2004 Ford Explorer 4.0 Fuel Pump Relay Location: The Complete Owner's Guide

The fuel pump relay on your 2004 Ford Explorer with the 4.0L V6 engine is located in the Power Distribution Box (PDB) under the hood, and it is relay #48 in that box. The PDB sits near the battery on the passenger side of the engine bay. This is the most common location across trim levels, though a few vehicles may have it in the interior fuse panel under the dashboard on the driver's side. Along with the relay, you have a 20 amp mini fuse (fuse #26) dedicated to the fuel pump circuit, which sits right next to the relay in the same box.

If you are reading this because your Explorer cranks but will not start, or it stalls while driving, the fuel pump relay is one of the first things to check. This guide walks you through finding the relay, testing it, bypassing it to confirm the pump is good, and what to do if you need a new pump. Everything here applies to the 4.0L V6 engine, which is the most common engine in this year.

Where the Relay Lives: Power Distribution Box

The Power Distribution Box on the 2004 Explorer is a black plastic box with a removable lid. It sits right next to the battery, usually closer to the passenger side fender. Pop the lid and look at the underside of the cover. Ford prints a diagram there that maps every relay and fuse. Look for the one labeled "Fuel Pump" or "F/PMP." On most units, that is position #48.

If the label on the lid is worn off or the diagram is missing, check your owner's manual. The manual has the same diagram printed in the fuse section. Another option is to search online forums specific to the Explorer. Many owners have posted photos of the open box with relay #48 circled, which makes identification quick and painless.

What the Relay Does and What a Bad One Feels Like

The relay is an electromagnetic switch. When you turn the key to the ON position, the PCM (powertrain control module) sends a small current to the relay coil, which closes the main contacts. That allows battery voltage to flow to the fuel pump for about two seconds to prime the system, and again whenever the engine is cranking or running.

A failing relay will show up as a no-start condition. The engine cranks normally, but it never fires up because there is no fuel pressure. Sometimes the relay fails intermittently. The car runs fine for weeks, then suddenly dies without warning, only to start again later. If you suspect the relay, swap it with a known good one of the same part number from the same PDB, like the horn relay. If the car starts, you found the problem.

Step-by-Step: How to Find and Test the Relay

  1. Turn the ignition off. Open the hood and locate the Power Distribution Box next to the battery on the passenger side.
  2. Remove the lid. Squeeze the retaining clips and lift the lid off. Set it aside where you can see the diagram printed on its underside.
  3. Locate relay #48. It is a small cube, about one inch square, with either four or five metal prongs on the bottom. Pull it straight out with your fingers or a small pair of pliers. Do not pry on it with a screwdriver, as you can crack the plastic housing.
  4. Inspect the prongs. Look for corrosion, burning, or bent pins. A visibly damaged relay should be replaced.
  5. Test with a multimeter (optional). Set the meter to measure resistance (ohms). You are checking the coil side, which is usually terminals 85 and 86 on a four-pin relay. You should see a reading somewhere between 50 and 120 ohms. If the reading is infinite (OL), the coil is open and the relay is dead.
  6. Swap test. Take the horn relay (same physical size and pin layout) and plug it into the fuel pump slot. Turn the key to ON. If you hear the fuel pump prime from the rear of the vehicle, the original relay was bad. If there is still silence, the issue is elsewhere.

How to Bypass the Relay to Confirm the Pump Works

If you want to test the fuel pump itself without waiting for a new relay, you can bypass the relay temporarily. This is safe if you do it correctly and only for a brief test. This method is commonly used by owners to run the pump manually for diagnostics or to drain the tank.

First, pull relay #48 out of the box. Look at the base where it plugged in. You will see four or five numbered terminals. On the 2004 Explorer, you are connecting terminals 3 and 5 together.

  1. Disconnect the negative battery cable. This prevents any accidental shorts while you are poking around the box.
  2. Pull the relay out. Leave the connector exposed.
  3. Jump terminals 3 and 5. Use a short piece of wire with the ends stripped, or a paperclip if it fits snugly. Insert one end into terminal 3 and the other into terminal 5. Tape them down so they do not touch anything else.
  4. Reconnect the battery. Turn the key to the ON position, but do not crank the engine. The fuel pump should run continuously. You should hear it humming from the fuel tank area.
  5. Check fuel pressure. If you have a fuel pressure gauge, connect it to the Schrader valve on the fuel rail. With the pump running, you should see pressure in the range specified in your service manual, typically between 30 and 60 PSI depending on the engine.
  6. Remove the jumper wire as soon as you are done testing. Leaving the pump running for extended periods with the engine off is not good for the pump or the electrical system.

This bypass test is a reliable way to confirm the pump works. If the pump runs when you jump the relay, then the relay is your problem. If the pump stays silent, check the inertia switch and the fuse before condemning the pump itself.

