1991 Ford Explorer Fuel Pump Relay Location: Your Complete Guide

The fuel pump relay for your 1991 Ford Explorer is located inside the engine compartment's main Power Distribution Box (PDB). Specifically, find the black box mounted on the driver's side inner fender wall, near the brake master cylinder and close to the battery. This critical relay powers the fuel pump when you turn the ignition key. Locating it is essential for diagnosing no-start, no-fuel conditions common in early Explorers. Let's break down exactly how to find it and work with it.

Understanding the Power Distribution Box (PDB)

  • What It Is: The PDB, often called the main fuse box or relay center, is a critical hub in your Explorer’s electrical system. It houses important fuses protecting major circuits and several key relays controlling high-amperage devices like the fuel pump, cooling fan, horn, and headlights. It’s typically a rectangular, black plastic box with a removable lid.
  • Finding the Box: Open your Explorer’s hood. Stand facing the engine. Locate the battery on the front driver's side (left side if you're sitting in the driver's seat). Directly behind the battery, bolted onto the inner fender wall (the vertical metal wall separating the engine compartment from the wheel well), you’ll find the PDB. It's positioned near the brake master cylinder and fluid reservoir.
  • Access: The PDB lid is secured by several plastic clips. Unclip these and carefully lift the lid straight off. You will see an array of fuses, relays, and possibly larger fusible links.

Locating the Fuel Pump Relay Inside the PDB

  1. Identify Relay Positions: Inside the PDB lid, you'll find a diagram or map showing the location and function of each fuse and relay slot. This is your primary guide. Match this diagram to the slots inside the box.
  2. Look for Labeling: The slots themselves may have labels embossed into the plastic next to them. Common labels near the fuel pump relay include "Fuel Pump," "FP," "EEC Relay," or "PCM Relay." The relay is usually a standard Ford Mini ISO relay, a small (approx. 1" cube), typically black plastic component with 4 or 5 metal prongs on the bottom. Crucially for the 1991 Explorer, the fuel pump relay slot is very likely labeled "EEC Power Relay," "EEC Relay," or sometimes "PCM Relay." This relay powers the vehicle's computer (EEC-IV module) AND the fuel pump. The computer controls the relay's ground, but the relay sends power to both.
  3. Confirm the Relay: There are usually several identical-looking relays in the PDB. Using the diagram on the lid and/or the embossed labels near each slot is the only reliable way to identify the correct relay. Never remove a relay solely based on its physical appearance.

Visual Aids to Help Identification (Descriptions as actual photos can't be shown):

  • Photo Idea 1: Close-up of a typical 1991 Ford Explorer PDB location behind the battery on the driver's side inner fender.
  • Photo Idea 2: Inside view of the PDB lid showing the fuse/relay diagram. The "EEC Power Relay" or "PCM Relay" would be circled.
  • Photo Idea 3: Inside view of the PDB itself, with arrows pointing to the specific relay slot(s), highlighting any embossed "EEC PWR" or similar labeling near it. A typical black Mini ISO relay would be shown in place.
  • Photo Idea 4: Close-up of a typical Ford Mini ISO relay showing its size and prong configuration.

Why the Fuel Pump Relay Matters

  • Its Role: The relay acts like a high-power electrical switch. The ignition switch sends a low-power signal to the relay's control circuit (coil) when you turn the key to "Run" or "Start." This energizes the coil, closing internal contacts that connect the battery's high current to the fuel pump circuit. Without this relay functioning, the fuel pump gets no power and the engine cannot start or run.
  • Symptoms of Failure: The most obvious symptom is the engine cranking but refusing to start because no fuel is being delivered. You might also notice the engine suddenly cutting out while driving, mimicking a fuel pump failure. Sometimes, a failing relay will cause intermittent starting issues – starting fine one minute, not the next. You might hear the relay clicking erratically or not at all when turning the key.

Testing the Fuel Pump Relay (Safely & Simply)

A bad relay is a common failure point. Testing it can save time and money before replacing the fuel pump or other components.

  • Method 1: The Swap Test (Safest & Easiest)
    • The PDB contains several identical relays controlling things like the horn, cooling fan, or air conditioning clutch.
    • Identify a relay with the same part number as the suspected fuel pump relay (using the lid diagram) – typically another Mini ISO relay. The horn relay is often a good candidate.
    • Ensure the ignition is OFF.
    • Carefully pull the known good relay out of its socket (it might take slight wiggling or leverage with a small screwdriver).
    • Pull the suspect fuel pump relay out.
    • Insert the known good relay into the fuel pump relay socket. Insert the suspected bad relay into the socket the known good relay came from (like the horn socket).
    • Turn the ignition key to the "Run" position (do not start the engine). You should hear the fuel pump prime for 1-2 seconds (a buzzing/humming sound near the fuel tank at the rear). If the pump now runs when it didn't before, the original relay is faulty. If the horn was working before, check the horn now with the swapped relay – if the horn doesn't work, the swapped relay is likely bad. Caution: Don't crank the engine long without fuel pressure present.
  • Method 2: Listening/Feeling Test:
    • Have an assistant turn the ignition key to the "Run" position (not Start).
    • Place your fingers firmly on the relay housing.
    • You should feel and hear a distinct "click" as the relay energizes when the key is turned "Run," and another "click" as it de-energizes about 1-2 seconds later. Lack of clicking suggests a problem with the relay or the control circuit feeding it.
  • Method 3: Basic Voltage Test (Multimeter Required):
    • Set multimeter to Volts DC.
    • Find a suitable ground point (bare metal on the chassis/engine).
    • Turn ignition to "Run."
    • Referencing the PDB diagram/lid, identify the specific slots in the relay socket. Typically, one pin should have constant battery power (B+) regardless of the key position (test between suspect pin and ground – should show ~12V). Another pin should be the "trigger" wire from the ignition switch – it should get battery voltage only when the key is in "Run" or "Start" (test with key on). If you have both power (B+) on one terminal and a good switched (ignition "Run") signal on the trigger terminal, but the fuel pump doesn't run, the relay is suspect. If power or the trigger signal is missing, the problem is elsewhere (fuse, wiring, ignition switch).
    • Note: Proper terminal identification requires a wiring diagram specific to the 1991 Explorer.

