1999 Ford Mustang Fuel Pump Relay: Your Complete Guide to Diagnosis, Location, and Replacement

If your 1999 Ford Mustang cranks but won't start, has sputtering power under acceleration, or exhibits intermittent starting issues, a failing fuel pump relay is among the most probable and critical culprits. This small, inexpensive electrical component acts as a vital switch, controlling power to your fuel pump. Ignoring relay problems leads directly to a stranded Mustang. Replacing a faulty fuel pump relay is a straightforward DIY repair for most owners. This guide provides exhaustive, practical information to definitively identify your 1999 Mustang's fuel pump relay location, accurately diagnose problems, perform reliable testing, choose the right replacement, and complete the installation correctly, ensuring your Mustang runs reliably again.

Understanding the Fuel Pump Relay's Crucial Role

The fuel pump relay is fundamentally an electronically controlled switch. Its primary job is to handle the significant electrical current required by the fuel pump motor. When you turn the ignition key to the "Run" position, the Powertrain Control Module (PCM) sends a relatively small control signal to the relay. This signal energizes an electromagnet within the relay, which physically closes a set of high-current electrical contacts. Only when these contacts close does battery voltage travel through the relay and reach the fuel pump, powering it on to pressurize the fuel system. Once the engine starts and the PCM sees a running signal, it typically maintains the relay closed. Turning the key off removes the control signal, the relay contacts open, and the fuel pump loses power. This design protects the ignition switch from handling the pump's heavy current load and allows the PCM precise control over pump operation.

Why the 1999 Mustang Relay Fails: Failure Modes and Symptoms

Fuel pump relays are electromechanical components with moving parts and electrical circuits, making them susceptible to wear and environmental factors common under the hood:

  1. Internal Contact Wear/Pitting: The primary failure mode. The high-current contacts inside the relay arc and pit over thousands of cycles. Eventually, they fail to make a solid connection, leading to intermittent power loss to the fuel pump.
  2. Burned/Melted Contacts: Sustained high current flow or a partially stuck relay can generate excessive heat, melting plastic housings or solder joints.
  3. Coil Failure: The electromagnet coil can burn out due to age, voltage spikes, or manufacturing defects, preventing the relay from energizing at all.
  4. Corrosion: Moisture ingress can corrode terminal pins and internal components, increasing resistance or causing open circuits.
  5. Sticking: Contacts can weld shut due to arcing, causing the fuel pump to run continuously even with the key off (draining the battery and posing a potential risk). Contacts can also stick open.

Symptoms specific to a failing fuel pump relay in your 1999 Mustang manifest as fuel starvation:

  • Engine Cranks But Won't Start: The most classic symptom. The starter turns the engine over normally, but no fuel is delivered.
  • Intermittent Starting Problems: The car starts fine sometimes, fails at other times, especially when the engine or relay is hot.
  • Engine Stall While Driving: Sudden loss of power and engine shutoff, often while under load or at speed.
  • Sputtering or Hesitation Under Load: Intermittent relay cutting power causes brief losses of fuel pressure, especially noticeable during acceleration.
  • No Fuel Pump Prime Noise: When you turn the key to "Run" (before cranking), you should hear a distinct whirring or buzzing sound from the rear of the car (the fuel pump priming) for 1-2 seconds. Complete silence is a strong indicator of relay, fuse, or pump failure.
  • Fuel Pump Runs Continuously: Less common, but if the relay contacts weld shut, you'll hear the pump run constantly, even with the key removed, leading to a dead battery overnight.

Precise Location: Finding the Relay Box in Your 1999 Mustang

For the 1999 model year, Ford located the central power distribution box, housing all major fuses and relays, in the engine compartment, on the driver's side. The specific identification and access procedure is crucial:

