1990 Chevy 1500 Fuel Pump Relay Test: A Complete Step-by-Step Diagnosis Guide

If your 1990 Chevy 1500 cranks but won't start, the fuel pump relay is one of the first things you need to check. The fuel pump relay controls power to the fuel pump, and when it fails, the engine won't get fuel. Testing this relay is straightforward and can save you time and money before replacing more expensive parts like the fuel pump itself. The relay is located in the under-hood relay center, and you can test it with a multimeter or by swapping it with a known good relay in just a few minutes.

Where Is the Fuel Pump Relay on a 1990 Chevy 1500?

The fuel pump relay on a 1990 Chevy 1500 is located in the under-hood relay control center. This black plastic box is mounted on the firewall on the passenger side of the engine compartment. To access it:

  1. Open the hood and locate the relay control center on the passenger side firewall, near the top of the engine compartment.
  2. Remove the cover by twisting off the plastic nuts or clips holding it in place. The cover usually has a diagram printed on the inside that shows the exact location of each relay.
  3. Identify the fuel pump relay using the diagram. It is typically labeled as "Fuel Pump" or "FP". On many 1990 models, it is a black or gray relay with four or five pins.

If the diagram is missing or unclear, look for a relay that matches the description in your owner's manual or a service manual for your specific model.

How to Test the 1990 Chevy 1500 Fuel Pump Relay

There are several methods to test the fuel pump relay. The most reliable method involves using a multimeter to check continuity and voltage. Here is a step-by-step guide for each method.

Method 1: Listen for the Relay Click

This is the simplest and quickest test. It does not require any tools.

  1. Sit in the driver's seat and turn the ignition key to the "ON" position (do not crank the engine).
  2. Listen carefully for a clicking sound coming from the under-hood relay center. You should hear a single click when the relay energizes.
  3. Have someone help you if needed. One person turns the key while the other listens near the relay center.

If you hear a click, the relay coil is likely working. If you do not hear a click, the relay may be faulty, or there may be a problem with the control circuit from the PCM.

Method 2: Swap the Relay with a Known Good One

This is a practical test that does not require any special tools. Many relays in the relay center are identical, such as the horn relay or A/C relay.

  1. Locate the fuel pump relay and another relay of the same type (e.g., the horn relay).
  2. Swap them by pulling each one straight out of its socket and plugging the other into the fuel pump relay slot.
  3. Turn the ignition to "ON" and listen for the fuel pump to prime. If the fuel pump now works, the original fuel pump relay is faulty.
  4. Test the other component (e.g., the horn) to confirm the swap is valid. If the horn no longer works, you have confirmed the original relay was bad.

Method 3: Test with a Multimeter

For a more definitive test, use a multimeter to check the relay's internal components. This method requires the relay to be removed from the vehicle.

Step 1: Remove the Relay

Disconnect the negative battery cable before removing the relay to avoid electrical hazards. Pull the relay straight out of its socket.

Step 2: Check the Coil Resistance

  1. Set your multimeter to the ohms (Ω) setting.
  2. Touch the probes to the two coil terminals on the relay. These are usually the smaller terminals, often labeled 85 and 86.
  3. You should read a resistance between 50 and 150 ohms. This range is typical for a fuel pump relay coil.
  4. If the reading is zero ohms, the coil is shorted. If the reading is infinite (OL), the coil is open. Either case means the relay is bad and needs to be replaced.

Step 3: Test the Switch Contacts (Unpowered)

  1. With the relay still removed, set your multimeter to the continuity (beep) mode or ohms setting.
  2. Touch the probes to the two switch terminals, usually labeled 30 and 87.
  3. With no power applied, the multimeter should show infinite resistance (no continuity). This means the contacts are open, which is normal for an unpowered relay.

Step 4: Activate the Relay and Test the Contacts

  1. Apply 12 volts to the coil terminals. Connect the positive lead of a 12V battery (or a jumper wire from the vehicle battery) to terminal 86, and the negative lead to terminal 85.
  2. You should hear a distinct "click" as the relay activates.
  3. While power is applied, touch the multimeter probes to terminals 30 and 87 again. You should now see very low resistance (close to zero ohms) and hear a beep for continuity.
  4. If you do not hear a click, or if the contacts do not show continuity when powered, the relay is faulty.

Method 4: Check for Power and Ground at the Relay Socket

If the relay itself tests good, the problem may be in the wiring or the PCM. You can test the relay socket with the relay removed.

