1987 Mustang Fuel Pump Relay: Complete Guide to Diagnosis, Location, and Replacement

If your 1987 Ford Mustang cranks but won’t start, the fuel pump relay is the first and cheapest thing to check. A failing fuel pump relay is one of the most common causes of no-start conditions on Fox Body Mustangs, and diagnosing it takes only a few minutes with a basic voltmeter. The fuel pump relay for the 1987 Mustang is located under the driver’s seat, and when it fails, the fuel pump won’t receive power to prime the system or keep running once the engine starts . This guide walks you through exactly how to test, locate, and replace the relay step by step, without wasting time or money on parts you don’t need.

Where Is the Fuel Pump Relay on a 1987 Mustang?

The fuel pump relay on a 1987 Mustang is located under the driver’s seat. For 1986-1991 Fox Body Mustangs, this is the standard location across the board . You’ll need to slide the driver’s seat all the way forward, then look underneath the seat frame. The relay is mounted to a small bracket with a wiring harness plugged into it. It’s a small, rectangular component, usually black or gray in color. On 1992-1993 models, the relay moved to the passenger side inner fender or below the Mass Air Flow (MAF) sensor, but for the 1987, it’s strictly under the driver’s seat .

How Does the Fuel Pump Relay Work?

The electrical circuit for the fuel pump has two separate paths: a control path and a power path .

Control path: The control path consists of the inertia switch, the computer (EEC), and the fuel pump relay coil. It turns the fuel pump relay on or off under computer control. The switched power (red wire) from the EEC relay goes to the inertia switch (red/black wire), then from the inertia switch to the relay coil, and then from the relay coil to the computer (tan/light green wire). The computer provides the ground path to complete the circuit. This ground causes the relay coil to energize and close the contacts for the power path .

Power path: The power path picks up from a fuse link near the starter relay. The feed wire from the fuse link (orange/light blue wire) goes to the fuel pump relay contacts. When the contacts close because the relay energizes, power flows through the contacts to the fuel pump (pink/black wire). The fuel pump has a black wire that supplies the ground to complete the circuit .

Important: The computer does not source any power to actuators, relays, or injectors. It provides the ground necessary to complete the circuit. This means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit .

Symptoms of a Bad Fuel Pump Relay

Before you start digging, here are the most common signs that your fuel pump relay is failing:

  1. Engine cranks but won’t start – This is the most obvious symptom. The starter turns the engine over, but you never hear the fuel pump prime when you turn the key to the ON position .
  2. No fuel pump priming sound – Normally, when you turn the ignition to ON, the fuel pump should run for 1-3 seconds to build pressure. If you don’t hear that faint whirring sound from the rear of the car, the relay might not be engaging .
  3. Fuel pump runs constantly – If the relay sticks in the closed position, the fuel pump may run continuously even with the key off, which can drain the battery .
  4. Car dies after running for a while – A failing relay can intermittently lose connection, causing the fuel pump to shut off after the engine has been running for 20-30 minutes .

Step-by-Step Diagnosis: How to Test the Fuel Pump Relay

You’ll need a basic voltmeter or test light, and a small jumper wire (a paper clip works fine).

Step 1: Check the Inertia Switch First

Before you touch the relay, check the inertia switch. The inertia switch is located under the plastic trim covering the driver’s side taillight on hatchback models . On Fox Body Mustangs, the inertia switch cuts off power to the fuel pump relay on 1986-1990 models, and directly to the pump on 1991-1993 models . Use your voltmeter to check for power on both sides of the switch. You should have 12 volts on the red wire going in and the red/black wire coming out. If you have power on the red wire but not on the red/black wire, the inertia switch is tripped or faulty. Press the reset button on top of the switch .

Step 2: Listen for the Relay Click

Turn the ignition switch to the ON position. Listen for a single click from under the driver’s seat. That click is the fuel pump relay engaging. If you hear the click, the relay is likely working, and the problem is elsewhere. If you don’t hear the click, proceed to the next step .

Step 3: Use the Fuel Pump Test Connector

Find the EEC test connector in the engine bay. It’s usually located near the passenger side hood hinge. Jump the connector in the upper left-hand corner (or lower right-hand corner depending on year) to a good body ground using your jumper wire. Turn the ignition to ON. The fuel pump should start running. If it does, the relay and power path are working, and the problem is in the computer control circuit. If it doesn’t, you have a power path issue .

