1973-1991 Dual fuel tank systems theory of operation

[Ford Assassin]

Originally Posted by hatzie

[*]You can see below that... On the 1987 RV series TBI trucks the selected fuel pump receives Power on the wire from the switch so it will activate because it has power and the shared frame ground. The UN-Selected pump receives Ground on the wire from the switch thus shutting it down because it now has two ground wires and no power.

1987-1991 RV series TBI Fuel Pump Relay for TBI tank switch and fuel pumps.[/SIZE]
Year specific information can be found in the 1987-1991 Emissions and Driveability service publications. You can find them in my manuals post.

The TBI fuel pump relay (AC Delco 158240 Multi-Purpose SPDT Relay) can be found on the firewall between the transmission tunnel and the Heater/AC box.
Fuel Pump relay without the transmission kickdown relay. You can see the pump test terminal dangling beside the split loom.

I have a stand alone harness with its own fuel pump relay and power wire. Any reason I can't just plug that fuel pump power wire from the LS harness into the 'test' wire on the factory TBI relay on the firewall? Trying not to butcher the factory harness, nor make more work for myself.