Replacing the Fuel Pump Relay

  • Sourcing a Replacement: You have several options:
    • Ford Dealer: Best for an exact OEM specification part (part number usually printed on the old relay). Most reliable, potentially higher cost.
    • Auto Parts Stores: Relay is standard. Ask for a "Mini ISO relay" for a 1991 Ford Explorer fuel pump/EEC circuit. Counter personnel can look it up. Brands like Standard Motor Products (SMP), BWD, or Bosch are reliable aftermarket options. Bring the old one!
    • Online Retailers: Search by year, make, model, and "fuel pump relay" or use the part number off the old relay. Ensure compatibility.
  • Installation (Simple):
    • Ensure the ignition is OFF.
    • Align the pins on the new relay correctly with the slots in the socket. These relays only fit one way.
    • Press the relay firmly into place until it clicks or feels fully seated. Avoid forcing.
    • Replace the PDB lid securely.

Beyond the Relay: Related Components and Checks

A lack of fuel pump operation doesn't always mean the relay is bad. Consider these steps:

  1. Check the Inertia Switch: This safety switch cuts fuel pump power during an impact, but can also trip accidentally from bumps or jolts. It's located on the passenger side kick panel near the firewall inside the vehicle or sometimes behind the passenger side kick panel trim. Press the reset button firmly on its top. Try starting again.
  2. Inspect Fuses: Inside the PDB, find the fuse associated with the fuel pump circuit, often labeled "ECU," "PCM," or "Fuel Inj." Check the fuse visually (look for a broken filament) or test it with a multimeter for continuity. Also, check the main battery fuses/fusible links in the PDB and starter solenoid fuse.
  3. Listen for the Fuel Pump: Turn the ignition key to "Run" (don't crank). Listen carefully at the rear of the vehicle near the fuel tank. You should hear a distinct buzzing/humming sound lasting 1-2 seconds. If you hear it, the pump is at least getting power momentarily (relay/fuse/switch might be working, pump could still be failing). If you don't hear it, the problem is further upstream (relay, fuse, inertia switch, wiring, ignition switch).
  4. Fuel Pressure Test: If the pump primes (you hear it), the problem might be insufficient fuel pressure due to a clogged filter, failing pump, or leaking injectors/regulator. A fuel pressure gauge test port is located on the fuel injector rail. Testing pressure requires a gauge.
  5. Wiring and Connectors: Check for visible damage, corrosion, or loose connections at the PDB, fuel pump relay socket, inertia switch connector, and fuel pump electrical connector (back near the tank).

Important Safety Precautions

  • Disconnect Battery: Always disconnect the negative (-) battery cable before performing any electrical testing or component replacement beyond simple relay swaps. This prevents shorts or accidental sparks.
  • Fuel System Awareness: Understand that the fuel system may be pressurized even with the engine off. Avoid excessive force on fuel lines, and have rags ready to catch fuel if depressurizing the system is necessary (follow service manual procedures). Do not smoke or have open flames nearby.
  • Electrical Safety: Use proper tools, insulated screwdrivers, and respect high-amperage circuits.
  • When to Call a Pro: If electrical diagnosis is beyond your comfort level, or if basic checks (relay, inertia switch, fuses) don't resolve the issue, consult a qualified mechanic. Diagnosing wiring faults or fuel system problems requires expertise and specialized tools.

Conclusion: Pinpointing Power for Your Fuel Pump

Knowing the 1991 Ford Explorer fuel pump relay location inside the Power Distribution Box on the driver's side inner fender is the first step in tackling a no-start condition. Remember to look for it labeled as "EEC Power Relay," "PCM Relay," or similar markings. Using the diagram under the PDB lid is crucial for positive identification. Testing via relay swapping is a straightforward diagnostic step. If a faulty relay is identified, replacement with a standard Mini ISO relay is simple. However, always remember to perform basic safety checks (inertia switch, fuses) and be aware that the lack of fuel pump operation can stem from issues beyond the relay itself, including wiring faults or a failing pump. Following these steps methodically will help you get your Explorer back on the road.

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