1990 Mustang Fuel Pump Relay Location: Your Complete Guide to Finding and Fixing It

If your 1990 Ford Mustang cranks but won't start, and you don't hear the fuel pump hum for two seconds when you turn the key to the ON position, the fuel pump relay is likely the problem. The fuel pump relay for a 1990 Mustang is located in the engine compartment fuse box, mounted on the driver's side inner fender well, near the strut tower. This small black or gray relay controls power to the fuel pump. When it fails, the pump gets no electricity, and your engine starves for fuel. Replacing this relay is a simple, inexpensive fix that can save you a tow truck bill. Below, I walk you through exactly where to find it, how to test it, and how to swap it out in under 15 minutes.

Understanding the 1990 Mustang Fuel Pump Relay

The 1990 Ford Mustang uses an electric fuel pump mounted inside the fuel tank. The fuel pump relay acts as a switch. When you turn the ignition key to the ON position, the engine computer (EEC-IV) sends a signal to the relay, which closes and sends battery voltage to the pump. The pump runs for about one to two seconds to pressurize the fuel system, then stops until the engine starts. If the relay is bad, the pump gets no power, and the engine won't start.

This relay is part of the Ford EEC-IV power relay system. It works together with the EEC power relay and the inertia fuel shutoff switch. The EEC power relay supplies power to the computer and the fuel pump relay. If the EEC relay is bad, the fuel pump relay won't get the signal to close. Always check both relays if you have a no-start condition.

Exact Location of the 1990 Mustang Fuel Pump Relay

1. Engine compartment fuse box, driver's side inner fender.
Open the hood. Look at the driver's side (left side) of the engine bay. You will see a black plastic box mounted on the inner fender well, near the battery and strut tower. This is the engine compartment fuse and relay box. It has a removable lid.

2. Open the fuse box lid.
The lid may have a diagram on the inside showing the location of relays and fuses. If the diagram is missing or faded, do not worry. The fuel pump relay is typically the middle relay in the row of three relays. The three relays are usually labeled or color-coded. The EEC power relay is often the one closest to the battery, the fuel pump relay is the middle one, and the third relay may be for the horn or other accessory.

3. Identify the relay.
The relay is a small, square, plastic component with four or five prongs on the bottom. It is usually black or gray. On a 1990 Mustang, the fuel pump relay is often marked with a "FP" or "Fuel Pump" label on the fuse box cover. If not, look for a relay that has a dark green or orange wire leading to it. The wire colors can vary, but the relay is always in the middle position.

4. Alternative location for some models.
On a very small number of 1990 Mustangs, particularly those built very early in the model year, the fuel pump relay may be located behind the passenger side kick panel, inside the car. But this is rare. The vast majority of 1990 Mustangs have the relay in the engine compartment fuse box. Check the engine bay first before tearing into the interior.

How to Test the 1990 Mustang Fuel Pump Relay

Before buying a new relay, you can test the old one. This saves you money and confirms the problem.

1. Listen for the pump prime.
Turn the key to the ON position (do not crank the engine). Listen from the rear of the car. You should hear a faint humming sound from the fuel tank for about one to two seconds. If you hear nothing, the relay or the pump is not getting power.

2. Check for power at the relay.
Use a multimeter set to DC voltage. With the key OFF, probe the relay socket. The socket has four terminals: 30 (common), 87 (normally open), 85 (coil ground), and 86 (coil power). With the key OFF, terminal 30 should have battery voltage (12V). If not, check the fuse. With the key ON, terminal 86 should have voltage. If you have voltage at 30 and 86, but the relay does not click, the relay coil is bad.

3. Test the relay itself.
Remove the relay from the socket. Apply 12V from a battery to terminal 86 and ground to terminal 85. You should hear an audible click. If no click, the relay is bad. If it clicks, test continuity between terminals 30 and 87 using the multimeter. With the relay energized, there should be continuity (close to zero ohms). If there is no continuity, the relay is faulty.

4. Check the EEC power relay.
The EEC power relay is located right next to the fuel pump relay. It is usually the relay closest to the battery. If the EEC relay is bad, the fuel pump relay will not get the signal. Test the EEC relay the same way. If both relays are good, the problem may be the inertia switch or the fuel pump itself.

How to Replace the 1990 Mustang Fuel Pump Relay

Replacing the relay is straightforward. You do not need any special tools.

