1999 Ford Mustang Fuel Pump Relay Location: Find, Test & Replace

Finding the fuel pump relay in your 1999 Ford Mustang is quick and straightforward: it’s located in the Central Junction Box (CJB), also known as the main interior fuse panel, under the dashboard just behind the lower edge of the instrument cluster, near the driver's left knee. The fuel pump relay itself is one of several identical black plastic relays in this box, typically identified as Relay Position #10. Its fuse, Fuse #14 (15A), shares the same panel.

Forget complicated searches under the hood or in the trunk. The fuel pump relay in your fourth-generation (SN-95) 1999 Ford Mustang resides inside the passenger compartment. Knowing its exact location is critical when troubleshooting a non-starting engine due to a silent fuel pump.

Understanding the Central Junction Box (CJB)

The Central Junction Box is the primary hub for many of the Mustang's critical electrical components. Positioned low on the driver's side dashboard, this panel sits almost directly behind the lower part of the instrument cluster bezel, accessible near your left knee while seated. It's enclosed by a removable plastic cover featuring finger grips. To access the fuses and relays inside, you simply pull this cover straight down towards the floor; no tools are typically needed.

Identifying the Fuel Pump Relay

Once you've removed the cover, you'll see a rectangular array consisting of multiple fuses and several identical-looking black plastic relays plugged into sockets. These relays generally look the same:

  • Physical Appearance: Black square plastic cube-shaped components.
  • Terminals: Four metal terminals underneath correspond to four pins in the socket (usually arranged in a square pattern).
  • Standardized Design: They follow a common automotive relay form factor.

To positively identify the correct relay specifically for the fuel pump, you need to reference the fuse box diagram. This diagram is usually printed on the inside surface of the cover you just removed or sometimes on a sticker adhered to the main fuse panel itself facing upwards (though wear and tear might make this difficult after 25 years). Locate the diagram and look for:

  • The Relay Section: The diagram will have a section clearly labeled "Relays" or show the relay positions.
  • Position #10: In the vast majority of 1999 Mustangs (V6 and V8 models), the fuel pump relay is designated as Relay 10.
  • Labeling: It might explicitly say "Fuel Pump," "FP," "PWR," or sometimes just "10" next to the relay symbol in position 10. The crucial factor is identifying Relay #10 based on the panel legend for your specific car.
  • Associated Fuse: Note that the fuse for the fuel pump circuit is almost always Fuse #14, a 15 Amp fuse located elsewhere within the same CJB panel. It protects the relay's switching circuit. Checking this fuse should always be part of the initial troubleshooting step if you suspect relay issues.

Accessing and Removing the Relay

  1. Key Off / Vehicle Off: Ensure the ignition switch is completely off. Remove the key.
  2. Access CJB: Locate the CJB panel as described above. Use your fingers to grip the access cover firmly and pull it straight down towards the driver's footwell floor. Set the cover aside.
  3. Find Relay #10: Using the diagram on the cover or panel, visually confirm the position of Relay 10 within the array of relays. The sockets are typically numbered faintly on the plastic panel.
  4. Grip Firmly: Place your thumb and index finger on the top sides of the Relay #10. Relay positions are usually quite snug to prevent vibration-related issues.
  5. Pull Straight Out: Apply firm, steady pressure and pull the relay directly out of its socket. Avoid excessive wiggling or twisting, as this could damage the relay terminals or the socket pins. The relay should come free smoothly.
  6. Inspect: With the relay removed, visually inspect the terminals on both the relay and inside the socket for any signs of corrosion, melting, burning, or bent pins.

The Critical Role of the Fuel Pump Relay

Simply knowing its location is useful, but understanding why it matters is key. The fuel pump requires significant electrical current to operate – far more than the low-current signals sent from the Powertrain Control Module (PCM). The fuel pump relay acts as a heavy-duty remote-controlled switch:

  1. PCM Command: When the ignition key is first turned to "Run" or "Start," the PCM sends a small ground signal to the relay's switching coil circuit.
  2. Internal Magnet: This signal energizes an electromagnet inside the relay.
  3. High Current Flow: The energized magnet pulls down internal metal contacts, closing the relay's high-current circuit. This connects the power source (through a large fuse like the battery junction box fuse) directly to the fuel pump motor.
  4. Pump Activation: With high current flowing, the fuel pump energizes, pressurizing the fuel rail so the engine can start.
  5. PCM Control: The PCM keeps the relay energized as long as the engine is running or cranking. It typically de-energizes the relay within a few seconds after turning the ignition off.

A faulty relay prevents this crucial high-current connection, leaving your fuel pump silent and your engine unable to start.

