2004 Ford Explorer Sport Trac Fuel Pump Relay: Location, Testing, and Replacement Guide

The bottom line: On a 2004 Ford Explorer Sport Trac, the fuel pump relay is located in the Power Distribution Box (PDB) under the hood, on the driver's side near the battery. It is relay #46B, and the fuel pump fuse is fuse #30 (20 amp), also in the same box. If your Sport Trac cranks but won't start, and you don't hear the fuel pump hum when you turn the key to the ON position, the relay is one of the first things you should check. This guide walks you through finding the relay, testing it, bypassing it for diagnostics, and replacing it when needed.

Where Exactly Is the Fuel Pump Relay on a 2004 Ford Explorer Sport Trac?

The 2004 Ford Explorer Sport Trac has its fuel pump relay inside the under-hood Power Distribution Box (PDB) , which is sometimes called the battery junction box (BJB). This box is mounted on the driver's side of the engine compartment, positioned near the firewall and behind the battery . It is a rectangular black plastic box with a removable cover.

Inside this box, the fuel pump relay is socket #46B. This is confirmed by the factory fuse and relay diagram for the 2004 Explorer Sport Trac, which lists "46B Relay – Fuel pump relay" . The matching fuel pump fuse is #30, rated at 20 amps . Some sources also indicate the relay may be labeled as relay #48 in earlier documentation, but the 2004 Sport Trac specific diagram clearly shows #46B as the fuel pump relay . If you have the standard Explorer (non-Sport Trac) from 2004, the relay is in the same under-hood box, often referenced as relay #48 . Always confirm using the diagram on the underside of the PDB cover or in your owner's manual.

To access the relay:

  1. Turn off the engine and remove the key from the ignition.
  2. Open the hood and locate the black box on the driver's side, behind the battery.
  3. Press the release tabs on the sides of the box and lift the cover off.
  4. Look at the diagram printed on the underside of the cover. Identify relay socket 46B for the fuel pump relay and fuse 30 for the fuel pump.

What Does the Fuel Pump Relay Do?

The fuel pump relay is a small electrical switch that controls the power supply to the fuel pump. When you turn the key to the ON position, the Powertrain Control Module (PCM) sends a signal that energizes the relay. This closes the circuit and sends battery voltage to the fuel pump, which then pressurizes the fuel system . The relay works alongside the fuel pump driver module (FPDM), which further controls the pump's speed and operation . If the relay fails, the fuel pump won't receive power, and the engine won't start.

Common Symptoms of a Failing Fuel Pump Relay

When the fuel pump relay goes bad, you will likely experience one or more of these issues:

  • Engine cranks but won't start: This is the classic symptom. The starter turns the engine over, but since no fuel is being delivered, there's no combustion .
  • No fuel pump noise: When you turn the key to the ON position (without starting), you should hear a faint hum or whirring sound coming from the fuel tank area for about 2 to 3 seconds. This is the fuel pump priming. If you hear nothing, the relay could be the culprit .
  • Intermittent starting issues: The vehicle might start fine sometimes, but fail to start other times. This points to an intermittent relay failure .
  • Engine stalls while driving: A failing relay can cut power to the fuel pump while the engine is running, causing it to stall. This can happen suddenly and without warning .
  • Poor engine performance: In some cases, the engine might run but with reduced power, sputtering, or hesitation, due to inconsistent fuel pressure .

Keep in mind that a bad fuel pump fuse, a failed fuel pump, a faulty inertia switch, or a corroded fuel pump driver module can also cause similar symptoms . So, proper testing is essential.

How to Test the Fuel Pump Relay on a 2004 Explorer Sport Trac

Before you buy a new relay, you can test the existing one. A simple method is the swap test, and a more precise method uses a multimeter.

Method 1: The Swap Test

This is the easiest method. The fuel pump relay is the same type as several other relays in the PDB.

  1. Find a similar relay: Look for another relay in the same box with the same part number and identical pin configuration. Common candidates in the 2004 Sport Trac include the horn relay (#35 circuit) or the fog lamp relay (#45A) .
  2. Swap the relays: Pull out the suspected fuel pump relay and insert the known-good relay (e.g., the horn relay) into the fuel pump socket.
  3. Test: Try to start the engine. If the engine starts, you have confirmed that the fuel pump relay was faulty. If the engine still won't start, the relay is likely fine, and the problem lies elsewhere (fuse, pump, inertia switch, FPDM, or wiring).
  4. Check the swapped relay: Now, take the suspected fuel pump relay and put it into the horn relay socket. Test the horn. If the horn doesn't work, the relay is indeed bad.

Method 2: Testing with a Multimeter

If you have a multimeter, you can test the relay's coil and switching circuit.

  1. Remove the relay from the PDB.
  2. Identify the pins: A typical 4-pin relay has pins numbered 30, 85, 86, and 87. Pins 85 and 86 are the coil terminals. Pins 30 and 87 are the switch terminals .
  3. Test the coil: Set your multimeter to measure resistance (ohms). Place the probes on pins 85 and 86. You should read a resistance value, typically between 50 and 120 ohms. If you read infinite resistance (OL), the coil is burnt out, and the relay is faulty.
  4. Test the switch: Set the multimeter to measure continuity. With the relay unpowered, there should be no continuity between pins 30 and 87. Now, apply 12V power to pin 86 and ground to pin 85. You should hear a click, and the multimeter should now show continuity between pins 30 and 87. If the relay clicks but there is no continuity, the switch contacts are bad. If the relay doesn't click, the coil is faulty.

