1990 Chevy Truck Fuel Pump Relay Location: The Complete Guide to Finding, Testing, and Replacing It

If your 1990 Chevy truck cranks but won't start, the fuel pump relay is one of the first components you should check. On most 1990 Chevy trucks, including the C/K series, the fuel pump relay is located under the hood inside the engine compartment fuse and relay center, typically mounted on the passenger side of the firewall. On some models, depending on the specific configuration, the relay may also be found under the dash on the passenger side, above the ECU or computer. Knowing exactly where to look and how to test this relay can save you hours of diagnostic time and expensive tow truck bills.

Understanding the Fuel Pump Relay in Your 1990 Chevy Truck

The fuel pump relay is an electromagnetic switch that controls power to the fuel pump. When you turn the ignition key to the ON position, the computer (ECM) momentarily grounds the relay coil for about two seconds, which allows the relay to close and send 12 volts of power from the battery to the fuel pump. This primes the fuel system before you start the engine. Once the engine is running, the ECM continues to ground the relay as long as it detects engine RPMs above 400. If the relay fails, the fuel pump won't receive power, and your truck won't start.

Where to Find the Fuel Pump Relay on a 1990 Chevy Truck

There are two primary locations where the fuel pump relay can be found on a 1990 Chevy truck, depending on the specific model and engine configuration.

Location 1: Under the Hood, Engine Compartment Fuse Box

The most common location for the fuel pump relay on a 1990 Chevy truck is inside the under-hood fuse and relay center. This box is typically located on the passenger side of the engine bay, mounted on the firewall near the master cylinder. To access it:

  1. Open the hood and locate the rectangular black plastic fuse and relay box on the passenger side firewall.
  2. Remove the plastic cover by pressing the tabs or lifting the latches.
  3. Look at the diagram printed on the underside of the cover. The fuel pump relay is usually labeled as "FUEL PUMP," "FP," or "FUEL."
  4. In some 1990 models, the relay is mounted under the engine fuse box, not inside it. You may need to lift the entire fuse box assembly to see the relay underneath.

Location 2: Under the Dash, Above the ECU

On some 1990 Chevy trucks, particularly those with certain engine options, the fuel pump relay is located inside the cab. It is mounted under the dashboard on the passenger side, above the ECU (Electronic Control Unit) or computer. To access this location:

  1. Remove the kick panel on the passenger side of the dashboard.
  2. Remove the glove box if necessary to gain better access.
  3. Look for a black relay with a small metal bracket attached to it.
  4. The relay is usually held in place by a single 10mm bolt.

Identifying the Correct Fuel Pump Relay

Once you have located the relay center, identifying the correct relay is critical. The fuel pump relay is typically a rectangular module with four or five pins. Here are some tips to identify it:

Check the color. On many 1990 Chevy trucks, the fuel pump relay is color-coded. The green relay is often the fuel pump relay, while the brown relay is the EEC (computer) relay. If your truck has air conditioning, a black relay may be present for the WOT (wide open throttle) function.

Check the label. Look at the diagram on the fuse box cover or the relay itself. The fuel pump relay is usually labeled with "FUEL PUMP," "FP," or a similar designation.

Check the wire colors. On a 1991 Chevy truck (which is very similar to the 1990 model), the fuel pump relay is identifiable by its wire colors: Gray, Tan/White, Orange, and Black/White. The Gray wire is the power output to the fuel pump, the Orange wire is the constant 12V battery power input, the Tan/White wire connects to the oil pressure switch, and the Black/White wire is the ground control from the ECM.

How to Test the Fuel Pump Relay

Before you jump or replace the relay, you should test it to confirm it is faulty. Here is a simple step-by-step testing procedure.

Step 1: Listen for the Fuel Pump Prime

Turn the ignition key to the ON position (do not crank the engine). Listen carefully near the rear of the truck, close to the fuel tank. You should hear a 2-3 second hum from the fuel pump. If you hear this sound, the relay is likely working correctly at that moment, and the problem may be elsewhere. If you hear nothing, the relay or another component in the circuit may be faulty.