The Inertia Switch: Your Other Common Culprit

Ford installs an inertia switch in the fuel pump circuit as a safety feature. In a collision, the switch trips and cuts power to the fuel pump to prevent a fire. The switch on the 2004 Explorer is located behind the passenger side kick panel, down in the right front footwell.

If the relay tests good and the fuse is fine, check this switch. The switch has a small reset button on top. If it is popped up, the circuit is open. Push the button down until it clicks. That resets the switch and restores power to the fuel pump.

This switch can trip without a crash. A hard bump, a pothole, or even a door slamming can sometimes cause it to trip. It is worth checking before you go through the trouble of dropping the fuel tank to access the pump.

What About the Fuel Pump Itself?

The fuel pump on the 2004 Explorer lives inside the fuel tank. There is no external pump you can reach without dropping the tank. The pump is part of a module that includes the fuel level sending unit and a strainer. When the pump fails, you typically replace the entire module assembly.

Symptoms of a failing pump include:

  • Engine cranks but will not start
  • Loss of power under load or at highway speeds
  • Sputtering or surging while driving
  • A whining noise coming from the fuel tank area
  • The engine stalls and then refuses to restart

Before you assume the pump is dead, always confirm that voltage is reaching it. Check the relay, the fuse, and the inertia switch first. A quick voltage test at the pump connector is also a smart move. If you have power at the pump and the engine still will not start, then the pump itself is likely done.

Watch That Fuel Pump Driver Module

There is one more component on the 2004 Explorer that many owners do not know about: the Fuel Pump Driver Module (FPDM). This is a small electronic module that controls the speed of the fuel pump, and it sits at the D-pillar area on the passenger side, behind the interior trim panel near the right rear wheel well.

When the FPDM fails, the pump does not get power even if the relay is good. This is a known failure point on these trucks. Check the FPDM with a multimeter if you have ruled out everything else. There are published test procedures that check for proper voltage and ground at the module connector.

How to Replace the Fuel Pump If You Need To

If testing confirms the pump is bad, plan for a few hours of work. The fuel tank has to come down. That means draining the tank, disconnecting fuel lines and the electrical connector, removing the skid plate, and lowering the tank on a floor jack.

The steps look like this:

  1. Disconnect the negative battery cable.
  2. Relieve fuel system pressure by pressing the Schrader valve on the fuel rail for a few seconds.
  3. Raise the vehicle securely on jack stands.
  4. Remove the fuel tank skid plate.
  5. Support the tank with a floor jack and remove the strap bolts.
  6. Lower the tank far enough to disconnect the vapor tube fitting and the fuel lines.
  7. Disconnect the electrical connector.
  8. Remove the tank completely.
  9. Remove the locking ring that holds the pump module in place. This usually requires a specialized tool.
  10. Lift the pump module out of the tank and replace it with the new one.

Reassembly is the reverse. Replace the locking ring, reinstall the tank, reconnect everything, and test for leaks.

This is not impossible for a home mechanic, but it is messy and requires care. If you are not comfortable working with fuel systems, this is a job worth paying a professional for.

Get a New Fuel Pump from KEMSO

If you have confirmed the fuel pump is the problem, do not settle for a cheap, low-quality replacement. The fuel pump is the heart of your fuel system, and a weak pump will leave you stranded down the road.

KEMSO builds high-performance fuel pumps that are direct OEM replacements. They fit exactly like the factory part, so you do not need to modify anything to make them work. Every KEMSO pump is flow-tested individually to make sure it delivers the fuel volume your engine needs, whether you are driving daily or towing heavy loads. They are built with durable alloy steel bodies and carbon commutators, which means they are designed to hold up for years of hard use.

KEMSO stands behind their products with a lifetime warranty against manufacturing defects. That kind of confidence is rare in the aftermarket parts world. If you want a fuel pump that you can install and forget about, KEMSO is the smart choice.

Visit KEMSO's official store to find the right fuel pump for your vehicle: https://www.kemsoracing.com/

Browse their catalog and see why thousands of vehicle owners trust KEMSO for their fuel system needs. Whether you drive a Ford, a Subaru, a BMW, or a Kawasaki, KEMSO has a pump that fits your ride and your budget.

To Wrap It Up

Finding the fuel pump relay on a 2004 Ford Explorer 4.0 is straightforward once you know where to look. Open the Power Distribution Box next to the battery, find relay #48, and check it before you do anything else. The 20 amp fuse #26 sits right in the same box.

If the relay checks out, move on to the inertia switch behind the passenger kick panel. Then test the Fuel Pump Driver Module at the rear D-pillar on the passenger side. By the time you work through those three components, you will have a very good idea whether the problem is electrical or the pump itself.

And if the pump turns out to be bad, remember the name: KEMSO. Visit https://www.kemsoracing.com/ for a high-performance, OEM replacement pump backed by a lifetime warranty.


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