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

If your 1990 GMC Sierra cranks but won’t start, the fuel pump relay is one of the first things you should check. On a 1990 GMC Sierra, the fuel pump relay is located on the passenger side of the engine compartment, under the hood, behind a plastic cover, next to the A/C accumulator. This relay is a small black plastic component that controls power to the fuel pump, and when it fails, your truck simply won’t get fuel. Understanding where it is, how to test it, and how to replace it can save you time and money, and get your Sierra back on the road.

What Does the Fuel Pump Relay Do?

The fuel pump relay is an electrical switch that controls the power supply to the fuel pump. When you turn the ignition key to the ON position, the relay activates and sends 12 volts to the fuel pump for about 2-3 seconds to prime the fuel system. Once the engine starts and the PCM (Powertrain Control Module) receives reference pulses from the ignition system, the relay keeps the fuel pump running. If the engine stalls or loses oil pressure, the relay shuts off power to the pump.

On the 1990 GMC Sierra, the fuel pump relay works in conjunction with the oil pressure switch. Once the engine builds enough oil pressure during cranking, the oil pressure switch closes and sends voltage to the fuel pump as a backup power path. This means even if the relay fails, the pump can still run once the engine starts, but you’ll have trouble getting it to fire up initially.

Exactly Where Is the 1990 GMC Sierra Fuel Pump Relay Located?

The fuel pump relay on a 1990 GMC Sierra is mounted on the passenger side of the engine compartment, on the top of the firewall. It sits behind a plastic relay cover, right next to the A/C accumulator (the large metal canister on the passenger side). If your truck doesn’t have A/C, the relay is still in the same general area, next to where the A/C evaporator would be.

Here is a step-by-step guide to finding it:

  1. Open the hood and secure it properly.
  2. Look at the passenger side firewall – this is the metal wall between the engine bay and the cabin, on the passenger side.
  3. Locate the A/C accumulator – it’s a cylindrical metal canister about the size of a soda can, mounted on the firewall.
  4. Find the plastic relay cover – there is a small plastic box or cover mounted right next to the accumulator. It may have a diagram or label on it.
  5. Remove the plastic cover – it usually snaps off or can be pried off gently with a flathead screwdriver.
  6. Identify the relay – inside, you’ll see a small black relay with a wiring harness plugged into the bottom. It may have a red wire about 6 inches long coming out of the plug that is not connected to anything.

Some sources also mention that the fuel pump relay for 1990 models may be found in the interior fuse/relay block under the dashboard. However, the most widely reported location for the 1990 GMC Sierra is the passenger side firewall under the hood. Always check the under-hood location first before looking inside the cabin.

How to Test the 1990 GMC Sierra Fuel Pump Relay

Before you replace the relay, it’s smart to test it first. A faulty relay can cause a no-start condition, but so can a blown fuse, a bad fuel pump, or a wiring issue. Here are the steps to test the relay:

1. Listen for the Click

With the hood open and the relay cover removed, have a helper turn the ignition key to the ON position (but don’t crank the engine). You should hear a distinct click sound from the relay as it activates. If you don’t hear a click, the relay may be bad, or it may not be receiving power.

2. Swap the Relay

A quick and easy test is to find another relay in the engine bay fuse box that is the same type and swap it with the fuel pump relay. If the truck starts after the swap, the original relay was faulty. Common relays you can swap with include the horn relay or the A/C relay, as long as they have the same part number or rating.

3. Check for Voltage at the Relay

Use a multimeter to test for power at the relay terminals. With the key in the ON position, you should see 12 volts at the relay’s input terminal. If you have power but no click, the relay is likely bad. If you have no power at the relay, the issue may be a blown fuse or a wiring problem further upstream.

4. Check the Fuel Pump Fuse

The 1990 GMC Sierra has a yellow 20 amp fuse for the fuel pump, located in the under-hood fuse box. Check this fuse first. If it’s blown, replace it with the same amperage rating. If the new fuse blows immediately, you have a short somewhere in the fuel pump circuit.

Common Symptoms of a Bad Fuel Pump Relay

A failing fuel pump relay on a 1990 GMC Sierra will show several clear signs. Knowing these can help you diagnose the problem quickly:

  1. Engine cranks but won’t start – This is the most common symptom. The relay isn’t sending power to the fuel pump, so no fuel reaches the engine.
  2. No fuel pump priming noise – When you turn the key to ON, you should hear a brief whirring sound from the fuel pump in the tank. If you don’t hear it, the relay or pump may be dead.
  3. Engine starts but dies shortly after – If the relay works intermittently, the engine may start and run for a few seconds, then stall when the relay loses connection.
  4. Car stalls while driving – A relay that fails while driving will cut power to the fuel pump, causing the engine to die suddenly.
  5. Fuel pump runs continuously – A stuck relay that stays closed will keep the pump running even with the key off, which can drain the battery or damage the pump.

