to let it start. it may be improperly adjusted, as the stock reverse lights have always been a PITA to operate, or that might mean that it just needs a new one. i have it bypassed for now