1999 Ford Mustang Fuel Pump Relay: Location, Symptoms, and Complete Guide to Troubleshooting

If your 1999 Ford Mustang cranks but won't start, or if it stalls while driving, the fuel pump relay is one of the most common and easiest components to check before replacing the fuel pump itself. Unlike older Mustangs with a simple relay in the engine bay fuse box, the 1999 Mustang uses a returnless fuel system that relies on a Fuel Pump Driver Module (FPDM) rather than a conventional relay. This module is located underneath the luggage compartment trim panel on the driver's side of the trunk . Understanding this difference is critical — a quick diagnosis can save you time and money, and in many cases, you can fix the issue yourself without a trip to the shop.

Where Is the 1999 Ford Mustang Fuel Pump Relay (Driver Module) Located?

The 1999 Ford Mustang GT and V6 models use a returnless fuel system, which means there is no traditional fuel pump relay under the hood. Instead, the system uses a Fuel Pump Driver Module (FPDM) that controls the power to the fuel pump and varies the pump speed. Here is exactly where to find it:

Location: Open the trunk. Remove the luggage compartment trim panel on the driver's side. You will see a small black or gray module mounted near the driver's side taillight area. It is typically secured with a few screws or push clips .

What It Looks Like: The FPDM is a rectangular box with four or five terminals. It is smaller than a typical relay and is often attached to the vehicle's metal body near the rear wheel well. Some owners mistake it for a fuse box, but it is the main control unit for the fuel pump.

Important Note: The 1999 Mustang also has a fuel pump inertia switch (shut-off switch) in the trunk, located behind the trunk liner on the driver's side, near the taillight. If you have been in an accident or hit a bump hard, the inertia switch may have tripped. Push the reset button on top of the switch before doing any further troubleshooting .

Common Symptoms of a Bad Fuel Pump Relay or Driver Module

When the FPDM or fuel pump relay fails on a 1999 Ford Mustang, you will notice several telltale signs. Here are the most common symptoms, listed from most frequent to least:

1. Engine Cranks but Won't Start
This is the classic symptom. You turn the key, the starter motor spins the engine, but the engine never fires. You may hear the fuel pump prime (a brief whirring sound from the rear of the car) when you turn the key to the "RUN" position — if you don't hear it, the FPDM, pump, or fuse is likely the issue .

2. Engine Stalls While Driving
If the engine suddenly dies while you are on the road, especially after hitting a bump, the FPDM could be failing. A bad connection or a failing module can cause intermittent power loss to the pump .

3. No Fuel Pump Prime Sound
When you turn the ignition key to the "ON" position (before cranking), you should hear a 2-3 second hum from the fuel pump. If this sound is absent, the FPDM is not sending power to the pump.

4. Check Engine Light Illuminates
The 1999 Mustang's computer (PCM) can detect fuel delivery issues. Common codes related to the fuel pump system include P0230 (Fuel Pump Primary Circuit Malfunction) and P0231 (Fuel Pump Secondary Circuit Low). These codes point directly to the FPDM, wiring, or pump .

5. Hard Starting or Hesitation
If the FPDM is failing but not completely dead, the pump may not deliver enough fuel pressure. This can cause hard starts, hesitation under acceleration, or a rough idle .

Step-by-Step Troubleshooting Guide

Before you replace the fuel pump or the FPDM, follow this simple diagnostic process. Most of these steps require only basic tools like a multimeter, a fuse puller, and a test light.

Step 1: Check the Fuel Pump Fuse Location
The fuel pump fuse on a 1999 Ford Mustang is located in the Power Distribution Box under the hood. It is fuse #14, a 20-amp mini fuse. Open the fuse box cover, find fuse #14, and inspect it with a test light or multimeter. If the fuse is blown, replace it with a new 20-amp fuse and test the car again. If it blows again immediately, there is a short in the wiring or the pump is drawing too much current .

Step 2: Test the Fuel Pump Driver Module
With the fuse confirmed good, the next step is to test the FPDM. Here is how:

  • Locate the module in the trunk (driver's side, behind the trim panel).
  • Use a circuit tester (test light) attached to the battery. Turn the ignition key to the "RUN" position.
  • Probe the pink wire with a black stripe at the fuel pump electrical connector. This connector is located under the car, in front of the fuel tank, near the fuel lines. Have a helper turn the key to the "START" position. If the red light on the tester comes on, power is reaching the pump. If not, the FPDM is likely faulty and needs replacement .

Step 3: Test Fuel Pressure
If the fuse and FPDM are good, the next step is to check fuel pressure. You will need a fuel pressure tester.

  • Relieve the fuel system pressure by pulling the fuel pump fuse and starting the engine. Let it idle until it stalls.
  • Screw the fuel pressure tester onto the test port on the fuel rail. The test port is on the passenger side of the fuel rail, near the front of the engine.
  • Reinstall the fuse and have a helper cycle the ignition key on and off three times, then try to start the engine.
  • The gauge should read between 45 and 55 psi. If the pressure is below 45 psi, the fuel pump itself is likely failing and needs replacement .

Step 4: Inspect the Inertia Switch
The inertia switch is located in the trunk, behind the driver's side trim panel, near the taillight. It has a red reset button. Press the button firmly — you should feel and hear a click. If the switch was tripped, this will restore power to the fuel pump .