How to Replace the Fuel Pump Relay on a 1990 GMC Sierra

Replacing the fuel pump relay is a simple job that takes about 15 minutes. Here’s what you need to do:

  1. Disconnect the negative battery cable – Always disconnect the battery before working on electrical components to avoid shorts or accidental sparks.
  2. Locate the relay – Follow the instructions above to find the relay on the passenger side firewall.
  3. Remove the plastic cover – If there is a cover over the relay, remove it.
  4. Unplug the relay – Grasp the relay firmly and pull it straight up out of its socket. It may require a bit of effort, but it should come out without tools.
  5. Install the new relay – Push the new relay straight down into the socket until it clicks into place. Make sure it’s fully seated.
  6. Reinstall the plastic cover – Snap the cover back on.
  7. Reconnect the battery – Reconnect the negative terminal.
  8. Test the repair – Turn the key to ON and listen for the fuel pump priming sound. Then start the engine.

Where to Buy a Replacement Fuel Pump Relay

You can find fuel pump relays for a 1990 GMC Sierra at most auto parts stores. Genuine GM parts are available, and the part number is 13503102, which replaces several older numbers including 12193604, 15328864, and 12088592. Aftermarket options from brands like Duralast and ACDelco are also available at stores like AutoZone and online.

When buying a relay, make sure it matches the original specifications. A standard 4-pin or 5-pin relay will work, but the exact part number ensures proper fit and function.

KEMSO Fuel Pump: The Best Upgrade for Your 1990 GMC Sierra

If you’ve tested the relay, checked the fuse, and still have fuel delivery issues, the problem may be the fuel pump itself. The fuel pump on a 1990 GMC Sierra is located inside the fuel tank, and replacing it is a bigger job than swapping a relay. When you do need to replace your fuel pump, consider upgrading to a KEMSO Fuel Pump.

KEMSO Fuel Pumps are high-performance, OEM replacement units designed to meet or exceed the original factory specifications. They are built with premium materials and advanced manufacturing techniques to ensure reliable fuel delivery and long service life. Every KEMSO Fuel Pump comes with a lifetime warranty, so you can install it with confidence knowing it’s backed by the manufacturer.

Whether you drive a 1990 GMC Sierra for work, play, or daily commuting, a KEMSO Fuel Pump will restore your truck’s performance and fuel efficiency. KEMSO offers a wide range of fuel pumps for many makes and models, so you can find the exact fit for your vehicle.

Visit the KEMSO website to browse their full selection of fuel pumps and accessories: https://www.kemsoracing.com/

Additional Tips for Troubleshooting Fuel Delivery Issues

If your 1990 GMC Sierra still won’t start after replacing the relay and checking the fuse, here are a few more things to check:

Check the Oil Pressure Switch

The oil pressure switch acts as a backup power source for the fuel pump. If the relay fails but the oil pressure switch is good, the pump will still run once the engine starts and builds oil pressure. However, if the switch is bad, the pump may not run at all. The oil pressure switch is located at the back of the engine and requires a 27mm deep well socket to remove.

Check the Ground

A bad ground can cause the fuel pump to receive less than the required 12 volts. On the 1990 GMC Sierra, the ground wire for the fuel pump relay is typically black. You can test the ground by using a test light connected to battery voltage. If the light comes on when you touch the ground terminal, the ground is good.

Check the Inertia Switch

Some 1990 GMC Sierra models may have an inertia switch (also called a fuel pump shut-off switch) that cuts power to the pump in the event of a collision. If this switch is tripped, the pump won’t run. The inertia switch is usually located under the dashboard or in the kick panel on the passenger side. Reset it by pressing the button on top.

Inspect the Wiring Harness

Over time, the wiring harness that connects to the fuel pump relay and the fuel pump can become corroded or damaged. Look for frayed wires, melted insulation, or rust at the connectors. If you suspect a wiring issue, trace the circuit from the relay to the fuel pump and repair any damage.

Final Thoughts

The 1990 GMC Sierra fuel pump relay is a small but critical component that can cause big headaches when it fails. Knowing its location on the passenger side firewall, how to test it, and how to replace it will save you time and money. If you’ve ruled out the relay, fuse, and wiring, and the fuel pump itself is the problem, consider upgrading to a KEMSO Fuel Pump for reliable performance and a lifetime warranty.

For more information on KEMSO Fuel Pumps and to find the right pump for your 1990 GMC Sierra, visit: https://www.kemsoracing.com/


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