  1. Open the Hood: Securely prop the hood.
  2. Locate the Box: Stand on the driver's side, looking near the rear of the engine bay, close to the firewall (the vertical wall separating the engine from the passenger compartment). You will see a large, rectangular, usually black plastic box with a lid secured by clips or simple latches.
  3. Identify the Cover: The lid is clearly marked. Look for labels like "POWER DISTRIBUTION BOX," "FUSE/RELAY CENTER," or similar phrasing. Most have an under-hood fuse diagram sticker on the lid itself.
  4. Remove the Cover: Release the securing clips or latches (squeeze and lift) and carefully remove the cover. Set it aside safely.
  5. Understand the Layout: Inside, you'll find an array of fuses (mostly smaller, multi-colored plastic) and several larger, usually cube-shaped relays plugged into sockets. The specific arrangement can vary slightly depending on whether you have a V6 or V8 engine and optional equipment. DO NOT RELY SOLELY ON MEMORY OR GENERAL DIAGRAMS. FIND THE SPECIFIC MARKING.
  6. Locate the Fuel Pump Relay: This is the critical step. Examine the underside of the lid you just removed. There must be a detailed diagram listing every fuse and relay position by function and rating. Physically locate the socket corresponding to the "FUEL PUMP" relay based on this diagram. Common locations within the box, but always verify:
    • Near the center or toward the front of the box.
    • Often close to other major relays like the EEC (PCM) Relay.
    • The relay socket might be labeled "F/PMP" or similar on the box itself adjacent to the socket, but the lid diagram is the authoritative source. The relay itself may have a factory marking, but this is not reliable as replacements often lack it. Match the position on the diagram. It's typically a standard ISO Mini or ISO Micro relay form factor.

Accurate Diagnosis: Confirming the Relay is the Problem

Never replace parts solely on suspicion. Proper diagnosis saves time and money:

  1. Initial Checks: Before blaming the relay, rule out the simplest causes:
    • Fuel Level: Is there sufficient fuel in the tank?
    • Inertia Switch: Located in the trunk (usually passenger side kick panel) or passenger footwell. A bump can trigger it, cutting fuel pump power. Check if its button is popped up; press it down firmly to reset.
    • Main Fuel Pump Fuse: Consult the same under-hood fuse diagram. Find the fuse (typically 15A or 20A, but check diagram!) dedicated to the fuel pump circuit, often labeled "F/PMP" or "FUEL PUMP." Visually inspect it for a broken filament. Crucially, test it for continuity with a multimeter even if it looks intact. Replace blown fuses immediately and investigate why it blew (could indicate a pump short).
  2. Listen for the Fuel Pump:
    • Turn the ignition key to the "Run" position (do not crank the engine). Immediately listen near the rear of the car, particularly around the fuel tank. You should hear a distinct whirring/buzzing for 1-2 seconds. NO SOUND points heavily towards a problem in the control circuit: fuse, relay, inertia switch, pump ground, wiring fault, or dead pump.
  3. Physical Relay Inspection: With the ignition OFF, locate the relay as per the diagram. Carefully pull it straight out of its socket. Examine:
    • Housing: Cracks, melted plastic, or signs of overheating?
    • Terminals: Severe corrosion or burning?
  4. The Swap Test (Effective Preliminary Check):
    • Important: Find another relay in the power distribution box with the same part number and/or same terminal configuration as the suspected fuel pump relay. Common candidates are the Horn Relay or A/C Compressor Relay (again, verify form factor and rating via diagram/part number). Ensure these non-essential systems aren't needed for your test.
    • Swap the suspect fuel pump relay with the known good relay of the same type (e.g., put horn relay into fuel pump socket, fuel pump relay into horn socket).
    • Turn the key to "Run". Does the fuel pump now prime? Try starting the car. If it starts/runs, your original relay is bad. Try activating the system you borrowed the relay from (e.g., honk horn). If the swapped relay works there too, it confirms the diagnosis. If the problem moves (e.g., horn doesn't work with old relay), definitive proof. Warning: If the car still doesn't start and the borrowed system works, the problem is likely elsewhere (pump, wiring, etc.). Avoid swapping with critical relays like EEC/PCM.
  5. Multimeter Testing (Definitive Proof):
    • Equipment: A basic digital multimeter (DMM) set to measure DC voltage (20V range) and resistance (Ohms).
    • Identifying Terminals: Find the relay socket without the relay plugged in. Using the lid diagram or markings on the socket itself:
      • Identify the Control Circuit terminals: 85 (Ground Control) & 86 (Switch/Power Control). Typically, the PCM grounds Terminal 85 internally and sends +12V to Terminal 86 to trigger the relay.
      • Identify the Load Circuit terminals: 30 (Power In - Always Hot from battery via fuse) & 87 (Switched Power Out - To fuel pump/inertia switch). On a 4-pin relay, that's all. Some relays have 5 pins (87 & 87a) - unused pin is usually blanked off. Ensure you know the type (4 or 5 pin).
    • Testing Control Circuit Coil:
      • Measure resistance across Terminals 85 & 86.
      • A good relay coil usually reads between 50 to 150 Ohms. Near zero Ohms indicates a short. Infinite resistance (OL) indicates an open coil - relay dead.
    • Testing Load Circuit Contacts:
      • Relay OUT of socket. Set DMM to Ohms.
      • Measure resistance between Terminals 30 & 87.
      • Should be INFINITE (OL) resistance (contacts open). Tap the relay – resistance should remain infinite. Any reading other than OL (especially when cold) indicates failed contacts (stuck closed or severely contaminated).
    • Testing Function Under Load Control (Best Test):
      • Plug the relay BACK INTO its socket firmly.
      • Set DMM to DC Volts (20V scale).
      • Identify Terminal 87 in the socket. You may need to back-probe the wire using a thin pin or use probe tips carefully contacting the metal connector inside the socket plastic.
      • Ground the Black (-) DMM Lead: A clean bolt on the engine or chassis.
      • Key OFF: Probe Terminal 87. Should read Near 0 Volts (no power).
      • Turn Key to "RUN": Immediately probe Terminal 87. It should jump to Battery Voltage (Approx. 12V+) for 1-2 seconds during prime, then drop back to 0V if the engine isn't cranked. If voltage does NOT appear at 87 when commanded, and the fuse/control circuit are good, the relay contacts are failing.
    • Testing Control Signal at Socket (Verifies PCM Command):
      • Relay OUT of socket. Set DMM to DC Volts.
      • Key OFF: Probe Socket Terminal 85 (relative to ground). Should be ~0V. Probe Socket Terminal 86 (relative to ground). Should be ~0V.
      • Turn Key to "RUN": Immediately:
        • Probe Socket Terminal 85 (relative to ground). Should remain near 0V (PCM grounding it).
        • Probe Socket Terminal 86 (relative to ground). Should read Battery Voltage (Approx. 12V+) for 1-2 seconds. This confirms the PCM is sending the trigger signal. Lack of voltage here points to PCM, wiring, or fuse issue feeding terminal 86.