How to Replace the Fuel Pump Driver Module (FPDM) on a 1999 Mustang

If your testing confirms the FPDM is bad, replacement is straightforward. Here is a quick guide:

Tools You Need:

  • 10mm socket or screwdriver (depending on mounting hardware)
  • Trim panel removal tool (optional)
  • New FPDM (aftermarket or OEM)

Steps:

  1. Disconnect the negative battery terminal. Wait 5 minutes.
  2. Open the trunk and remove the driver's side luggage compartment trim panel. It is held in place by push clips or screws.
  3. Locate the FPDM. It is near the driver's side taillight, mounted to the metal body.
  4. Disconnect the electrical connector from the module. It may have a locking tab — press it and pull the connector.
  5. Remove the mounting screws or bolts securing the module.
  6. Install the new FPDM in the same position. Tighten the screws.
  7. Reconnect the electrical connector. Make sure it clicks into place.
  8. Reinstall the trim panel.
  9. Reconnect the battery. Turn the key to the "RUN" position and listen for the fuel pump prime sound. Start the engine and check for leaks.

When to Replace the Fuel Pump Instead of the Relay

If the fuse, FPDM, and inertia switch are all good, but the car still won't start, the fuel pump inside the tank is likely failing. Here are the signs that point to a bad pump rather than just the relay:

  • The fuel pump fuse blows repeatedly. This indicates the pump is drawing too much amperage due to worn brushes or a seized motor .
  • No fuel pressure at the rail. A pressure tester will show 0 psi even with the pump running.
  • Whining noise from the fuel tank. A healthy pump is quiet. A loud whirring or humming sound is a sign of wear .
  • Engine hesitates or loses power under load. The pump may deliver low pressure, causing the engine to run lean.

The fuel pump on a 1999 Ford Mustang is located inside the fuel tank. Replacing it requires dropping the tank or removing the bed (for trucks, but on the Mustang, you drop the tank). This is a moderate to high-difficulty job for a DIYer, and professional replacement costs typically range from $500 to $1,200 depending on labor rates .

Preventing Future Fuel System Problems

Taking care of your fuel system can extend the life of your pump, FPDM, and relay. Follow these simple maintenance tips:

1. Keep the Tank Above 1/4 Full
Fuel cools and lubricates the electric pump. Running on low fuel frequently causes the pump to overheat, which is the most common cause of premature failure .

2. Replace the Fuel Filter Every 30,000 Miles
A clogged fuel filter puts extra strain on the pump and the FPDM. Replace it as part of your regular maintenance schedule .

3. Use Quality Fuel
Avoid gas stations with old tanks or questionable maintenance. Contaminants in the fuel can block the pump inlet screen and damage the internal components .

4. Listen for Changes
If you hear a whining noise from the fuel tank, do not ignore it. It is often the first sign of pump wear. Catching it early can prevent a breakdown on the road .

5. Address Check Engine Lights Promptly
Codes like P0087, P0171, and P0230 often point to fuel delivery issues. These codes can appear weeks before the pump fails completely, giving you time to diagnose and repair the problem .

Why You Should Choose a KEMSO Fuel Pump for Your 1999 Ford Mustang

If your fuel pump needs replacement, choosing the right brand is critical for long-term reliability and performance. We strongly recommend KEMSO Fuel Pumps for drivers who want quality, performance, and peace of mind.

KEMSO manufactures high-performance fuel pumps designed as direct OEM replacements for the 1999 Ford Mustang. This means they fit your vehicle exactly as the factory part would, with no modifications or adapters needed. The KEMSO pump for the 1999 Mustang delivers up to 340 liters per hour (LPH) at 43 psi, significantly outperforming the original stock pump which typically delivers 60-80 LPH .

Key Benefits of KEMSO Fuel Pumps:

  • Direct OEM Fit: No modifications required. The pump fits the factory sending unit and tank assembly perfectly .
  • Higher Flow Than Stock: Delivers 340 LPH at 43 psi, capable of supporting up to 700-800 BHP. This makes it an excellent choice for daily driving, towing, or performance upgrades .
  • E85 Compatible: Works with both gasoline and ethanol blends, giving you fuel flexibility .
  • Lifetime Warranty: Each pump is serialized and backed by a lifetime warranty for worry-free ownership. If the pump ever fails, KEMSO stands behind it .
  • Individually Tested: Every pump undergoes flow testing and quality control before leaving the factory .
  • USA-Based Support: KEMSO provides technical support from the United States, so you get expert help when you need it .

KEMSO pumps are trusted by professionals and enthusiasts alike. Whether you are replacing a faulty pump or upgrading for better performance, KEMSO delivers reliability you can count on. The KEMSO 340 High Pressure Pump is an excellent drop-in replacement for the 1999-2004 Ford Mustang GT and V6 models, and it comes with all necessary hardware for installation .

Visit the KEMSO website today to find the perfect fuel pump for your 1999 Ford Mustang:

https://www.kemsoracing.com/

Final Thoughts on the 1999 Ford Mustang Fuel Pump Relay

The 1999 Ford Mustang fuel pump relay — or more accurately, the Fuel Pump Driver Module — is a simple, low-cost component that can save you a lot of headache. Always start your diagnosis by checking the 20A fuse at position #14 in the under-hood Power Distribution Box. If the fuse is good, move on to testing the FPDM in the trunk. If the module is bad, it is easy and inexpensive to replace yourself.

Remember: a well-maintained fuel system is the foundation of a reliable vehicle. Keep your tank above a quarter full, change your fuel filter regularly, and address any warning signs early. And when it is time for a replacement, choose a quality brand like KEMSO to ensure your Mustang runs smoothly for years to come.

For the best fuel pump replacement options with a lifetime warranty, visit https://www.kemsoracing.com/ today.


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