Gary I had a fusible link wire drive me crazy. It was almost broke in two. Chasing with a volt meter the wires would check out until you would load the circuit then the voltage would drop to zero. Kinda sounds like what you might have going on.
Gary, if you haven't solved the problem yet:
I think laserjock's advice is right on track to locating the problem which will be on the Starter Relay side of the main 8-pin GP Harness Connector. I have a 1987 GP harness (engine side and starter relay/main harness side) mostly unwrapped on the floor near my desk so I can refer to that. It should be very close to or identical to yours with possibly a different wire color somewhere.
There are two 20 ga. fusible links which are both in the power circuit to the components you mentioned where you are having the problem. Both of those Fusible links are spliced into a single red power wire that supplies the voltage through the 2 fusible links to the components you named, plus some others in the GP harness. Since other components are working one of those fusible links must be supplying voltage to() the FSS and that's where the problem will be. I'll look at the harness I have more closely after I post this and come back and tell you exactly which of the two fuse links goes to the FSS in case you can't see the defect but you probably will. I'll tell you exactly where to look.
On the Starter Relay side of the 8-pin connector there will be 2 DARK BLUE wires (which identify 20 ga. Fuse Links) and then 5 inches further back on each wire should be a larger moulded black cylindrical shape piece with a flat side where the identification reads "FUSIBLE LINK 20 GA." for each one. The wires change color on the other side of the molded parts to RED/LIGHT GREEN tracer (stripe) Or one of them might be only RED).
From the center of those moulded parts going back another 5 inches is where BOTH of the Red/Light Green stripe wires (or one of them red if that's the case) are spliced into one single larger RED power wire. That RED power wire comes out of the main engine compartment harness next to the passenger side battery. It comes from the Ignition circuit in the cab and that's what powers the components you mentioned and few others. That larger RED power wire does not connect to the Starter Relay (SR) but it runs in the same loom as the wires that run from the (+) post of the SR and go to the 8- pin GP harness connector.
So, you'll simply need to remove the plastic conduit and tape from the harness on SR side of the 8 pin GP harness connector far enough back to inspect the fuse link wires and back 10 inches to the splice if you need to replace it.
I have a few things to do outside (rain coming coming in tonight) and after that I'll look at the harness I have and figure out which of the 2 Fuse Links goes to the FSS.
Well today I drove off just fine. Then came home to get some stuff.Engine started right up and died. Cranked fine but no start.No stumble. Heard electric fuel pump running. Key on pulled fuel shutoff wire.. nothing.. No click. Made jumper wire from battery positive and cranked up to start. Ran fine.. So this now has happened twice. First time it burned a fuesable link behind the passenger side battery. This power to injection pump powers the water in fuel switch. the fast idle and the fuel shutoff solenoid. Also it powers the fuel filter heater but thats no longer connected. Any ideas whats overloading the system? My guess is the shutoff solenoid. any guess???
