1991 Ford F150 Fuel Pump Relay Location: Find It Fast & Fix Your Truck

The fuel pump relay on the 1991 Ford F150 is primarily located on the passenger side firewall near the brake booster. It's housed within a small, square plastic cover that simply lifts off, revealing a bank of several relays. Specifically, look for a brown-colored relay labeled "Fuel Pump" (or EEC Power Relay which often powers the pump too).

That's your answer. If your 1991 Ford F150 suddenly won't start or cranks but doesn't fire, and you suspect a fuel delivery issue like a silent fuel pump, finding the fuel pump relay is almost always the fastest first step. Knowing its location saves crucial time and effort compared to chasing down other potential causes blindly. Let's dive deeper into precisely how to find it, understand why it's important, and what to do once you locate it.

Why the Fuel Pump Relay Matters So Much

Before delving further into its exact spot, it's essential to understand what this small component does. The fuel pump relay acts as the high-powered switch for your truck's electric fuel pump. The fuel pump draws a significant amount of electrical current to operate. Trying to run all that power directly through the ignition switch or through the vehicle's computer (the EEC-IV module) would quickly overwhelm those sensitive components.

This is where the relay comes in. It uses a small signal current (often coming from the computer) to activate a powerful electromagnet inside the relay. That magnet closes robust internal electrical contacts, which then allow the high current needed by the fuel pump to flow directly from the battery, through the relay, and to the pump itself. In simple terms: the relay makes turning the heavy-duty fuel pump on and off safely possible using only a tiny control signal.

Pinpointing the Location: Passenger Side Firewall

For the vast majority of 1991 Ford F150 trucks (including most Flareside and Styleside models):

  1. Open the Hood: Access the engine bay.
  2. Go to the Passenger Side: Walk to the right side of the engine compartment (the side where the brake master cylinder and brake booster are located).
  3. Locate the Firewall: The firewall is the solid vertical metal wall separating the engine bay from the cab interior.
  4. Find the Plastic Box Near the Brake Booster: Look closely at the upper part of the passenger side firewall. Slightly above and/or slightly to the right (towards the fender) of the large, black vacuum brake booster, you should see a small, square, usually black or grey plastic cover. This is approximately 4-6 inches wide and is mounted directly to the firewall itself.
  5. Identify the Cover: This cover might be labeled "Relays" or just feature molded plastic clips holding it in place. It typically has a small finger-lift tab on the bottom edge or just pops off with gentle upward pressure.
  6. Remove the Cover: Carefully lift or pop the cover straight off. You'll now see several relays plugged into sockets underneath. Common relays in this box include:
    • PCM Relay (POWR RELAY on some diagrams - often yellow)
    • Fuel Pump Relay (often brown)
    • EEC Relay (EEC PWR RELAY - sometimes blue or black, and sometimes combined function with fuel pump)
    • Horn Relay (sometimes located here)
    • Wipers Relay (sometimes located here)
  7. Identify the Fuel Pump Relay: This is crucial. Look for a relay that is BROWN in color. On the relay itself, the plastic top (or the relay socket next to it) will usually be stamped or labeled "Fuel Pump" or "FP." If you see a brown relay, it's almost certainly the fuel pump relay. Double-check by reading any labels molded into the surrounding plastic panel near the socket or printed on the relay.

Verifying the Relay and Its Function

Once you've located what you believe is the fuel pump relay (the brown one labeled FP), you have a couple of easy, initial checks:

  1. The Click Test:
    • Turn your ignition key to the "Run" position (ON, but don't crank the starter). Listen carefully. You should hear a distinct, audible click coming from the relay area lasting about one second when you first turn the key to "Run". This click is the relay energizing. You should also hear the fuel pump itself whir or buzz momentarily (typically around 1-3 seconds) before stopping, as this pressurizes the fuel lines. This sequence primes the system for starting.
    • If you don't hear the relay click and don't hear the pump run for those first couple of seconds in "Run," it strongly points towards a problem with the relay circuit.
    • If you hear the relay click but don't hear the pump run, the problem might be the pump itself, the wiring to the pump, or the inertia shutoff switch (discussed later).
  2. The Swap Test (Safely): This bank often has another crucial relay: the EEC Power Relay (sometimes just called the "Computer Relay"). Important: Only swap an identical relay to test.
    • Find the EEC Power Relay (or PCM Relay). On many 1991 F150s, this is often a yellow relay. Check its label.
    • Crucially: Does it look exactly like the brown Fuel Pump relay? Same size, same number of pins (usually 5), same manufacturer markings? Only proceed if they are visually identical.
    • Turn the ignition OFF.
    • Unplug both relays.
    • Swap their positions. Plug the brown fuel pump relay into the EEC socket and the yellow EEC relay into the fuel pump socket.
    • Turn the ignition back to "Run" and listen carefully.
    • Interpretation:
      • If the fuel pump now runs with the swapped yellow relay: This means the original brown relay was faulty. The yellow relay took over the job successfully.
      • If the fuel pump still does not run with the swapped relays: The issue is likely not the relay itself. The problem could be elsewhere in the circuit (fuse, wiring, inertia switch, computer, pump).
      • Important Note: If the truck starts behaving strangely after swapping or doesn't crank at all, swap the relays back immediately. While the EEC relay is often similar, some minor variations exist, and mismatched relays can sometimes cause issues. This test is generally safe with identical relays and performed quickly only in the "Run" position.