Step 2: Swap Test

If you have another relay in the fuse box that is the same type (such as the horn relay), you can perform a swap test. Remove the fuel pump relay and swap it with the identical relay. If the fuel pump now works and the horn stops working, the original fuel pump relay was faulty.

Step 3: Bench Test the Relay

Remove the fuel pump relay from the socket. Look at the diagram printed on the side of the relay to identify the terminals. Apply 12 volts to the coil terminals (typically pins 85 and 86) using a battery or a power source. Use a multimeter to check for continuity between the switch terminals (pins 30 and 87). When the relay is activated, there should be continuity between these terminals. If there is no continuity, the relay is faulty and needs replacement.

How to Jump the Fuel Pump Relay

Jumping the fuel pump relay is a diagnostic technique that bypasses the relay and sends power directly to the fuel pump. This helps you determine if the fuel pump itself is working or if the problem is in the relay or control circuit. Important: This is a temporary test only, not a permanent fix.

Tools You Will Need

  • A 12-gauge insulated jumper wire or a paperclip
  • Safety gloves
  • A multimeter (optional but recommended)

Step-by-Step Jumping Procedure

Step 1: Safety First. Ensure the truck is in Park or Neutral with the parking brake engaged. Disconnect the negative battery cable before making any connections to avoid electrical shorts or sparks. Work in a well-ventilated area away from any fuel vapors.

Step 2: Locate the Relay. Locate the fuel pump relay in the under-hood fuse box or under the dash, as described above.

Step 3: Remove the Relay. Gently rock the relay back and forth while pulling upward to remove it from its socket.

Step 4: Identify the Terminals. Look into the relay socket. You will see four or five slots. The two largest slots are typically terminals 30 and 87. Terminal 30 is the constant battery power input. Terminal 87 is the load output to the fuel pump. Use a multimeter to confirm that terminal 30 has constant 12V power when the ignition is on.

Step 5: Connect the Jumper Wire. Take your jumper wire and insert one end into the slot for terminal 30 and the other end into the slot for terminal 87. You should immediately hear the fuel pump hum or gurgle from the rear of the truck near the fuel tank. If you hear the pump running, the fuel pump itself is likely working, and the problem is the relay or the control circuit.

Step 6: Attempt to Start the Truck. With the jumper wire firmly in place, get into the cab and attempt to start the engine. If the truck starts and runs perfectly, you have confirmed that the fuel pump and wiring are healthy, and the issue lies in the relay itself or the control signal from the ECM.

Step 7: Remove the Jumper Wire. Once the test is complete, remove the jumper wire immediately. Do not leave the jumper wire in place for extended periods, as it will keep the fuel pump running continuously, even when the engine is off. This can drain the battery, overheat the fuel pump, and create a serious fire hazard in the event of a collision.

Common Symptoms of a Bad Fuel Pump Relay

Knowing the symptoms of a failing fuel pump relay can help you diagnose the problem quickly. Here are the most common signs:

1. Engine cranks but won't start. This is the most obvious symptom. You can hear the starter motor turning the engine over, but the engine never fires up because no fuel is being delivered.

2. No fuel pump prime sound. When you turn the key to the ON position, you should hear a 2-3 second hum from the fuel tank. If you hear nothing, the relay may be faulty.

3. Truck starts only after extended cranking. If the relay is failing intermittently, the engine may only start after you crank it for a long time. During this extended cranking, the engine may build enough oil pressure to close the oil pressure switch, which provides an alternate power path to the fuel pump.

4. Truck dies immediately after releasing the key from "Start" to "Run." If the engine starts but dies as soon as you let go of the key, the oil pressure switch path may be faulty, or the relay is not staying engaged.

5. Intermittent stalling. The truck may run fine for a while and then suddenly stall without warning. This can happen when the relay overheats and fails while driving, then works again after it cools down.

Other Potential Causes of Fuel Pump Issues

While a faulty fuel pump relay is a common cause of fuel delivery problems, it is essential to consider other potential issues. If the relay tests good but the pump still doesn't run, check these components:

Blown fuel pump fuse. Locate the fuel pump fuse (usually a 20-amp fuse) in the under-hood fuse box. Remove it and inspect the element visually. If it is blown, replace it.

