Been chasing this awhile, but the frustration is getting the better of me. I bought this old truck locally for cheap. It is an old soil conservation fleet truck with the stepside bed and slant six engine. The problems started with the battery draining down overnight. Finally found that the emission control harness (the emission controls had already been disabled) had fallen down behind the engine and shorted to the block, draining the battery and smoking the voltage regulator on the firewall. I removed the harness and replaced the regulator and life was good........for a while. Then it would not want to crank once in a while. You could start it by jumping the wires at the starter relay, so I replaced the relay and life was good........for a while. Now it will not crank and none of the dash lights will come on. the only thing that comes on are the hazard flashers. The battery is fully charged and I can make it crank by jumping the starter relay, but it will not fire. Just about at wits end with this. Any help is greatly appreciated. Thanks

 

I am not positive about this but in 79 and 80 I believe almost all power went thru the starter relay. There are also several fusible links in that system. This kind of a problem requires you to start at the battery with a volt meter and move systematically thru the wires until you find where you lose voltage. Good luck.