Beyond the Firewall: Other Potential Locations (Less Common)

While the passenger firewall box is the main location, be aware of slight variations, especially on specific trim levels or trucks equipped differently:

  1. Under the Dash - Main Interior Fuse Panel: A minority of 1991 F150s might have the fuel pump relay located within the primary interior fuse panel. This panel is located beneath the dashboard, on the driver's side, usually to the left near the door hinge. To access it:
    • Open the driver's door.
    • Look near the lower part of the dashboard, typically behind a small panel or cubby that pivots or pulls down.
    • Find the fuse panel cover and remove it.
    • Look inside. You might find the fuel pump relay (again, usually brown and labeled "Fuel Pump" or "FP") among the fuses and other relays.
  2. Engine Compartment Fuse Box (Under Hood): Some models primarily use the under-hood power distribution center for fuses and relays. On '91 F150s, this is not as common for the fuel pump relay specifically (that's usually in the firewall box mentioned first), but it's worth quickly scanning the labels if you have one. Do not confuse the fuse-only panel (often on driver's fender) with the relay box on the firewall. The firewall box houses the critical relays.
  3. California Emissions Vehicles: Trucks sold in California or meeting CA emissions standards sometimes have slight wiring differences. While the firewall box is still the primary location, always double-check labels meticulously.

Fuses: The Essential Companion Check

The relay controls the high power, but the electrical circuit supplying it starts with fuses. Never skip the fuses! Check these locations:

  1. Under Dash Fuse Panel: (Driver's side). Look for the fuse labeled "ECM" (Engine Control Module), "PCM," or "EEC." This fuse powers the truck's computer and is often critical for sending the signal to activate the fuel pump relay. A blown ECM/PCM/EEC fuse will usually prevent the relay from clicking and the pump from running.
  2. Cavity Fuse: Within the same under-dash fuse panel, specifically look for the fuse labeled "Fuel Pump" or "FP." Crucially, on many 1991 F150s, this fuse does NOT power the fuel pump motor directly. It often powers the fuel pump monitor circuit inside the computer. A blown FP fuse can still cause starting problems related to the computer's control, even though power isn't directly cut to the pump. However, if it's blown, replace it.
  3. Under-Hood Fuse Panel (If Equipped): Quickly scan for any fuel pump related fuses labeled "FP" or "Fuel Pump" or PCM/ECM/EEC. Replace any blown ones with the correct amperage rating.

Don't Forget the Inertia Safety Switch!

The 1991 Ford F150 features an inertia shutoff switch as a safety device. Its purpose is to cut power to the fuel pump automatically in the event of a collision to prevent fuel-fed fires. However, a sudden jolt (like hitting a large pothole very hard) or even an electrical fault can sometimes trigger this switch mistakenly. Knowing where it is and how to reset it is vital.

  • Location: Almost universally on the 1991 F150, the inertia switch is located on the passenger side of the vehicle, low on the firewall, above and slightly towards the engine from the passenger's feet.
    • Specifically: Open the passenger door. Look towards the front, low on the firewall where it meets the floor pan or kick panel. You will see a small, square-ish plastic module (roughly 1.5-2 inches square) mounted with the wiring plug on the bottom and a prominent red reset button sticking out of the top.
  • Resetting: If the inertia switch has been triggered, the red button will be visibly popped up. To reset it, simply press the red button firmly straight down until it clicks and sits flush again. Listen for the fuel pump relay click and pump whine when you turn the key to "Run" after resetting. Always check this switch if you suspect a no-fuel problem! It's a quick and free potential fix.