Step 4: Test the Relay Wiring

With the key ON, check for voltage at the fuel pump relay connector. The wiring colors for a 1987 Mustang are :

  • Orange/Light Blue – Constant 12V power (from the fuse link at the starter relay) – This should be hot all the time.
  • Red/Black – 12V switched power from the EEC relay (through the inertia switch) – This should be hot with the key ON.
  • Tan/Light Green – Ground from the computer (PIN 22 at the EEC) – This wire gets grounded by the computer to activate the relay.
  • Pink/Black – Power output to the fuel pump – This wire should show 12V when the relay is engaged.

If you have power on the orange/light blue wire but not on the red/black wire, the problem is in the control circuit (inertia switch, EEC relay, or wiring). If you have power on both the orange/light blue and red/black wires but the relay doesn’t click, the computer may not be providing the ground on the tan/light green wire. This could be a bad EEC computer, a broken wire, or a faulty PIP sensor .

Step 5: Check the Fuse Links

The fuse links are located in the wiring harness near the starter solenoid on the passenger side inner fender . Fuse links look like regular wires but are actually fusible links that protect the circuit. Check for 12 volts at the orange/light blue wire at the fuel pump relay. If you have no voltage or low voltage, the fuse link may be blown. Inspect the fuse links visually and with a voltmeter. They can corrode or break over time .

Common Problems Found During Diagnosis

Problem 1: Relay clicks but pump doesn’t run – This usually means the power path is broken. Check the pink/black wire at the relay for voltage when the relay is engaged. If you have voltage there, the problem is between the relay and the fuel pump, or the fuel pump itself is bad. Check the wiring under the car and at the fuel pump harness .

Problem 2: No power to the fuel pump harness, but power when jumped – This is a classic sign of a control circuit problem. The relay works when you force it on with the test connector, but the computer isn’t providing the ground. Possible causes include a bad PIP sensor (the computer uses the PIP signal to know the engine is running), a faulty EEC computer, or a break in the tan/light green wire between the relay and the computer .

Problem 3: Fuel pump runs constantly with the key ON – This happens when the tan/light green wire is grounded to the body instead of going through the computer. If a previous owner bypassed the computer ground, the relay will stay engaged as long as the key is on .

How to Replace the Fuel Pump Relay

Replacing the fuel pump relay on a 1987 Mustang is straightforward:

  1. Locate the relay – Under the driver’s seat, mounted to a bracket.
  2. Unbolt the relay bracket – Use a 10mm socket or wrench to remove the bolt holding the bracket to the seat frame.
  3. Disconnect the wiring harness – Unplug the connector from the relay.
  4. Remove the old relay – It simply pulls out of the bracket.
  5. Install the new relay – Push the new relay into the bracket, plug in the wiring harness, and reinstall the bracket bolt.

Make sure the replacement relay is the correct part for your 1987 Mustang. The relay should match the OEM specifications. Also, inspect the wiring connector for corrosion or damage. If the connector is corroded, clean it with electrical contact cleaner or replace the connector .

Upgrade Your Fuel System with KEMSO Fuel Pumps

If your diagnosis reveals that the fuel pump itself has failed, or if you’re looking for a reliable, long-lasting replacement, consider upgrading to a KEMSO Fuel Pump. KEMSO fuel pumps are designed as high-performance OEM replacements that meet or exceed factory specifications. Each pump comes with a lifetime warranty, giving you peace of mind that your Mustang’s fuel system is built to last. Whether you’re restoring a classic Fox Body or building a daily driver, KEMSO offers a direct-fit solution that delivers consistent fuel pressure and reliable performance. Visit KEMSO at https://www.kemsoracing.com/ to browse their full lineup of fuel pumps for your 1987 Mustang and other vehicles.

Final Tips

Always start with the simplest checks first. The inertia switch is the most common cause of a no-power-to-fuel-pump issue, and it’s free to reset. The fuel pump relay is the next cheapest and easiest component to test. If you’ve confirmed the relay is good, move on to the fuel pump itself. A fuel pressure test at the Schrader valve on the fuel rail should show 37-40 PSI with the pump running . If you have no pressure, and the pump is getting power, the pump is likely bad. And if you’re replacing the pump, remember that KEMSO offers a lifetime warranty and high-performance options at https://www.kemsoracing.com/.

One more thing: If you’ve checked everything and still can’t find the problem, look for any aftermarket modifications. Kill switches, alarm systems, and previous owner wiring changes can cause unexpected issues. One owner spent days troubleshooting a no-start condition only to discover a hidden kill switch wired into the fuel pump circuit .


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