How to Bypass the Fuel Pump Relay for Testing

If you want to test whether the fuel pump itself is working, you can bypass the relay temporarily. Important: This is a diagnostic step only and should not be done while driving. Ensure the vehicle is in Park or Neutral, the parking brake is engaged, and the ignition is OFF before touching any electrical connectors.

  1. Remove the fuel pump relay from the PDB.
  2. Locate the correct terminals: On the 2004 Sport Trac, you will need to bridge terminals 3 and 5 in the relay socket, as mentioned in forum discussions for the 2004 Explorer . Refer to the relay pin-out diagram printed on the PDB cover to confirm which terminals correspond to the switched power (30) and the load (87).
  3. Create a jumper wire: Use a short piece of wire with the ends stripped to fit the relay socket terminals.
  4. Insert the jumper: Carefully insert the jumper wire into the terminals for pins 30 and 87.
  5. Turn the key to the ON position (do not start the engine). The fuel pump should now run continuously, pressurizing the fuel system. You should hear it humming. You can also test fuel pressure at the Schrader valve on the fuel rail .
  6. Remove the jumper immediately after testing and reinstall the relay.

If the fuel pump runs with the jumper in place, then the pump is likely good, and the relay or its control circuit is at fault. If the pump still doesn't run, the problem is likely the pump, the fuse, the inertia switch, the FPDM, or the wiring .

Checking Other Components in the Fuel Pump Circuit

If the relay tests good, don't assume the fuel pump is the problem. The 2004 Sport Trac has several other components that commonly fail.

The Fuel Pump Fuse

The 20-amp fuel pump fuse is located in the same PDB, in slot #30 . A blown fuse is a common and cheap fix. Visually inspect it or test it with a multimeter. Replace it with a new 20-amp fuse if blown.

The Fuel Pump Inertia Switch

This safety switch is designed to cut power to the fuel pump in the event of a collision. It's located in the right front passenger footwell, behind the kick panel . If it has been triggered, it needs to be reset by pressing the button on top. In some cases, the switch itself can fail internally.

The Fuel Pump Driver Module (FPDM)

The 2004 Sport Trac is highly likely to be equipped with a Fuel Pump Driver Module (FPDM) . The FPDM is mounted externally on the frame rail, typically behind the driver's side rear wheel . This module is a known weak point in these vehicles, primarily due to corrosion that occurs when the module is mounted flat against the frame, trapping moisture . A corroded FPDM can cause stalling and no-start conditions. Inspect it for any signs of white, chalky corrosion or pitting. If you replace it, be sure to use a mounting kit with rubber standoffs to prevent the corrosion from recurring .

Replacing the Fuel Pump Relay

Replacing the relay is a straightforward DIY job.

  1. Purchase the correct relay: You can buy a new OEM-style relay from a Ford dealer, or an aftermarket relay that matches the specifications. They are generally inexpensive, typically costing between $30 to $50 .
  2. Disconnect the negative battery cable to prevent any accidental shorts .
  3. Remove the PDB cover.
  4. Pull out the old relay by grasping it firmly and pulling straight up. The relay for the fuel pump is in socket #46B .
  5. Insert the new relay in the exact same position, pressing it down firmly until it clicks into place.
  6. Reconnect the negative battery cable.
  7. Test the system: Turn the key to the ON position and listen for the fuel pump to prime. Then, start the engine.

Why Consider a High-Performance Fuel Pump from KEMSO?

If you've confirmed that the relay, fuse, and FPDM are all working, and you've determined the fuel pump itself has failed, you'll need a replacement. While you're addressing the fuel system, it's a good time to consider the quality of the parts you install.

For those looking for a reliable and durable replacement, we recommend the KEMSO brand fuel pumps. KEMSO fuel pumps are designed as high-performance, OEM replacement units. They are built to meet or exceed the original manufacturer's specifications, ensuring a perfect fit and reliable operation for your vehicle. We stand behind our product with a lifetime warranty, giving you peace of mind that is hard to come by with cheaper alternatives. You can browse their full range of fuel pumps and other performance parts at their official website: https://www.kemsoracing.com/

When to Call a Professional

Working on your fuel system involves gasoline, which is highly flammable. If you are not comfortable working with electrical systems or fuel components, or if you have tried the steps above without success, it is always best to consult a professional mechanic. The fuel pump replacement on an in-tank system is labor-intensive, and a professional can ensure the job is done safely and correctly. This type of repair typically costs between $600 to $900, including parts and labor .

A systematic approach to diagnosing a no-start condition is key. Start with the easiest and cheapest checks—the relay and fuse. Then move on to the inertia switch and FPDM. Only after those are ruled out should you suspect the fuel pump itself. Following this guide will help you get your 2004 Ford Explorer Sport Trac back on the road with confidence.

For more information on high-quality replacement fuel pumps and other automotive performance parts, be sure to visit https://www.kemsoracing.com/ today.


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