Troubleshooting Steps When You Suspect the Relay

Armed with the location, here’s a practical sequence when you have a no-start condition and suspect fuel delivery:

  1. Listen for the Pump: Turn ignition to "Run". Do you hear the pump prime for 1-3 seconds? If YES, the relay is working initially.
  2. Check Fuses: Visually inspect the EEC/PCM fuse and the Fuel Pump fuse in the under-dash panel. Replace any blown fuses (but understand why it blew later).
  3. Locate the Inertia Switch: Find it on the passenger kick panel/firewall. Press the red reset button if it's popped up.
  4. Locate the Relay: Go to the passenger firewall relay box. Remove the cover.
  5. Listen for Click: Have a helper turn the key to "Run" while you listen/feel for the relay clicking in the box (the brown FP relay).
  6. Perform the Swap Test (if possible): Safely swap the identical brown FP relay with the yellow EEC relay. Turn key to "Run". Did the pump start working? If YES, replace the brown relay.
  7. Check Power: If still no luck, use a multimeter (or test light if careful) to verify:
    • Constant Battery Power (Usually thick Red/Light Blue wire) at the relay socket.
    • Ignition Switch Signal (trigger power - Often Tan/Yellow wire) at the socket when key is turned to "Run".
    • Ground (Usually Black/White or Black wire) at the socket.
    • Output to Fuel Pump (Usually Dark Green/Yellow wire) at the socket during the prime cycle.
  8. Direct Jumper Test (Advanced - Use Extreme Caution): Only proceed if you understand the risks. Temporarily use a fused jumper wire (or a dedicated relay bypass tool) ONLY between the constant Battery Power socket terminal and the Output to Fuel Pump socket terminal in the FP relay socket. This directly powers the pump, bypassing the relay.
    • MAJOR WARNING: Only do this briefly to test the pump! Never drive or run the engine with a bypass jumper installed. It removes critical safety features (like the inertia switch and relay control), risks electrical fire if wiring is compromised, and keeps the pump running constantly.
    • If the pump runs with this jumper, it confirms power can reach the pump, pointing back to the relay or its control circuit (fuse, wiring, computer, inertia switch). If the pump doesn't run, it points to problems after the relay/power distribution (pump motor, pump wiring, inertia switch, ground).

When to Replace the Relay

Replacing the fuel pump relay is inexpensive and easy. Do it if:

  • It conclusively fails the swap test.
  • It shows signs of physical damage (melting, cracks, corrosion).
  • Testing shows it's not closing contacts when triggered.
  • You've ruled out fuses, the inertia switch, and have intermittent pump operation (a classic sign of failing relay contacts).

Always replace with an identical relay with the same manufacturer part number if possible (Motorcraft is the OEM brand). A standard automotive five-pin relay matching the original specs (contact rating, pinout) will also work. Plug it into the socket securely.

Beyond the Relay: Other Fuel Pump System Culprits

Finding the relay is the start, but it's not always the end. If you've checked the relay, fuses, and inertia switch and the pump still isn't running reliably, consider:

  1. The Fuel Pump Itself: After decades, the pump motor can wear out or fail.
  2. Fuel Pump Wiring: Corrosion, damage, or breaks in the wiring harness between the relay and the pump (especially near the fuel tank or frame rails) can interrupt power.
  3. Bad Grounds: A poor ground connection for the pump or the relay circuit can cause failure. Check and clean ground points.
  4. Failing Computer (EEC-IV Module): While less common than relay failure, the computer supplies the control signal to activate the relay. Faults elsewhere in the engine control system can prevent this.
  5. Ignition Switch Problems: A worn ignition switch might not send the "Run" power signal to the computer/relay correctly.
  6. Fuel Filter: An extremely clogged fuel filter won't stop the pump from running, but it can mimic symptoms if severe and should be part of regular maintenance. Listen to see if the pump sounds labored or straining.
  7. Low Fuel Pressure (Even if Pump Runs): A weak pump, clogged filter, or bad regulator can cause low pressure, leading to hard starts or poor performance even if the relay and pump are operating.

Conclusion: Knowledge is Power (and Fuel Delivery)

Knowing precisely where to find the 1991 Ford F150 fuel pump relay – right on the passenger side firewall near the brake booster under that small plastic cover – empowers you to tackle one of the most common causes of a sudden no-start condition quickly and effectively. Remember the key players: the brown relay, the EEC fuse, and that critically important inertia reset button on the passenger side floor.

Start with the simple checks: listen for the pump prime, reset the inertia switch, swap the relay, and check the fuses. Often, the solution is surprisingly straightforward and costs less than $20 for a new relay. Mastering this location gives you the insight to get your dependable '91 Ford F150 back on the road, proving yet again why this generation of truck remains a favorite among those who fix things themselves. If the relay proves fine, you're now armed with the knowledge to systematically investigate the rest of the fuel delivery system with confidence.

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