Selecting the Correct Replacement Relay

Using the correct relay is vital for reliability and safety:

  1. Remove the Old Relay: Use the diagram to pull out the faulty one.
  2. Critical Matching Criteria:
    • Form Factor/Shape: Must fit the socket (ISO Mini or ISO Micro are common). Physical fit is non-negotiable.
    • Terminal Layout (Pinout): The position of pins 30, 85, 86, 87 must exactly match the socket wiring. Compare the old relay's bottom pattern visually to the new one. DO NOT ASSUME! Use the old one as a pattern.
    • Current Rating: Typically 20-30 Amps for a 1999 Mustang fuel pump. This is critical; too low causes premature failure, too high is unsafe. It's usually printed/stamped on the relay housing (e.g., "20A" or "30A"). Match this rating precisely.
    • Voltage Rating: 12V DC is standard.
  3. Reputable Sources:
    • Ford Dealership (OEM): Guaranteed match, but highest cost. Part numbers like F5TZ-9345-A (or variants) were common, but ALWAYS cross-reference with your VIN or provide the old relay. Get the Motorcraft part number (e.g., RY13 - Check for supersessions!).
    • Auto Parts Stores: Standard replacement relays are widely available. Bring your old relay! Ask for:
      • Standard ISO relay matching form factor, pinout, and rating (e.g., 20A/30A, 12VDC). Brand examples: Bosch, Tyco/CPC, Omron, Standard Motor Products (SMP), BWD.
      • Ask for the fuel pump relay specifically for a 1999 Ford Mustang (give engine size - 3.8L V6 or 4.6L V8). Double-check physical match and rating.
    • Online Retailers: Confirm specifications rigorously before ordering.
  4. Avoid Bargain Relays: Extremely cheap relays often use inferior contacts prone to welding and premature failure. Stick with reputable brands.

