Well after lots of digging I cant figure out what that vacuum solenoid does? its definitely a valve though as opposed to a switch with contacts. the diagram below it for 79-81 has the same valve but labeled as "ESS solenoid" so that ESS relay in the 82-84 diagram is solely responsible for controlling that and nothing else. Heck I cant even figure out what ESS stands for since you say that that solenoid valve gets hot when plugged in and reads like a closed switch on a multimeter, I'm going to assume that it is shorted. since it is essentially connected across the coil but with a relay in between, it being shorted will pull the coil power down which killed your spark. this is a bit off subject but I honestly cant figure out why there even is a relay for the ESS solenoid since power flowing to both the relay and the solenoid is switched by the ignition switch. I think now that your truck is running without that part being connected the issue you will have with your spark should be along the lines of what Giovanni and Geezer are mentioning with the coil resistance and cooling. Hopefully someone will know what the deal with that solenoid is for. strangely it needs an ignition pulse the way its ground is tied with coil negative, which may be part of the reason it failed. It looks like it probably gets a high voltage kick back like a tachometer would which would probably break down the insulation on the windings over its 35+ year life I wonder if this is just a basic valve like an A/C idle up valve? that would make finding a replacement much easier. Maybe it just functions as some sort of emissions device and can be removed much like a persons appendix?
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