Step 4
Run the harness along the side of the carpet up to the firewall. Remove the main harness grommet. Make a slit in the grommet and run the harness through the grommet. We made a slit in a cross to give enough clearance for the positive side connector to make it through without leaving a hole. Don't damage the harness in any way! Note: You might need to remove the clip behind the 10mm nut inside the vehicle to gain access. You may also want to remove the strut tower bar to get better access underhood.
Step 5
Run the harness to the battery. Make sure the harness can be secured out of the way from anything that could damage it. Attach the positive side to the battery. Wait until the pump installation is finished to attach the negative terminal.
Step 6
Back inside the car, remove the fuel pump assembly access cover (driver-side cover) Underneath, you will find one connector, two fuel couplers and one hose.
Step 7
Remove the 8mm nuts that hold down the pump assembly. The top ring will come free from the assembly. You'll have to finesse the ring, twisting it to free it. Don't force it; it will come out in the right position. Remove the pump assembly and be cautious of any fuel that might drip.
Step 8
Remove the bottom cover of the fuel assembly. With a twisting motion, remove the pump from the assembly. Disconnect the electrical connector from the pump. The AP Boss fuel pump uses a different style connector so we have to cut off the stock connector and crimp the new pins and insert them into the new connector. Note: Do this one at a time so that you don't mix up which wire goes into which side. Add a bit of solder to the connector before you re-install to ensure that the wire doesn't come loose from the terminal. Remove the plastic o-ring from the stock pump and install it on the new pump. Install the assembly in this order: harness, pump, rubber grommet, aluminum spacer and bottom cover.
Step 9
Re-install the pump, making sure to not over-tighten the cover. Attach all of the connections. Before you put everything together, attach the battery to ensure there are no problems. Prime the pump by turning the key on and off a couple of times. Once the car starts, put everything back together and you're done!
Here are some of the tools and supplies you will need for the install. Make sure you buy a high-quality relay. The difference in price is only a couple of dollars, and will keep you from having to go back and forth to get parts.
Nissan S13 Custom Fuel Pump Relay Harness Install:
Tools and supplies you'll need: 10mm deep socket, 10mm combination wrench, wire strippers, crimpers, soldering iron, voltmeter, screwdriver set, coat hanger, Bosch relay, premade relay bottom, 12-gauge wire (enough to run to the battery), 12-gauge female terminal, 12-gauge ring terminal, 16-gauge ring terminal, inline fuseholder, 15-amp fuse and shrink-wrap.
Step 1
Locate the fuel pump harness. For the S13, it sits right by the cap to the pump assembly in the trunk and to the right of the spare tire. Remove the cover to reveal the harness. With your multimeter, locate the trigger wire for the fuel pump and the ground. The wire should momentarily show voltage when the key is turned to the "on" position. The 12V should be blue with a red wire; the ground is white with a purple wire. Note: This vehicle is already outfitted with an aftermarket pump, so we're just adding in the relay harness and upgrading the wiring harness.
Make a slit in a cross pattern so that the positive connector can clear the grommet but not leave a hole. Be very cautious of your main harness as you are doing this. | 
Keep any debris that's sitting on top of the fuel tank from falling in after you remove the pump assembly by blowing some compressed air on the area. Have a rag handy to soak up any fuel that may leak out from removing the fuel lines. | 
Keep trying combinations until you can get the pump assembly free. Don't force it; it will come out. Keep enough rags handy to avoid spilling fuel in your car while moving the assembly and cover up the fuel tank access hole. |
The AP Boss pump uses a different style connector. You will need to cut the old connector off and crimp on the new one. Place a dab of solder on your crimps to prevent them from coming apart. | 
When installing the battery leads, move from positive to negative. Check that you properly torque the terminals. | 
The factory 12V supply is blue with red. The ground, a white with purple wire, is on the opposite end. |