1. Buy the correct relay.
The 1990 Mustang uses a standard Ford 4-pin relay. You can find them at any auto parts store for about $10 to $20. The part number is usually F4AB-9345-AA or equivalent. Make sure the relay is rated for 30 amps or higher. Do not use a generic 20-amp relay, as it may overheat.

2. Disconnect the battery.
This is a safety step. Remove the negative battery cable before working on the fuse box. This prevents accidental shorts or sparks.

3. Remove the old relay.
Grasp the relay firmly and pull straight up. It may be tight, but it will come out. Do not twist or pry it, as this can damage the socket.

4. Install the new relay.
Align the prongs of the new relay with the socket. Push it straight down until it clicks into place. Make sure it is fully seated.

5. Reconnect the battery.
Reattach the negative battery cable. Tighten it securely.

6. Test the system.
Turn the key to the ON position. Listen for the fuel pump prime sound from the rear. If you hear it, the relay is working. If not, recheck your connections or test the EEC relay.

Other Common Problems That Mimic a Bad Relay

Sometimes the fuel pump relay is not the issue. Here are other things to check before you spend money on a new relay.

1. Inertia fuel shutoff switch.
The 1990 Mustang has an inertia switch in the trunk, usually on the driver's side, behind the trunk liner. This switch cuts power to the fuel pump in a crash. If the switch is tripped, the pump will not run. Press the reset button on top of the switch to reset it. If you hear a click, the switch was tripped.

2. Blown fuse.
The fuel pump circuit has a fuse in the engine compartment fuse box. It is usually a 20-amp fuse. Check the fuse with a multimeter or a test light. Replace it if blown.

3. Bad fuel pump.
If the relay tests good, the inertia switch is reset, and the fuse is good, the fuel pump itself may be dead. You can test the pump by applying 12V directly to the pump connector at the fuel tank. If the pump runs, the wiring is the problem. If it does not run, replace the pump.

4. Corroded or loose wiring.
Check the connector at the fuel pump relay socket. Corrosion can cause a poor connection. Clean the terminals with a small wire brush or contact cleaner. Also, check the ground wire for the relay. A bad ground can prevent the relay from closing.

Tools and Parts You Will Need

• New fuel pump relay (Ford 4-pin, 30 amp)
• Multimeter or test light
• 10mm socket or wrench (to disconnect battery)
• Small flathead screwdriver (to pry relay if stuck)
• Contact cleaner or electrical cleaner (optional)

Estimated Time and Cost

Replacing the 1990 Mustang fuel pump relay takes about 10 to 15 minutes. The relay costs between $10 and $20 at most auto parts stores. If you buy a relay from a dealer, it may cost $30 to $40. This is one of the cheapest and easiest repairs you can do on your Mustang.

KEMSO Fuel Pump: The Best Upgrade for Your 1990 Mustang

If you find that your fuel pump is the root cause of your no-start issue, do not just buy any cheap replacement. Choose a brand that stands behind its products. KEMSO offers high-performance fuel pumps that are built to OEM replacement standards. Every KEMSO fuel pump is a direct fit for your 1990 Mustang, meaning no modifications, no cutting, and no wiring headaches. The pump comes with a lifetime warranty, so you never have to worry about buying another one again. KEMSO pumps are engineered to deliver consistent fuel pressure and flow, which is critical for your Mustang's engine performance and longevity. Whether you have a stock 5.0L V8 or a modified engine, KEMSO has the right pump for you. Visit their website to see the full range of products: KEMSO Racing Fuel Pumps

Summary of the 1990 Mustang Fuel Pump Relay Location

• Location: Engine compartment fuse box, driver's side inner fender, middle relay.
• Appearance: Small black or gray square relay with four prongs.
• Testing: Listen for pump prime, check voltage at relay socket, test relay with 12V.
• Replacement: Pull out old relay, insert new relay, test.
• Related parts: EEC power relay, inertia switch, fuel pump fuse.

If your 1990 Mustang does not start, start with the fuel pump relay. It is the most common cause of a no-fuel condition in these cars. The fix is simple, cheap, and quick. You can do it yourself with basic tools. If the relay is good, move on to the inertia switch, fuse, and pump. And if you need a new fuel pump, remember that KEMSO provides a lifetime warranty and OEM-quality fitment. Check out their selection at KEMSO Racing