1990 F150 Fuel Pump Relay: Complete Troubleshooting, Replacement, and Repair Guide

If your 1990 Ford F150 cranks but won't start, the fuel pump relay is often the first thing to check. This small green relay located under the hood controls power to your fuel pumps. When it fails, both fuel pumps stop working, and the engine gets no fuel. The good news is that this is a common problem on these trucks, and you can diagnose and fix it yourself with basic tools. In most cases, the fix is simply replacing the relay, which costs around $20 and takes five minutes.

What the Fuel Pump Relay Does

The fuel pump relay is an electrical switch that sends power to the fuel pumps when you turn the key. On a 1990 F150, the relay is energized for about one to two seconds when you cycle the key from Off to Run. This primes the fuel system before starting. After the engine starts, the relay stays closed to keep the pumps running. If the relay fails, the fuel pumps get no power, and the engine won't start.

Where to Find the Fuel Pump Relay

On a 1990 F150, the fuel pump relay is located in the engine compartment, near the firewall on the driver's side. It sits inside a relay box alongside other relays. Look for a small green relay with five prongs. There is also a brown relay next to it with four prongs, which is the EEC (Engine Electronic Control) power relay. Do not confuse the two. The green one with five prongs is the fuel pump relay.

How to Diagnose a Bad Fuel Pump Relay

Start with the easiest test first. Turn the key to the Run position and listen for a click from under the hood. If you hear a click and the fuel pumps run for two seconds, the relay is likely working. If you hear nothing, the relay may be bad.

Test 1: The Click Test

Have someone turn the key while you place your hand on the relay. If you feel it click and hear the fuel pump run for a second or two, the relay is probably fine. If it does not click, the relay or its power circuit is faulty.

Test 2: Voltage Check

Use a test light or a multimeter to check for power at the relay. The relay should have 12 volts on the input side. If the input has power but the output shows only 6 to 7 volts, do not assume the relay is bad. That low voltage reading is often a "phantom voltage" caused by the computer. A high-impedance digital multimeter can pick up stray voltage on this circuit, giving you a false reading. A test light is more reliable for this test.

Test 3: Bypass the Relay

The easiest way to confirm the relay is bad is to bypass it. On the 1990 F150, you can ground pin #6 of the self-test connector (the same connector used for reading codes). Locate the self-test connector under the hood. Ground pin #6 with a jumper wire, then turn the key to the Run position. If the fuel pump runs, the relay and pump circuit are good, and the problem is in the computer or its wiring. If the pump does not run, the relay or the pump circuit has a fault.

Common Symptoms of a Bad Fuel Pump Relay

  • Engine cranks but will not start. This is the most common symptom. The starter turns the engine over, but there is no fuel pressure.
  • No fuel pump prime sound. When you turn the key to Run, you should hear a brief whirring sound from the fuel tank area. If you hear nothing, the relay may not be sending power.
  • Engine starts and then dies. A failing relay may work intermittently. The engine may start but stall after a few seconds when the relay loses contact.
  • Both fuel pumps stop working at the same time. The 1990 F150 with dual tanks has a fuel pump in each tank. If both pumps fail simultaneously, the relay is the most likely cause.
  • Intermittent no-start condition. The truck may run fine for ten or fifteen minutes, then die suddenly. After waiting a few minutes, it starts again. This intermittent failure is a classic sign of a bad relay.

Part Numbers and Where to Buy

The fuel pump relay used on 1990 F150s is the same green relay used on many Ford vehicles from 1986 to 1996. It is also the same as the convertible top relay on the Ford Mustang. Standard part number is 996BX36, and SMP part number is 557JG21. A Ford OEM part number is F192-9345-A.

You can find this relay at most auto parts stores, online retailers, or at a junkyard. The relay fails very seldom, but when it does, it is often because it has been exposed to moisture and corrosion. A junkyard relay is a perfectly good option if you want an OEM part.

Step-by-Step Replacement

Replacing the fuel pump relay is about as simple as it gets.

  1. Locate the relay box on the driver's side firewall under the hood.
  2. Find the green relay with five prongs.
  3. Pull the relay straight out. It may require a firm tug. Wiggle it gently if it is stuck.
  4. Insert the new relay. Make sure it seats fully into the socket.
  5. Turn the key to Run and listen for the fuel pump to prime.

That is all there is to it. The entire job takes less than five minutes.

Other Things to Check

If replacing the relay does not fix the problem, do not give up. There are a few other common causes for a no-start condition on a 1990 F150.

