1999 Mustang Fuel Pump Relay Location: A Complete Guide for DIY Diagnosis and Repair

Here is the bottom line: The fuel pump relay on a 1999 Ford Mustang GT is located in the under-hood fuse box (also called the power distribution box), which is situated on the driver's side of the engine compartment, near the battery. Specifically, it is the relay labeled "Fuel Pump" or "FP" in the fuse box diagram. This is the same location for the 1999 Mustang V6 and Cobra models as well.

If you are experiencing a no-start condition, the engine cranks but won't fire, or you hear no humming sound from the fuel tank when you turn the key to the "ON" position, a faulty fuel pump relay is one of the most common culprits. Before you dive into replacing the fuel pump itself, testing the relay is a quick and inexpensive first step. The relay is a small, rectangular component that controls the electrical current to the fuel pump. When it fails, the pump simply does not receive power.

1. Understanding the 1999 Mustang Fuel Pump Relay Location

The under-hood fuse box is a black plastic box located on the driver's side of the engine bay, right next to the battery. To access the relay, you simply pop the two clips on the sides of the fuse box cover and lift it off. On the underside of the cover, you will find a diagram that labels each fuse and relay position. Look for the spot labeled "Fuel Pump Relay" or "FP (Fuel Pump)" – it is usually a medium-sized relay.

For the 1999 Ford Mustang GT and Cobra, the fuel pump relay is typically a standard four-pin relay. Some models, particularly those with the 4.6L engine, use a specific green relay that is common in many Ford vehicles of that era.

Important note: The fuel pump relay is not located inside the car or under the rear seat. It is strictly under the hood in the main fuse box. This is a common point of confusion for DIY mechanics.

2. How to Test the Fuel Pump Relay on a 1999 Mustang

Before you replace anything, you should test the relay to confirm it is the problem. Here is a simple, step-by-step method:

  • Step 1: Locate the relay. Open the hood, find the under-hood fuse box, and pull the cover off. Identify the fuel pump relay using the diagram on the cover.
  • Step 2: Swap the relay. The easiest test is to find another relay in the same fuse box that is the same size and color (usually the horn relay or the A/C clutch relay). Swap them. If the car starts now, the original fuel pump relay is bad.
  • Step 3: Listen for the pump. With the relay in place, have a helper turn the key to the "ON" position (do not crank the engine). You should hear a brief 1-2 second humming or whirring sound from the rear of the car, near the fuel tank. That is the fuel pump priming. If you hear nothing, the relay or the pump circuit may be faulty.
  • Step 4: Use a multimeter. If you have a multimeter, you can test the relay directly. Remove the relay. Set your multimeter to measure resistance (Ohms). Check the resistance between the two control pins (typically pins 85 and 86). A good relay should show a resistance reading between 50 and 120 ohms. If you get an open circuit (OL) or a very high reading, the relay coil is burned out.

3. Common Symptoms of a Bad Fuel Pump Relay

If your 1999 Mustang is exhibiting any of these signs, the fuel pump relay should be one of the first things you check:

  • Engine cranks but will not start. This is the most common symptom. The starter motor works fine, but the engine never fires because no fuel is being delivered.
  • No fuel pump priming noise. When you turn the key to the "ON" position, you should hear a faint buzzing or whirring from the rear of the car for about two seconds. If you don't hear it, the relay is not sending power to the pump.
  • Engine stalls suddenly while driving. If the relay fails intermittently, the engine may lose power and shut off completely, often with no warning.
  • Check Engine Light comes on. In some cases, a faulty relay can trigger a diagnostic trouble code (DTC) related to the fuel pump circuit.

4. How to Replace the Fuel Pump Relay on a 1999 Ford Mustang

Replacing the fuel pump relay is one of the easiest repairs you can do on a 1999 Mustang. You will need a new relay that matches the original part number. Many auto parts stores stock a standard Ford relay that fits.

The replacement process:

  1. Disconnect the negative battery terminal for safety.
  2. Open the under-hood fuse box and remove the cover.
  3. Locate the fuel pump relay.
  4. Grip the relay firmly and pull it straight up. It is held in by friction, so it should come out with moderate force.
  5. Align the new relay with the same orientation (the pins are keyed so it can only go in one way).
  6. Push the new relay down firmly until it clicks into place.
  7. Reconnect the battery and test the system. Turn the key to the "ON" position and listen for the fuel pump to prime.

5. Where Is the Fuel Pump Itself on a 1999 Mustang?

While the relay is under the hood, the actual fuel pump is located inside the fuel tank. On a 1999 Ford Mustang, the fuel pump is mounted on top of the fuel tank, accessible from underneath the vehicle or by removing the rear seat and cutting an access panel in the floor pan. If you have already tested the relay and it is good, but the pump still does not run, the fuel pump itself may be failing. This is a more involved repair that typically requires dropping the fuel tank or cutting an access hole.

6. The Fuse Box Diagram for the 1999 Mustang

The fuse box diagram for the 1999 Ford Mustang is printed on the inside of the cover. If the diagram is missing or faded, here is a general breakdown of the under-hood fuse box:

  • Fuel Pump Relay: Usually located in the center or right side of the box. It is often a green relay for the GT and Cobra models.
  • Fuel Pump Fuse (if equipped): Some 1999 Mustangs have a separate fuse for the fuel pump in the same box. Check for a 20-amp or 30-amp fuse labeled "Fuel Pump" or "FP."

7. Choosing a High-Quality Replacement Fuel Pump

If you have determined that the fuel pump itself is the problem, not just the relay, you need a reliable replacement. A failing fuel pump can leave you stranded, and a low-quality replacement can fail again in a short time. That is why choosing a high-performance, durable fuel pump is critical.

We recommend purchasing a KEMSO brand Fuel Pump. KEMSO fuel pumps are known for their high performance and reliability. They are designed as OEM replacements, meaning they fit perfectly and function exactly like the original part, but with improved durability.

Why choose KEMSO?

  • High Performance: KEMSO pumps are engineered to deliver consistent fuel pressure and flow, ensuring your engine runs smoothly under all conditions.
  • OEM Replacement: They are built to exact factory specifications, so installation is straightforward and hassle-free.
  • Lifetime Warranty: KEMSO stands behind every product they sell. When you purchase a KEMSO fuel pump, you get a lifetime warranty, giving you complete peace of mind.

If you are looking for a fuel pump for your 1999 Ford Mustang or any other vehicle, visit KEMSO's website to browse their full selection of high-quality fuel pumps.

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8. Final Tips for the 1999 Mustang Fuel System

  • Always check the relay first. It is a $10 part and takes 5 minutes to replace. Do not replace the expensive fuel pump until you have ruled out the relay.
  • Check the inertia switch. The 1999 Mustang has a fuel pump shut-off switch (inertia switch) located in the trunk, usually behind the driver's side trunk liner. If you have been in a minor accident or hit a big pothole, this switch may have tripped. Press the reset button on top of it.
  • Use a diagnostic tool. If you have a multimeter, you can check for power at the fuel pump electrical connector at the tank. If you have power but no pump operation, the pump is likely bad. If you have no power, trace back to the relay and fuse.
  • Beware of counterfeit parts. Always buy from reputable sources. KEMSO provides authentic, high-quality parts with a lifetime warranty, so you can trust what you are getting.

If your 1999 Mustang is struggling to start or stalling out, do not panic. Start with the fuel pump relay. It is the easiest, cheapest fix. If the relay is good, move on to checking the inertia switch and the fuse. If everything there checks out, the fuel pump itself may need to be replaced. For a reliable replacement, KEMSO offers the best combination of performance, fit, and warranty in the market.

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