Step-by-Step Replacement Procedure

  1. Ensure Safety: Work in a well-lit, ventilated area. Key OFF. Disconnect the NEGATIVE (-) battery cable. Allow systems (like airbags) a few minutes to power down.
  2. Locate the Relay Box: Hood open, driver's side near firewall.
  3. Remove Power Distribution Box Cover: Release clips/latches, lift off.
  4. Identify Fuel Pump Relay Socket: Use the under-hood fuse diagram on the lid. Locate the exact socket.
  5. Remove Old Relay: Grasp it firmly near its base (avoid pulling wires) and pull it straight up and out of its socket. It should release with moderate force. Avoid twisting.
  6. Prepare New Relay: Remove from packaging. Visually compare TERMINAL LAYOUT (pin configuration) and overall SHAPE to the old relay. They MUST match exactly. Confirm current/voltage rating. Inspect for damage.
  7. Install New Relay: Orient it precisely as the old one was installed (usually the terminals only plug in one way due to keying). Align it over the socket and press firmly and evenly straight down until it seats fully. You should hear/feel a positive click. Do not force it!
  8. Reattach Power: Reconnect the Negative (-) battery cable. Ensure tight connection.
  9. Test the System:
    • Turn the ignition key to the "Run" position. Listen CAREFULLY for the fuel pump priming whine/hum (1-2 seconds) from the rear of the car. This sound confirms the relay triggered.
    • Attempt to start the engine. It should start normally if the relay was the sole issue.
  10. Reinstall the Power Distribution Box Cover: Snap it securely into place.
  11. Final Check: Ensure smooth engine operation and drive the car to verify no sputtering/stalling symptoms return.

Preventative Maintenance and Troubleshooting Tips

  • Buy Quality: Investing in a well-known brand relay (Motorcraft, Bosch, SMP, BWD) significantly increases longevity compared to generic discount parts. Keep the old one as an emergency spare if it’s swapped out preventatively.
  • Visual Inspection: Whenever you're under the hood for service (oil change, etc.), quickly pop the power distribution box cover and visually inspect all fuses and relays, including the fuel pump relay, looking for any obvious damage, melting, or corrosion. Check for any loose relays – firmly press them down.
  • Connector Care: When removing or installing relays, handle carefully. Never force it. If a socket feels loose, inspect for broken locking tabs or loose wires (unlikely but possible). Ensure connectors are clean and corrosion-free.
  • Understanding Root Causes: If you experience repeated relay failure shortly after replacement, it signals a deeper problem:
    • Fuse Blowing? Indicates an electrical short circuit downstream (fuel pump motor failing, damaged wiring near tank/inertia switch).
    • Relay Contacts Melted/Burned? Points to severely excessive current draw (failing fuel pump pulling too many amps).
    • Relay Coil Burned Out? Could indicate a wiring issue feeding terminal 86 (voltage spike/short) or a rare PCM driver problem.
  • Voltage Checks: If diagnosis is tricky, systematically verifying voltage at key points (battery voltage at terminal 30 socket always, voltage at terminal 86 socket during prime, voltage at terminal 87 socket during prime) will isolate the failing component section (control side vs. load side vs. pump circuit).
  • Reference Documentation: Keep or download the 1999 Mustang's official wiring diagrams for the fuel pump circuit. These are invaluable for complex electrical troubleshooting.

(Conclusion)
The 1999 Ford Mustang fuel pump relay, while small and inexpensive, is an indispensable component for delivering fuel to the engine. Failure results in frustrating and potentially dangerous no-start or stalling situations. By understanding the relay's role, precisely locating it using the under-hood power distribution box diagram, performing methodical diagnostics (including the swap test and multimeter verification), and carefully selecting and installing the correct replacement, you can reliably restore your Mustang's performance. For most owners, this repair is well within the scope of achievable DIY maintenance, ensuring your pony keeps running strong.

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