Wiring Fuel Pump Relay: How to wire a fuel relay?


When starting your car, your engine needs the fuel to turn on. For that, the vehicle uses the mechanism of the Amp Relay to automate the task. Let’s dig into the wiring fuel pump relay and how it works.

Table of Contents

What is a Fuel Pump Relay?

As you turn the car’s key to start, there is so much happening in the chamber. Here, your car’s engine control module (ECM) communicates with electric fuel pumps, instructing it to deliver the correct quantity of diesel or petrol to the engine’s combustion chamber.

To achieve this, the ECM manipulates a relay known as the Amp Relay. When the gasoline pump requires electricity, the relay will switch on. Any time it doesn’t, the fuel pumps will stop working.

How does the Fuel Pump Relay Work?

Like a light switch, a fuel pump relay activates the pump. The key difference is that the switch is not physically touched but activated when enough power builds up.

The fuel pump is accountable for controlling the engine’s internal pressure. An increase in pressure initiates fuel injection into the engine. Inadequate fuel pressure could cause a misfire in the engine by preventing it from receiving enough gasoline.

Instead of directly activating the internal switch, this method activates the circuitry responsible for that function. The electromagnetic field produced by this coil subsequently serves to connect the relay’s metal contacts physically. The gasoline pump is turned on via a regulated circuit that includes the metal contacts.

Working principle of fuel pump relays

Caption: Working principle of fuel pump relays

Fuel Relay Location

Fuel pump relays can be found in a few different places, depending on the vehicle, but both are good places to start looking:

  • In the hood of the car, near the firewall, where the engine is located; or
  • Under the dashboard on the passenger side of the car.

Locating the fuel pump always involves locating the circuit breaker panel. Removing the lid from the fuse box is necessary to access the fuel pump relay.

The gasoline pump relay probably won’t have any indicator lights. However, the underside of the fuse box might contain a diagram explaining what each relay and fuse is for if you’re lucky. If your car doesn’t have a fuse box diagram, you’ll have to look out for the location of the fuel pump relay online.

Symptoms of a bad fuel pump relay

  • The engine fails to start as you turn the key since there is no power supply to the fuel pump.
  • Or the engine stalls when your car is running.
  • Even though the car is working correctly, a sudden loss of power can cause higher risks due to a failed fuel pump.
  • Moreover, you will not hear any noise from the fuel pump.

How to diagnose a bad fuel pump relay

  • Visually inspect the signs of corrosion or overheating in the relay
  • If you cannot identify these situations, you can test them with a digital multimeter.

What is the reason for a Fuel Pump Relay failure?

What follows are the two most common reasons for fuel pump relay failure.

Very Old Automobile

The likelihood of a fuel relay failure increases if your vehicle has high mileage and has run over 200,000 kilometers. The electrical wiring of a car becomes tangled and unsafe. Most parts have reached the end of their useful lifespan.

Having a week’s battery

A weak battery is the second most common cause when the fuel pump relay fails. A weak battery can produce insufficient voltage. Your car’s fuses, relays, and other electrical components can only function properly with a steady voltage source.

As well as preventing enough voltage from entering the electrical components, a poor battery can create electrical power spikes that can destroy relays. For this reason, a weak battery is usually the root cause of a fuel pump relay that won’t work.

Fuel Relay Replacing Cost

The fuel pump relay usually costs around $45 and $165. Since it is not very expensive, most auto stores will have it for $20 to $50. However, replacing the fuel pump relay will cost between $25 and $120 in labor.  

Changing out the pump relay is a simple task. You need to go to any auto store and buy a new fuel pump relay, find your car’s fuse box, look at the wiring diagram, and replace it with the new one. Most mechanics charge more than this small job because it takes longer. Even if you have a luxury car, the cost will go up.

How to rewire a fuel pump relay?

Once you find something wrong with the Electric Fuel Pump Harness or other electrical accessories, you should rewire the relay.

Before wiring a fuel pump relay

First, you’ll need to gain access to the wiring inside the fuel pump. Locating the 12v input cable from the ECU to the fuel pump in the wiring system is essential. Once that wire is located, cut it in half so that one end is connected to the fuel tank and the other is connected to the original ECU.

Choosing a Relay

There should be four terminals on the relay. They are typically referred to as 85, 30, 86, and 87. The four-prong relay with 14 gauge wiring is the best choice. You don’t want to be stuck on the roadside because the aftermarket fuel pump couldn’t handle the amperage.

You may find a fifth terminal 87a. It is an alternative to 87 and a normally closed contact. So power is required without energizing the relay. 

4-Pin-Relay-Wiring-Diagram vs. 5-Pin-Relay-Wiring-Diagram

Caption: 4-Pin-Relay-Wiring-Diagram vs. 5-Pin-Relay-Wiring-Diagram

Source: https://www.easycarelectrics.com/ 

Wiring in the Fuel Pump Relay

  • For the stock ECU to power the fuel pump, it will now power this relay. Start by connecting relay pin 85 to the 12v supply line from the stock ECU you severed. Connecting this 12v cable to the relay’s coil will supply it with the power it needs to activate.
  • Connect pin 86 to the car’s reliable ground wire. If you don’t want to fry the relay, make sure the ground is clean and of sufficient size. You should use the right-sized wire throughout.
  • Third, connect pin 30 of the relay directly to your power source.
  • Four, sever the factory wire leading to the gasoline pump and connect pin 87 to the open end of the wire.

It is the job of the ECU to close the fuel pump relay, allowing 12V from the battery to flow to the fuel pump when you operate the pump. If the fuel pump can’t produce the correct fuel pressure at maximum power, the engine won’t be able to run correctly. If the fuel mixture is too lean, engine damage is possible. The fuel consumption of a 500-horsepower engine is about 210 pounds per hour. The ratios of air or fuel are sensitive to variations in fuel temp since fuel density decreases with rising temperature. Take every precaution to keep fuel cold. The gasoline pump’s mounting location is critical, and you must set it lower to the tank pump or surge vessel outlet.

Fuel Pump Wire Diagram

Caption: Fuel Pump Wire Diagram

Source: https://www.enginebasics.com/

Can A Fuel Pump Run Directly To A Battery?

Electricity from the car’s system is what powers the fuel pump. The propulsion mechanism is typically an electric motor, which you can install inside or outside the vehicle. Connecting a fuel pump directly to a battery is possible, although we do not recommend it. A gasoline pump’s lifespan is short if it is subjected to excessive strain by being powered directly by a battery.

For the fuel pump to work, you must fix it into the car’s electrical system. However, you can power some petrol pumps solely by a car’s battery. For starters, before deciding the gasoline pump itself is broken, make sure you have checked all of the connections.

Additionally, a fire could break out if you do not wire the gasoline pump correctly. When wiring a fuel pump to a battery, it is best to get the advice of a skilled mechanic or electrician.

fuel pump amp circuit

Caption: fuel pump amp circuit

Source: https://completecar.ca/maintenance/bad-fuel-pump-relay-symptoms/ 


The fuel pump relay allows the amount of oil to reach the engine, which will help the car’s proper operation. However, if it fails to do so, chances are you need a fuel pump assembly. And, wiring a fuel pump is not difficult, so you can do it yourself or go for a certified mechanic. Here at Cloom, we offer auto-wiring harnesses and cable assemblies for your connection. Contact us now to know more Product Details.