Symptoms of a Failing Fuel Pump Relay

When the fuel pump relay malfunctions, it usually presents clear symptoms:

  • No Start Condition: The most common sign. The engine cranks normally but refuses to start. There is no sound of the fuel pump priming when the key is turned to "RUN" (before cranking).
  • Intermittent No Start: The problem might occur randomly. The car starts fine sometimes, then fails to start on other occasions without warning. This is often linked to failing internal relay contacts.
  • Silent Fuel Pump: As mentioned, you should hear a distinct whirring or buzzing sound for a few seconds when you turn the ignition key to "ON" (without cranking). This is the fuel pump priming the system. Absence of this sound points strongly to a power interruption – often at the relay or fuse.
  • Stalling While Driving: Less common than a total no-start, but possible. If the relay contacts suddenly open while driving, it immediately cuts power to the fuel pump, causing the engine to die as if the key was turned off.

Important Note: Many of these symptoms overlap with other problems: a completely dead fuel pump itself, blown fuses (especially Fuse #14 or the main high-current pump fuse in the Battery Junction Box under the hood), wiring harness damage, fuel pump driver module issues (GTP models), ignition switch problems, or PCM failures. Diagnosing the specific cause is crucial, not just replacing the relay.

Testing the Fuel Pump Relay

Locating the relay is the first step, but testing determines if it's truly faulty before replacement:

  • Swapping Test: The simplest method, assuming you identify identical relays performing less critical tasks (like the horn relay or blower motor relay). Swap Relay #10 with a known identical relay (using the diagram to find another relay with the same part number/configuration). Try starting the car. If the problem moves or resolves, the original fuel pump relay is likely bad. If the problem remains, the relay might be okay, or another identical relay swapped in might also be faulty.
  • Basic Bench Test (Jumper Wire Caution - For Experienced Users Only):
    1. Remove the fuel pump relay.
    2. Identify the relay's terminal pins. Consult the CJB cover diagram or relay marking (like "30," "87," "85," "86") which generally corresponds to:
      • 85 & 86: The coil control circuit (low-current).
      • 30: Permanent Battery Power Input (high-current).
      • 87: Switched Power Output to Fuel Pump (high-current).
    3. Carefully obtain a fused jumper wire or create one with appropriate gauge wire and an inline fuse (recommended). This bypasses the relay and directly applies power to the pump circuit very briefly ONLY for diagnostic purposes.
    4. Insert one end into the relay socket terminal known to be the fuel pump output (terminal #87, often labeled as such).
    5. Insert the other end into the relay socket terminal supplying constant battery power (terminal #30). BE ABSOLUTELY SURE YOU ARE CONNECTING TO 30 and 87 ONLY. Connecting power incorrectly can cause damage.
    6. Important: Turn the ignition key to "ON" (not start) after connecting the jumper. If the relay is faulty and the fuse/pump/ground are good, you should immediately hear the fuel pump turn on loudly and continuously until you turn the key off or remove the jumper. If you hear the pump run with the jumper, the relay or its control circuit (fuse #14, wiring to PCM) is faulty. If the pump doesn't run with the jumper, the problem lies elsewhere (main pump fuse under hood, pump ground, pump wiring, or the pump itself).
  • Multimeter Test (Ohms): Remove the relay. Set multimeter to Ohms (Ω). Test resistance across terminals 85 and 86 (the coil circuit). You should measure a resistance, typically between 50 and 100 ohms for most automotive relays. An "OL" (Open Loop) reading means the coil is broken; the relay is dead. A reading of 0 ohms indicates a shorted coil. Note: A good coil reading doesn't guarantee the main power contacts inside are functioning.
  • Multimeter Test (Continuity): With multimeter set to Continuity or Diode Test (emits a beep):
    1. Identify terminals 30 (power in) and 87 (power out). There should be NO continuity (no beep, "OL") between these pins when the relay is at rest.
    2. Apply 12 volts (using a bench power supply, a 9V battery, or car battery with leads) across terminals 85 (usually positive) and 86 (usually ground/negative). Observe polarity based on markings if available. You should hear and feel a distinct "click" as the relay energizes.
    3. While voltage is applied to 85/86, check continuity between terminals 30 and 87. Now there should be continuity (beep). Release power from 85/86; the continuity should disappear again (back to "OL").
  • Professional Diagnostic Tools: Advanced scan tools can monitor the fuel pump relay command signal from the PCM and potentially actuate the relay for testing.

