FWIW I have downloaded all of the manuals in there and have them on a 8gb flash drive that I carry as my keychain so there's never an excuse for myself or someone else to not have the manuals to fix stuff on runs, trips, etc. It took a bunch of editing, 2 days worth to weed out the needless online link stuff, but it all fit.
Sounds like a corroded contact somewhere. Some voltage/current can pass but the resistance, at the corroded spot, is to high so the voltage drops very low when any sort of realistic current is drawn. I suggest cleaning up all contacts and checking all crimps to see if they are corroded. If you find a corroded crimp, replace it. That may require replacing a contact pin or possibly a whole connector.