1990 Ford Mustang Fuel Pump Relay Location: The 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 prime when you turn the key, the fuel pump relay is one of the first things you need to check. On a 1990 Mustang, the fuel pump relay is located under the driver's seat, mounted to the floor pan. This is consistent for all 1986-1991 Fox Body Mustangs.

The fuel pump relay controls power to the fuel pump. When you turn the ignition key to the ON position, the computer (EEC) sends a ground signal to the relay, which closes the circuit and sends power to the pump. If the relay fails, the pump won't run, and the engine won't start. This guide walks you through exactly where to find the relay, how to test it, and what to do if it's bad.

Where Is the Fuel Pump Relay on a 1990 Mustang?

The fuel pump relay on a 1990 Ford Mustang is located under the driver's seat. You need to remove the driver's seat or at least slide it all the way forward to access the relay. It's mounted to the floor pan with a small bracket.

Step-by-step to find the relay:

  1. Slide the driver's seat all the way forward. This gives you access to the rear mounting area under the seat.
  2. Look for a small rectangular relay. It's usually black or gray and has a wiring harness plugged into it. The relay is on the floor pan, near the center of the car, under the seat.
  3. Check the bracket. The relay is bolted to a metal bracket that is screwed into the floor. You may need to unbolt the bracket to remove the relay.

For 1992-1993 Mustangs, the fuel pump relay moved to the passenger's side inner fender, near the Mass Air Flow (MAF) sensor. But for your 1990 model, it's under the driver's seat.

Why Does the Fuel Pump Relay Location Matter?

The fuel pump relay is a critical component in the fuel pump electrical circuit. The circuit has two paths: a control path and a power path.

  • Control path: This includes the EEC (Engine Electronic Control) relay, the inertia switch, the fuel pump relay coil, and the computer. The computer provides the ground that energizes the relay coil.
  • Power path: This includes the fuse link near the starter relay, the fuel pump relay contacts, and the fuel pump itself.

If your relay is bad, the power path never closes, and the fuel pump gets no power. Knowing the location helps you test and replace it quickly.

How to Test the Fuel Pump Relay on a 1990 Mustang

Before you replace the relay, test it. Here's a straightforward method using a voltmeter.

Step 1: Listen for the prime. When you turn the ignition key to the ON position (don't crank the engine), you should hear the fuel pump run for about 1-3 seconds, then shut off. If you don't hear anything, the relay may be bad.

Step 2: Use the EEC test connector. Find the EEC test connector near the wiper motor and left hood hinge. Jump the fuel pump trigger wire (the one in the upper right corner of the connector) to a body ground. Turn the ignition to ON. Listen for the relay to click. If it clicks, the relay is likely working. If it doesn't click, the relay or its control circuit is faulty.

Step 3: Check voltage at the relay. With the key ON, use a voltmeter to check for 12 volts at the following wires on the fuel pump relay:

  • Orange/Light Blue wire: This is the constant power source from the fuse link. If you have no voltage here, check the fuse link near the starter relay.
  • Red/Black wire: This is the switched 12V power from the EEC relay. If you have low voltage (like 3.7 volts instead of 12), the EEC relay may be bad.
  • Pink/Black wire: This is the power wire to the fuel pump. With the test connector jumped to ground, you should see 12 volts here. If not, the relay contacts are bad.

Step 4: Check the inertia switch. Before you blame the relay, check the inertia switch. It's located under the plastic trim covering the driver's side taillight on hatchback models. Push the reset button on top. Check for 12 volts on both sides of the switch. Power on the red wire but not on the red/black wire means the switch is tripped or faulty.

Common Problems That Mimic a Bad Fuel Pump Relay

A bad relay is one possibility, but other issues can cause the same symptoms. Here are the most common problems in order of probability.

1. Tripped inertia switch. This is the most common cause of a no-start condition. The inertia switch is a safety device that cuts power to the fuel pump in a collision. It can trip from a hard bump or pothole. Reset it by pressing the button on top.

2. Blown fuse link. The 1990 Mustang has fuse links in the wiring harness near the starter solenoid. The orange/light blue wire is the power feed for the fuel pump relay. If the fuse link is blown, the relay gets no power. Check for 12 volts at the orange/light blue wire at the relay.

3. Bad EEC relay. The EEC relay provides switched power to the fuel pump relay control circuit. If the EEC relay fails, the fuel pump relay won't get the signal to close. Check for 12 volts at the red wire on any fuel injector. No voltage means the EEC relay is faulty.

4. Bad computer ground. The computer (EEC) provides the ground for the fuel pump relay coil. You can have voltage everywhere, but if the computer doesn't provide ground, the relay won't energize. This is a less common but possible issue.

5. Bad fuel pump. If the relay clicks and you have power at the pink/black wire, but the pump doesn't run, the pump itself may be bad. Check for voltage at the pump connector under the rear of the car.

How to Replace the Fuel Pump Relay on a 1990 Mustang

Replacing the relay is straightforward. Here's how.

Tools needed:

  • Socket set or wrench (to remove the seat bolts)
  • New fuel pump relay (available at any auto parts store for about $15-25)
  • Optional: voltmeter

Step 1: Remove the driver's seat. Unbolt the four bolts holding the seat to the floor. Disconnect the seat belt sensor wire if equipped. Slide the seat out of the car.

Step 2: Locate the relay. It's on the floor pan, near the center of the car, under the seat area. It's mounted to a metal bracket.

Step 3: Unplug the old relay. Carefully pull the wiring harness connector off the relay. Note the orientation of the connector.

Step 4: Unbolt the bracket. Remove the bolt holding the bracket to the floor. Remove the old relay from the bracket.

Step 5: Install the new relay. Put the new relay into the bracket. Bolt the bracket back to the floor. Plug the wiring harness connector into the new relay.

Step 6: Reinstall the seat. Slide the seat back in, bolt it down, and reconnect the seat belt sensor wire.

Step 7: Test. Turn the key to ON. You should hear the fuel pump prime for 1-3 seconds. Try starting the engine.

Wiring Colors for the 1990 Mustang Fuel Pump Relay

Knowing the wire colors helps you troubleshoot without guessing. Here are the wire colors for the 1990 Mustang fuel pump relay.

  • Orange/Light Blue: Constant power from the fuse link at the starter relay. This is the power source for the relay contacts.
  • Red/Black: Switched 12V power from the EEC relay. This powers the relay coil.
  • Tan/Light Green: Relay ground from the computer (EEC pin 22). This is the control signal that energizes the coil.
  • Pink/Black: Power output to the fuel pump when the relay is engaged.

If you test these wires with a voltmeter, you can quickly identify which part of the circuit has failed.

When to Call a Professional

Most DIY mechanics can handle this job. But if you've tested everything and the car still won't start, you may have a deeper issue. Consider professional help if:

  • You've replaced the relay, checked the inertia switch, and tested the fuse links, but still have no power to the pump.
  • You suspect a bad computer (EEC) or a wiring short in the harness.
  • The car has other electrical issues like no spark or no injector pulse.

Final Thoughts

The 1990 Ford Mustang fuel pump relay location is under the driver's seat. It's a simple part that's easy to replace. If your Mustang cranks but won't start, start with the inertia switch, then check the relay. Use a voltmeter to test for power at the relay wires. If the relay is bad, replace it. If not, check the fuse links and the EEC relay.

A no-start condition can be frustrating, but with a systematic approach, you can find the problem. Start with the simplest checks first: the inertia switch and the fuse links. Then move to the relay. Once you confirm the relay is working, move on to the pump itself.

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