OK now turn the ignition to run, do not start the truck, pull the crank fuse and the relay and repeat those 4 measurements.

George

 

85 = 4.7v
86 = -0.04v
87 = 0.004v
30 = 12.5v

 

Here is what I think is happening. You have a short to power between the crank fuse and the relay energizing coil on 85 but it’s corroded and a crappy short. The voltage is not enough to energize the start relay but it is enough to keep it energized once the ignition switch engages it in the start position. This power source for the short is switched with the run position of the ignition switch. This partial short to power is either is the wire loom between the UHFB and the NSS or the NSS is hosed. Start by pulling the connectors at the NSS at the trans and repeat the 4 voltage measurement with the key in run and the crank fuse and start relay pulled.

George

 

BTW, you are now in the running for a hall of fame electrical problem that is weird AF. My absolute favorite type!

George

 

Thx for the help. That's what I was leaning towards as well, just been hopeful that there was some other magical phenomena I was missing like ECM wackiness or something of that nature. I'm going to do some more digging tomorrow and get back to you. NSS is new so probably crappy short lol

 

There are four likely causes: bad NSS, corrosion under the UHFB, a short in the wire run to the NSS or a short in the wire run back up to the start relay.

George

 

Did some borescoping and found what looked like a chaffed harness to the passenger side of the distributor. Moved some stuff and peeled the harness back to find this under the oil pressure sensor



Not content with that, I moved upstream to where I'd seen the harness chaffing on a bolt and found the short to power

 

Bingo, nice work! Diagnostics wins the day.

George

 

Problem solved, the truck is ready to get back to work! Thx for the help

 

Thats what I like to hear.

George