Replacing a Faulty Relay

If testing confirms the relay is defective, replacement is straightforward:

  1. Identify Correct Relay: Ford specifies a certain relay part number, but standardized automotive ISO relays are common replacements. The original is likely a Ford Part F7VY-14B192-AA or equivalent (often listed under Motorcraft SW-5079). Your local auto parts store can cross-reference this or your VIN. Ensure the new relay matches the configuration (4 terminals, usually ISO standard) and the amperage rating (typically 20A or 30A) of the old unit.
  2. Purchase: Obtain the new relay.
  3. Key Off: Ensure ignition is off.
  4. Confirm Location: Double-check you are replacing Relay #10 in the CJB.
  5. Align Terminals: Orient the new relay so its terminals align with the socket holes. Relays typically have keyed tabs to prevent incorrect insertion.
  6. Firm Pressure: Push the relay straight down firmly into its socket until it seats completely. You should feel or hear a click as it locks in place.
  7. Test Function: Turn the ignition key to "ON." You should hear the fuel pump prime for 1-3 seconds. If it does, this indicates the relay is functioning correctly at this point.
  8. Reinstall Cover: Snap the CJB access cover back into place.
  9. Try Starting: Attempt to start the engine. If the relay was the sole issue, the car should start normally.

Essential Safety Precautions

  • Battery Disconnection: While not always mandatory for relay access/testing, disconnecting the negative battery terminal adds a safety layer, preventing accidental shorts or sparks. Do this before extensive testing involving jumpers or meters.
  • Fuel System Pressure: If performing testing that involves direct connections or potentially running the fuel pump, be aware of high fuel pressure. Releasing pressure correctly via the Schrader valve on the fuel rail is necessary before disconnecting any fuel lines to prevent injury and fuel spray. Consult a service manual for proper procedures.
  • Electrical Shorts: Use extreme caution when working with jumper wires or probes near the fuse box. Ensure tools don't accidentally bridge different terminals. An inline fuse on any temporary jumper wire is highly recommended.
  • Avoid Sparks: Keep sparks, flames, and cigarettes away from any vehicle electrical work and especially any fuel system testing area.
  • Correct Tools: Use appropriate automotive electrical testing tools.

Beyond the Relay: Other Common Culprits

If the relay tests good, the problem lies elsewhere. Systematically check these related components:

  1. Fuel Pump Fuse (CJB Fuse #14): Check this essential 15A fuse first. It powers the relay's control circuit from the ignition switch to the PCM. A blown fuse indicates an overload in that circuit.
  2. Fuel Pump Inertia Switch: Designed to shut off fuel power in a collision. It can sometimes trip accidentally from a severe bump. Located in the trunk (hatchback models) or behind the kick panel on a coupe's passenger side. Push the reset button firmly if tripped.
  3. Main Fuel Pump Power Fuse: Located in the Battery Junction Box (BJB) under the hood (often labeled as fuse "R" or "C" for 30A fuse or similar). This fuse protects the high-current wire feeding the output side of the relay (terminal 30) and running to the pump. Check it visually or with a multimeter.
  4. Fuel Pump Ground: A poor ground connection for the pump or the relay coil can cause failure. Locate and clean the ground points (often near the fuel tank sender or on the body).
  5. Fuel Pump Wiring Harness: Inspect the wiring from the CJB relay out to the fuel pump (especially near the fuel tank) for chafing, damage, or corrosion. Check connections at the pump/sender unit and at the inertia switch.
  6. Fuel Pump Driver Module (FPDM - GTP Models): Some 1999 Mustang GTs with certain fuel pump configurations used an additional module to control pump speed. Its failure mimics relay failure.
  7. Fuel Pump Motor: The pump itself can fail mechanically or electrically. Confirming power at the pump connector when the relay should be active is the definitive test, requiring access to the fuel tank/sender.
  8. Ignition Switch: A worn ignition switch might not send the correct "Run" signal to the PCM or the fuse #14 circuit when the key is turned.
  9. PCM Failure: While less common, a faulty PCM module could fail to send the ground signal to activate the relay coil.

Conclusion

The fuel pump relay in your 1999 Ford Mustang is a critical yet easily accessed component. It’s definitively located within the Central Junction Box (CJB) under the driver's side dashboard, specifically in the slot labeled Relay Position #10 in the fuse box diagram. Knowing how to find it, remove it, and perform basic tests like swapping or jumping empowers you to diagnose common no-start scenarios efficiently. Remember that symptoms like a silent pump often point here, but always confirm with testing or consider other potential failures like fuses (CJB Fuse #14 and the main under-hood fuel pump fuse) or the inertia switch before replacing the pump itself. Accessing and servicing this relay requires minimal tools and knowledge of the correct CJB location and relay identification, making it a manageable DIY task for many Mustang owners looking to solve fuel delivery problems. Ensure safe electrical practices during testing.

Back to blog