Ranger Biodiesel Swap - Fuel Pump Override Switch

My Ranger is a 1999 model, equipped with a standard OBD2 system that monitors emissions and onboard mechanical diagnostics. To avoid this added complexity, most diesel swaps are performed on pre-OBD2 vehicles, or the builders will seek to remove/bypass/reprogram the ECU. 

For my build, I'm trying to retain as much of the ECU functionality as possible, and my goal is to pass the NJ MVC inspection honestly, with minimum codes and without an illuminated check engine light. 

In preparation for the swap, I'm making a number of modifications to the truck's electrical, fuel, and exhaust systems. The first of these will be described below, where I'm installing a fuel pump override switch.

The Ford Ranger ECU is programmed to run the fuel pump for approximately two seconds when the ignition is switched on, to prime the fuel injection system in preparation for engine startup. After this, the pump is shut-off until the ECU detects that the engine is running at sustained idle.

I'm replacing the original fuel system (single fuel line, regulator/return inside the tank, internal fuel pump) with a complete recirculating system featuring both sending and return lines, an external pump, and a water separator/filter. 

This type of system requires sustained operation of the fuel pump prior to startup, to circulate fuel and purge all trapped air from the lines. Rather than bypass the original fuel pump controls and wiring, I decided to install an override switch.

As shown on page 24-1 of the Ford Ranger Electrical Manual, the fuel pump is powered by the switched-side of the Fuel Pump Relay, through the Inertia Fuel Shutoff Switch, and on to connector C422 on page 24-7. 

The coil-side of the Fuel Pump Relay is connected to +12VDC on one side, and ECU Pin 80 on the -12VDC (Ground) side. To run the pump, ECU Pin 80 is switched to ground (logic output 0), which completes the circuit and triggers the relay coil. Therefore, an override switch simply needs to connect the existing Pin 80/Fuel Pump Relay wire to a switch, which can manually ground the circuit. 

Step 1: Shut off the truck, remove the connector from the ECU (engine bay on top of firewall), remove the protective covers and tape, and identify the wire connected to Pin 80.

Step 2: Carefully remove a 1/2" stretch of insulation from the wire on Pin 80 (do not cut the wire!!), wrap the stripped end of a new override switch wire around it, solder the joint thoroughly, and wrap the exposed conductors with electrical tape.

Step 3: Tuck the wires back inside the protective covers, including the new override switch wire. Route the new wire along the original harness and through the firewall of the truck, and rewrap the covers with electrical tape.

Step 4: Install a SPST toggle switch in the cab, and connect one side to the override switch wire that was just pulled though the firewall. Connect the other side to a wire that is grounded to the truck chassis.

This switch will now allow the driver to manually override the fuel pump at any time, without altering the original wiring or throwing any OBD2 codes.