Faulty fuel pump. The fuel pump itself may be worn out or damaged. If you jump the relay and the pump still doesn't run, the pump may need replacement.

Clogged fuel filter. A clogged fuel filter can restrict fuel flow to the engine, causing symptoms similar to a bad pump or relay.

Corroded wiring or ground. The fuel pump ground wire is exposed to road salt, moisture, and debris. A corroded ground can cause the pump to run slowly or intermittently. Measure the voltage drop between the pump ground and the battery negative terminal. It should be less than 0.2 volts.

Faulty ECM. In rare cases, a faulty ECM can prevent the fuel pump relay from being activated.

How to Replace the Fuel Pump Relay

Replacing the fuel pump relay is a straightforward process that requires no special tools. Here is a step-by-step guide:

  1. Purchase a new relay. Buy a new fuel pump relay that is compatible with your 1990 Chevy truck. Ensure the new relay has the same part number or specifications as the old relay. Do not try to use a different type of relay from another vehicle, as it may cause electrical issues or damage the system.

  2. Disconnect the negative battery cable. This is a safety precaution to prevent electrical shorts.

  3. Locate the old relay. Find the fuel pump relay in the relay center, as described above.

  4. Remove the old relay. Gently rock the old relay back and forth while pulling upward to remove it from its socket.

  5. Plug in the new relay. Insert the new relay into the socket, ensuring it is properly seated. Push down firmly until you feel it click into place.

  6. Reconnect the negative battery cable. Reattach the battery cable and tighten it securely.

  7. Test the system. Turn the ignition key to the ON position and listen for the fuel pump prime sound. Then try to start the engine. If the engine starts and runs smoothly, the replacement was successful.

Safety Precautions

Working on the fuel system of a 1990 Chevy truck involves inherent risks. Always follow these safety precautions:

Disconnect the battery. Before working on any electrical components, disconnect the negative battery cable to prevent electrical shocks and shorts.

Avoid fuel vapors. Work in a well-ventilated area. Do not smoke or have any open flames near the vehicle. Even a single spark near the fuel tank can cause a catastrophic explosion.

Use proper wire sizing. When jumping the relay, use a 12-gauge or heavier insulated wire. Never use a smaller gauge wire than the original factory specification, as undersized wire will overheat and increase resistance.

Do not leave the jumper wire in place. Jumping the relay is a temporary test only. Leaving the jumper wire in place can cause the fuel pump to run continuously, draining the battery and creating a fire hazard.

Inspect for fuel leaks. After working on the fuel system, check for any fuel leaks before starting the engine. Wrap a rag around fuel fittings when loosening them to prevent gasoline from spraying.

Why Choose KEMSO Fuel Pump for Your 1990 Chevy Truck

When it comes time to replace the fuel pump itself, you want a product that delivers reliable performance and long-lasting durability. KEMSO offers high-performance fuel pumps that are designed as direct OEM replacements for a wide range of vehicles, including 1990 Chevy trucks. All KEMSO fuel pumps are built to meet or exceed original equipment specifications, ensuring a perfect fit and reliable operation. Every KEMSO fuel pump comes with a lifetime warranty, giving you complete peace of mind. If you ever have an issue, KEMSO stands behind their product. Visit the KEMSO website at https://www.kemsoracing.com/ to browse their selection of fuel pumps and other automotive products. Whether you need a replacement for your 1990 Chevy truck or any other vehicle, KEMSO has the right solution for you.

Final Thoughts

The fuel pump relay on a 1990 Chevy truck is a critical component that can cause starting and drivability issues when it fails. Knowing the 1990 Chevy truck fuel pump relay location under the hood on the passenger side firewall, or under the dash on some models, allows you to diagnose and fix the problem quickly. By following the testing and jumping procedures outlined in this guide, you can determine if the relay is faulty or if the issue lies elsewhere in the fuel system. Remember to always prioritize safety when working with electrical components, and replace the relay with a compatible part. If you need a new fuel pump, consider KEMSO for high-quality, OEM-replacement fuel pumps backed by a lifetime warranty. Visit https://www.kemsoracing.com/ for more information.


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