  1. Set your multimeter to DC voltage (20V scale).
  2. Check for battery voltage at the power terminal (usually pin 30) with the ignition on. You should see around 12 volts.
  3. Check the ground terminal (usually pin 85) by setting the multimeter to continuity and touching one probe to the terminal and the other to a known good ground on the chassis. You should see close to zero ohms.
  4. Check the control signal from the PCM. With the ignition on, the control terminal (usually pin 86) should show battery voltage for a few seconds as the PCM commands the relay to energize.

If you have power and ground at the socket but the relay does not activate when you plug it in, the relay is likely bad. If the relay activates but the fuel pump does not run, the problem may be in the wiring from the relay to the fuel pump, or the fuel pump itself.

Common Symptoms of a Bad Fuel Pump Relay

Knowing the symptoms helps you diagnose the problem faster. Here are the most common signs that your fuel pump relay is failing:

  • Engine cranks but will not start: This is the most common symptom. The engine turns over, but there is no fuel pressure because the fuel pump is not getting power.
  • No fuel pump priming sound: When you turn the key to "ON," you should hear a faint whirring or humming sound from the fuel tank for about two seconds. If you do not hear this sound, the relay may not be sending power to the pump.
  • Intermittent starting issues: The truck may start sometimes and not others. This is a classic sign of a failing relay with worn contacts.
  • Engine stalls while driving: If the relay fails while the engine is running, the fuel pump loses power, and the engine dies immediately.
  • Fuel pump runs continuously: In rare cases, a stuck relay can keep the pump running even after the engine is off. This can drain the battery.

Other Potential Causes of No Start

If the fuel pump relay tests good, do not assume the relay is the only problem. Other issues can cause similar symptoms. Here are other common causes to check:

  • Blown fuel pump fuse: Check the fuse for the fuel pump in the under-hood fuse box. A blown fuse will prevent power from reaching the pump.
  • Faulty fuel pump: The pump itself may be worn out or seized. A fuel pressure test can confirm this.
  • Clogged fuel filter: A severely clogged filter can restrict fuel flow and cause the engine to stall or not start.
  • Wiring problems: Corroded or damaged wires between the relay, the fuel pump, and the PCM can interrupt power.
  • Faulty PCM: In rare cases, the PCM fails to send the signal to activate the relay. This is less common but possible.

How to Replace the 1990 Chevy 1500 Fuel Pump Relay

Replacing the fuel pump relay is a simple process that takes about five minutes. Here is how to do it:

  1. Disconnect the negative battery cable to prevent any electrical shorts.
  2. Locate the fuel pump relay in the under-hood relay center as described above.
  3. Pull the old relay straight out of its socket. Use a pair of needle-nose pliers if it is stuck.
  4. Insert the new relay into the same socket. Make sure it is fully seated and clicks into place.
  5. Reconnect the negative battery cable.
  6. Test the fuel pump by turning the ignition to "ON" and listening for the priming sound. If you hear it, the replacement was successful.
  7. Start the engine to confirm everything works.

Always purchase a relay that matches the original specifications. The part number should be the same as the one you removed. You can find replacement relays at any auto parts store or online.

Recommended Fuel Pump: KEMSO

If you discover that the fuel pump itself is faulty after testing the relay, you need a reliable replacement. KEMSO offers high-performance fuel pumps that are designed as direct OEM replacements. Every KEMSO fuel pump is built to meet or exceed original equipment specifications, ensuring a perfect fit and reliable operation for your Chevy 1500. All KEMSO fuel pumps come with a lifetime warranty, giving you peace of mind that your investment is protected. Whether you drive a truck, car, or powersports vehicle, KEMSO has a fuel pump solution for you. Visit their website to explore their full lineup and find the right pump for your needs: https://www.kemsoracing.com/

Final Thoughts

Testing the 1990 Chevy 1500 fuel pump relay is a straightforward job that any DIY mechanic can handle. Start with the simple listening test, then move to the relay swap, and finally use a multimeter for a definitive diagnosis. Most of the time, a bad relay is the culprit, and replacing it is cheap and easy. If the relay is good, move on to checking the fuse, wiring, and fuel pump itself. With this guide, you can confidently diagnose and fix the problem, getting your truck back on the road without unnecessary trips to the shop.

For more automotive guides and high-quality replacement parts, including fuel pumps and relays, visit KEMSO Racing at https://www.kemsoracing.com/


Back to blog