September 20, 2021

Fuel Pump Rewire

I decided to finally rewire my fuel pump. The rewire only takes 30 minutes max depending on what all you need to remove in order to get to the fuel pump. In my case with #PoisonIvy, the car is stripped and the fuel pump is visible as soon as you open the hatch.

The stock wiring can and will get you further than you would think, but it will become a restriction eventually. From the factory, the wire that supplies power to the fuel pump runs all the way from the front of the car all the way to the fuel pump in the rear. The voltage drop is significant as the voltage has to travel that long distance through a smaller gauge wire. By the time the power gets to the fuel pump, your fuel pump will have less voltage than it could. 

Words By: SPOOLIGAN   |   Images By: SPOOLIGAN

The biggest reason I kept putting it off was because I wanted to rewire it directly to the pump itself, not just the wiring outside of the tank/fuel hanger, even though the voltage drop is very minimal, if measurable at all. I converted to e85 a little while ago and my fuel system is struggling to keep up with the 35% or so additional fuel flow required. Now, the fuel pump rewire is much needed whereas on 93 octane, it was fine for my setup. 

Alright, let’s get started and go over a few things you are going to need in order to properly rewire your fuel pump. You are going to use a standard automotive relay. You can get them at any parts store, any electronics store, even Wal-Mart would probably have them. You can also get them on Amazon or eBay for really cheap. I suggest getting one that comes with a pigtail to simplify things. 

The relay will have 4 pins, some have 5 but the extra will not be used. On the relay, each pin is labelled with a number. 85, 86, 87, and 30. And if you have a fifth pin, it would be 87a, but again, this pin will not be used.



There are two different ways you can rewire your fuel pump.

Option 1

You can rewire it so that all of the changes in wiring are inside the harness, outside of the fuel tank. This is by far less time consuming and it is the easiest method as there aren’t any modifications needed to the fuel pump itself or the sending unit. The fuel pump does not need to be removed, all the wiring is done above the tank.

Option 2

You can improve the wiring that goes down into the tank directly to the fuel pump.

I chose to do option 2, just because. The difference in voltage drop between the two options is very minimal, if anything at all. So option 1 will work great and it is easier. I wanted to do option 2 so I could show you guys what is involved if you decided to rewire it all the way to the pump.

Below is the diagram used when rewiring the fuel pump and relates to both options. You are going to cut the factory fuel pump hot signal wire. On a 1G AWD, this wire is black with a white stripe. You are going to want to cut this wire on the car side of the harness, before it plugs into the fuel pump harness. Otherwise, you won’t be able to remove the fuel pump without cutting the wire. The wire labeled “12V PUMP” in the diagram below goes to the fuel pump side of that wire. The other end of that wire (car side) simply goes to pin 86 on the relay.




Pin 30- Run a wire from pin 30 directly to the battery (12V supply).

Pin 85- Run a wire from pin 85 straight to ground.

Pin 86- Connect the car side of that black/white wire you cut to pin 86.

Pin 87- Connect the other end of that wire to pin 87. If you have to hard wire your fuel pump because you do not have a connector, run a wire from the hot side of the pump directly to pin 87.


I also strongly suggest running a fuse in the battery feed wire. I am using an inline 20 amp fuse.

If you chose to go with option 1, you would already be done. For option two, there are some extra steps. First and foremost, remove your fuel pump. I always disconnect the fuel line first to avoid any kinking or snapping of the hardlines. I used some WD-40 on my line and fuel pump nuts.

The line requires a 19MM wrench and a 14MM wrench. If you do not have one of each, go get one. Do not attempt to use a wrench that isn’t quite the correct size. Do not try to use vice grips. You will strip and/or snap shit doing this.

Next, remove the nuts. These are an 8MM head. Loosen these with caution as the studs are notorious for snapping. Lastly, unplug the fuel pump harness and remove the pump from the tank with care.



If you are upgrading the fuel pump in the process, I will show you how to remove it. First, unscrew this screw and remove the fuel pump hanger. For this tutorial, I am replacing my Walbro 255 fuel pump and swapping to a Walbro 450.



Next, unplug your fuel pump if it is equipped with a harness like mine. Then, pull the fuel pump nozzle out of the sending unit and remove the pump.



If you are removing the old, factory fuel pump wiring, unbolt the positive wire from the underside of the sending unit top hat. Then, remove the screw holding the ground wire to the sending unit. I forgot to take a picture of the ground, but it is located down lower on the sending unit near where the pump sits. The ground wire comes off the pump and connects to the sending unit via a screw. If it is stripped like mine was, just cut the wire and leave the screw in place. Then, drill a new hole and install your own wire with a ring terminal.





The next step is where most of the modification is done when rewiring the fuel pump all the way to the pump. The power wire you just unbolted from the bottom of the top hat, there is more on the top of the top hat you need to remove. On the top, there is a circular rubber grommet thing. You need to pop this off and it is not exactly easy. I used a flat head screwdriver to pry it off.




As you can see with the rubber part removed, the terminal is riveted into the top hat. This is easy to remove. You can simply pound it out with a few taps of a hammer. If it does not come out easy, you can drill it out. Some fuel pumps may have a bolt here and not be riveted.


Once the black rubber piece is removed and the rivet is popped out, there will be a large hole left behind. You can use the hole in one of two ways. 

  1. You can install a brass bolt through the hole. Then, using a brass nut and washer below the hat, you can attach your fuel pump power relay from the pump wire using a ring terminal. On the top of the hat on the bolt head side, you can attach a wire with a ring terminal here and run this to your relay.
  2. Keep the hole and run your power wire directly from the fuel pump, up through the hole, and to the relay. I used this option as I didn’t have any perfect bolts and nuts. I will eventually be using the first method. But for now, I used a rubber grommet to protect the wire and then sealed the hole and wire with RTV.




Now you can begin wiring up the fuel pump. I won’t spend a lot of time discussing the many ways you can do this. Ideally you would want to run 10awg all the way to the pump. My pump had 12awg wiring coming out of it so I went from 10awg to the 12awg pump wires. The ground wire will bolt to the inside of the sending unit. 

When installing my 450 pump, I eliminated the o-ring design that uses though orange-ish little hat pieces. I cut the fat end of the sending unit tube off and used a piece of rubber fuel line hose to connect the fuel pump to the sending unit tube. If you do this, use an appropriate length of hose so you can retain the fuel pump mount that secures the pump to the sending unit.



For my relay, I mounted it in the left rear corner of the hatch.



If you ran a direct wire from the pump to the relay, you can use some kind of wiring connector so you can unplug the wire. Otherwise, you won’t be able to remove the fuel pump from the car without cutting the wire. I used a nice two pin connector that I found in the towing section of AutoZone.


Doing this, it is difficult to tell if it hands down makes a difference. And, i like results. So, I measured the voltage before the rewire at idle and at WOT. At idle, my voltmeter displayed mid to high 11 volts. At cruise, it was around 11.5. At WOT, it dipped all the way down into the high 10’s.

I have not yet checked the voltage after the rewire. But, I was running into lean issues when WOT before the rewire. When I would go WOT, I would go lean. So lean that I had to turn the boost down. Now, after the rewire, my WOT AFR stays right where I want it. But, I also upgraded my fuel pump when I did the rewire so that may have helped as well. I will post the after voltage results when I get them.