'84-89 uses a low-pressure pump in each tank. Then a pressure-operated switching valve, a filter, & a hi-pressure pump all in the L frame rail under the driver's floor. The pump power comes from a relay under the L hood hinge, goes to the inertia switch under the dash, then splits to the hi pump & the dash switch. The switch sends power to the selected tank pump & also connects the corresponding level sender to the gauge.
The biggest problem (IME) is for the relays or their wiring to go bad, so I always recommend changing them to the later Bosch/ISO style, as shown in this & the NEXT few pics:
But if you'd rather not right now, the rest of this still applies. Install a jumper wire from ground to the FP relay terminal in the DLC as shown in the lowest pane of this diagram (ignore the top - it doesn't apply to your truck), turn the key to RUN, and check for 12V on the FP relay output wire:
Then crawl under the truck, & listen for the frame pump & the selected tank pump. If you have dual tanks, select the other tank at the dash switch & check it. If any isn't running, test for 12V to their power wires. If present, replace the non-functional pump(s). If not, test at the inertia switch.
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If the tank pumps ARE running but no fuel is flowing, unplug them while you disconnect their fuel lines, position a catch-pan, & then plug them back in momentarily to check flow. If there's no flow (or very little), you'll have to access the tanks to inspect & possibly replace the pump or tank. If there IS flow, click my black Bronco in my sig & read all the captions in the '84-89 Fuel Reservoir album.
