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How to re-wire a fuel pump off a relay

Fuel Pump Re wiring Installing Relay Modification Performance GPH LPH

Installing a dedicated voltage supply line for your fuel pump

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When putting in an aftermarket fuel pump or looking to get more fuel out of your factory pump, one mod worth doing is re-wiring that fuel pump. The purpose of this is to make sure that the fuel pump is getting a good, clean voltage that won’t fluctuate or change based on other electrical conditions the car is going through. What I am saying is that we want to make sure that our fuel pump is receiving 14 volts when in use to be sure that our fuel pump is working as efficiently as possible. Also, with the average fuel pump replacement cost between $300 and $700 dollars, making sure the wiring is functioning properly is essential.

The factory wiring of the fuel pump is tied in with many other parts of the cars electrical system. If there are any fluctuations in voltage in this system, the fuel pump will also be affected. Just think about when you have the radio turned up, or running the high beam lights, or maybe when the alternator starts to weaken. When we are running a performance car at the limits, having a fuel pump start to lose voltage, and therefore lose flow, can cause catastrophic problems. Motors running right on the edge of tuning could go slightly lean enough to start detonation in the motor and cause a total failure. The best way to avoid this is to wire the fuel pump to have a dedicated voltage line right off the battery so that it consistently performs.

How to start Re-wiring your fuel pump

The steps of re-wiring your fuel pump are to first get to your fuel pumps wiring. At the wiring you need to find the 12v supply wire from the ECU to the fuel pump. Once that wire has been found cut that wire so that you now have half of that wiring running to the fuel pump, and the other half running to the factory ECU.

Choosing a Relay

The relay you need should be a four-prong relay with at least 14-gauge wiring. You do not want to get a relay that cannot handle the amperage that your fuel pump might pull causing the relay to fail, and you to be stranded on the side of the road.

Fuel Pump Re wiring Installing Relay Modification Performance GPH LPHFuel Pump Re wiring Installing Relay Modification Performance GPH LPHFuel Pump Re wiring Installing Relay Modification Performance GPH LPH

Wiring in the Fuel Pump Relay

The relay should have 4 pins on it. They are usually labeled 30, 85, 86, and 87. Steps:

1. Wire pin 85 of the relay to the 12v supply wire from the stock ECU that you just cut. This 12v wire will now energize the relay to turn on. So now instead of the stock ecu energizing the fuel pump, it will energize this relay.

2. Wire pin 86 to a good ground on the car. Be sure it is a good clean ground, and use appropriately sized wire so you don’t burn up the relay. In fact you should be using appropriately sized wire for all of this.

3. Run a dedicated wire from your battery to pin 30 on the relay.

4. Run a wire from pin 87 to your factory wire that you cut that runs to the fuel pump.

Now to explain what you just did. The relay is just like a light switch. Anytime it receives 12 volts on pin 85, it will turn on the switch and make continuity between pin 86 and pin 87 on the relay. Being that pin 86 is wired straight to the battery this will send a nice, clean voltage straight to the battery with no other obstructions to the fuel pump. Once the car is shut off, pin 85 will lose its 12 volts causing the coil inside the relay to close and the light switch will then be turned off cutting the continuity between pins 30 and 87, which will then cut the voltage supply to the fuel pump. Ingenious right?

If this is too much for you, this modification has become so important that companies sell ready to wire kits:

If you have any more questions on how to do this particular modification to your cars factory wiring harness, click the contact us button and send us an email.

Questions we have recieved:

From Reader Barry:

how to re-wire a fuel pump off a relay. on your web site it shows this diagram with #85 going to ground. in the wiring in the fuel pump relay #2. it states # 86 goes to ground. i am a non-wiring person, i have a bosch relay # 0 332 209 150-896. please advise.

#85 and #86 are the relay coil.  It doesn't matter which one you take to ground as long as the other you have as your triggered 12v signal.

Example:  - If you take #85 to ground than make #86 your 12v signal in.
                 - If you take #86 to ground than make #85 your 12v signal in.

Make sense? Team



Was reading your fuel pump rewire, and was wondering what you do if you are running two fuel pumps. I have an in tank fuel pump, and a inline fuel pump. Any help would be great. Thanks

Dear Reader,

IF you ran large enough wire and a relay to handle the current (amps) of both pumps than you would run the 12v out of the relay to both pumps to turn them on simultaneously. Only perform this if you calculated the gauge of wire run to match the amp draw of both pumps though and the relay size as well.

Good luck,

Enginebasics Team!

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