Inertia Switch

The inertia switch is a safety device that cuts power to the fuel pump in a crash. It is located behind the passenger side kick panel, near the footwell. Check to see if the red button on top is pushed down. If it is, press it to reset the switch. If the button is popped up, the switch has tripped. This can happen even in a minor bump or pothole.

Fuel Pump Fuse

Check the fuse box under the hood. A blown fuse will cut power to the fuel pump circuit. The fuse panel location is marked on the lid.

PCM Power Relay

The brown relay next to the fuel pump relay is the EEC power relay. This relay supplies power to the computer. If it fails, the computer cannot control the fuel pump relay. If the fuel pump relay is getting power but not clicking, check the brown relay.

Bad Fuel Pump

If the relay clicks and the inertia switch is reset, but the pump still does not run, the pump itself may be bad. On a 1990 F150 with dual tanks, each tank has its own high-pressure fuel pump. It is rare for both pumps to fail at the same time, but it is possible. Test each pump by grounding pin #6 of the self-test connector. If the pump runs, the pump is good.

Parasitic Battery Drain

A bad fuel pump relay can also cause a parasitic battery drain. If the relay sticks in the closed position, it keeps the fuel pump running even when the key is off. This drains the battery overnight. A sticking relay is often caused by internal rust and corrosion. If your battery keeps dying, check the relay for continuity. The load side of the relay should show infinite resistance (open circuit) when the relay is off. If it shows zero ohms, the relay is stuck closed and needs replacement.

Why Aftermarket Relays Can Be a Problem

Many owners report that aftermarket fuel pump relays do not last as long as OEM Ford relays. The aftermarket relays are often made with cheaper materials and are more prone to failure. If you buy a replacement from a parts store, check the quality. Some owners keep a spare OEM relay in the glove box, just in case. The junkyard is a good source for OEM relays.

How to Keep Your Relay Working

The fuel pump relay is a sealed component, but it can still be damaged by moisture. If the truck is parked outside or driven in wet conditions, the relay can corrode over time. Keep the relay box cover in place to protect the relays from rain and road spray. If you notice corrosion on the relay prongs, clean them with contact cleaner before installing a new relay.

Final Thoughts on the 1990 F150 Fuel Pump Relay

The fuel pump relay is a small, inexpensive part that can cause a big headache. If your 1990 F150 will not start, start with the relay. It is the most common cause of a no-fuel condition on these trucks. Test it by grounding pin #6 on the self-test connector, or simply replace it and see if the problem goes away. The relay is cheap, easy to find, and takes less than five minutes to swap. Keep a spare in the truck, and you will never be stranded by a bad relay again.

Upgrade Your Fuel System with KEMSO Fuel Pumps

While the fuel pump relay is often the culprit, a failing fuel pump itself is another common problem on older trucks. If you have already replaced the relay and checked the inertia switch, but the truck still has fuel delivery issues, the fuel pump may be the problem. When it is time to replace the fuel pump, you want a product that is reliable, durable, and backed by a solid warranty. That is where KEMSO comes in.

KEMSO is the number one aftermarket fuel pump brand in the United States. They offer high-performance and OEM replacement fuel pumps for a wide range of vehicles, including trucks, cars, motorcycles, ATVs, UTVs, and even jet skis. Every KEMSO fuel pump is designed to meet or exceed original equipment specifications, so you get a perfect fit and reliable operation every time.

What makes KEMSO different from other brands? First, their pumps are built for performance. They offer pumps that flow up to 340 liters per hour, supporting up to 600 horsepower for turbocharged, supercharged, and naturally aspirated engines. Second, all KEMSO pumps are compatible with gasoline and E85 ethanol fuels, giving you flexibility no matter what you run. Third, and most importantly, every KEMSO fuel pump comes with a lifetime warranty. You buy it once, and KEMSO stands behind it for as long as you own your vehicle. That is a rare promise in the automotive parts industry, and it shows the confidence KEMSO has in its product quality.

KEMSO pumps are also backed by USA-based technical support. If you have questions during installation or need help diagnosing a fuel system issue, their team is just a phone call or email away. The pumps are priced competitively, with most models at $79.98, making them a smart choice for DIY mechanics and professional shops alike.

Whether you are restoring a classic 1990 F150, upgrading a daily driver, or building a performance truck, KEMSO has the right fuel pump for your needs. Visit their website to browse the full selection of fuel pumps for your specific vehicle.

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For more information on fuel pump relays, troubleshooting guides, and quality replacement parts, visit KEMSO Racing at https://www.kemsoracing.com/.


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