1999 Mustang Fuel Pump Relay: Location, Testing, and Replacement Guide

Here is the bottom line: The 1999 Ford Mustang GT uses a returnless fuel system that relies on a Fuel Pump Driver Module (FPDM) rather than a traditional fuel pump relay. This module is located underneath the luggage compartment trim panel in the trunk, not in the under-hood fuse box where you might expect to find a standard relay . If your Mustang cranks but won't start, or stalls unexpectedly, the FPDM is often the first component to check before assuming the fuel pump itself has failed.

Understanding the 1999 Mustang Fuel System

The 1999 Ford Mustang GT features a returnless fuel system. This design is different from older Mustangs that used a traditional fuel pump relay. Instead of a simple relay that switches power on and off, the 1999 model uses a Fuel Pump Driver Module (FPDM) . This electronic module controls the voltage and speed of the fuel pump, allowing the pump to run at variable speeds based on engine demand. The FPDM is located underneath the luggage compartment trim panel in the trunk, on the driver's side near the taillight area . If you have a V6 model, the system is similar, but always verify with your specific vehicle's manual.

Where to Find the Fuel Pump Driver Module (FPDM)

Locating the FPDM is straightforward. Follow these steps:

  1. Open the trunk of your 1999 Mustang.
  2. Remove the luggage compartment trim panel on the driver's side. This panel covers the area behind the taillight and the side of the trunk.
  3. Look for a small black box with an electrical connector. This is the Fuel Pump Driver Module .
  4. The inertia switch (fuel pump shut-off switch) is also located in the same area, behind the trunk liner on the driver's side, near the driverside taillight. There is an access hole for it .

Before you start digging into the trunk, check the under-hood fuse box (Power Distribution Box) on the driver's side fender near the battery. Look for a 20 amp mini fuse labeled for the fuel pump (fuse #14) . A blown fuse is a common and easy fix. If the fuse is good, move on to the FPDM.

Testing the Fuel Pump Driver Module and Fuel Pump

If your Mustang cranks but won't start, or if it stalls while driving, the FPDM or the fuel pump itself could be the problem. Here is a simple testing procedure:

Step 1: Listen for the Fuel Pump

Turn the ignition key to the "ON" position (do not start the engine). You should hear a faint humming sound coming from the rear of the car, near the fuel tank. This sound indicates the fuel pump is priming the system. If you hear nothing, the pump is not getting power .

Step 2: Check the FPDM

If the pump is silent, test the FPDM. You can use an automotive circuit tester. With the key in the "RUN" position, check for power at the FPDM's electrical connector. If power is present at the module but not at the fuel pump, the FPDM is likely faulty. Replace the Fuel Pump Driver Module if power is not present .

Step 3: Test for Voltage at the Fuel Pump

If the FPDM tests good, the next step is to check the fuel pump itself. The fuel pump is located inside the fuel tank, so you will need to access the electrical connector under the car, in front of the fuel tank, near the fuel lines. Have a helper turn the key to the "START" position. Use a circuit tester to check for power at the connector. If power is present but the pump does not run, the pump is likely defective .

Step 4: Check Fuel Pressure

If the pump runs but the engine still struggles, check the fuel pressure. The threaded test port is located on the passenger side of the fuel rail, near the front of the engine. Attach a fuel pressure tester. The pressure should be at least 45 psi. If the pressure is below 45 psi, replace the fuel pump assembly .

Common Symptoms of a Failing Fuel Pump Relay (or FPDM)

Knowing the symptoms helps you diagnose the problem quickly. Here are the most common signs:

  • Engine cranks but will not start: This is the most obvious symptom. The engine turns over, but there is no fuel reaching the injectors .
  • Intermittent stalling: The car runs fine for a while, then suddenly dies without warning. This is a classic sign of a failing relay or FPDM .
  • No fuel pump noise: When you turn the key to "ON," you should hear the pump hum for a couple of seconds. If you hear nothing, the relay, FPDM, or pump is not working.
  • Hard starting: The engine takes longer than usual to start, especially after sitting for a while.
  • Loss of power under load: The car may hesitate or lose power when accelerating, climbing hills, or towing.

How to Replace the Fuel Pump Driver Module

Replacing the FPDM is a simple DIY job. Here is what you need to do:

  1. Disconnect the negative battery terminal. This is a critical safety step to prevent electrical shorts .
  2. Access the FPDM by removing the luggage compartment trim panel in the trunk.
  3. Unplug the electrical connector from the old module.
  4. Remove the old FPDM by unscrewing the mounting bolts or clips.
  5. Install the new module in the same position. Make sure it is securely mounted.
  6. Reconnect the electrical connector until it clicks into place.
  7. Reinstall the trim panel and reconnect the battery.
  8. Test the system by turning the key to "ON" and listening for the fuel pump to prime.

Important Safety Precautions

Working on a fuel system is dangerous. Always follow these safety rules:

  • Disconnect the battery before working on any electrical components .
  • Relieve fuel pressure before opening any fuel lines. Pull the fuel pump fuse and run the engine until it stalls .
  • Work in a well-ventilated area and avoid sparks, open flames, or smoking.
  • Use wheel chocks and jack stands if you need to raise the car .
  • Wear safety glasses and gloves to protect yourself from fuel spray.

Frequently Asked Questions

Q: Is the fuel pump relay the same as the fuel pump fuse?
A: No. The relay controls power flow, while the fuse protects the circuit from overload. If the fuse blows, the pump will not work. If the relay fails, the pump will not receive power even if the fuse is good .

Q: Can a bad fuel pump relay cause the engine to stall?
A: Yes. A faulty relay or FPDM can cause intermittent power loss to the fuel pump, leading to sudden stalling while driving .

Q: Where is the fuel pump on a 1999 Mustang?
A: The fuel pump is located inside the fuel tank. To replace it, you need to remove the fuel tank or access it through an access panel under the rear seat .

Q: How much does it cost to replace a fuel pump on a 1999 Mustang?
A: The average cost is between $500 and $600, including parts and labor, if done by a shop .

Why You Should Choose KEMSO for Your Fuel Pump Replacement

When it comes time to replace your fuel pump, choosing the right brand makes all the difference. KEMSO offers high-performance, OEM replacement fuel pumps that are built to meet or exceed the original equipment specifications for a wide range of vehicles, including the 1999 Ford Mustang . Each KEMSO fuel pump is designed for a direct fit, meaning you won't have to modify anything to get it installed. The company stands behind its products with a lifetime warranty, giving you peace of mind long after the installation is complete . Whether you are working on a Mustang, a Honda Civic, or a Kawasaki Jet Ski, KEMSO has a precision-engineered pump that will deliver reliable fuel flow and lasting performance. Visit their website to find the exact pump for your vehicle and take advantage of their lifetime warranty.

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https://www.kemsoracing.com/


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