The Official Guide to Finding the 1991 GMC Sierra Fuel Pump Relay Location (With Testing & Fixes)

Don't waste time searching! The fuel pump relay on your 1991 GMC Sierra C/K 1500, 2500, or 3500 series truck is almost always located in the primary underhood electrical center, mounted on the passenger side inner fender near the brake master cylinder. Look for a small black plastic box – the relay is inside, specifically identified as a brown or tan square relay, typically labeled "FUEL PUMP," "F/P," or "FP."

This relay is the critical electrical switch controlling power to your truck's fuel pump. When it fails or its connections get dirty, your engine might crank but won't start due to lack of fuel pressure. Knowing its exact location is essential for troubleshooting and quick fixes. Let's get you straight to the source so you can check or replace it.

Precise Location in the Underhood Electrical Center

  1. Open the Hood: Secure the hood prop.
  2. Go to the Passenger Side: Stand on the passenger side of the engine compartment.
  3. Locate the Master Cylinder/Brake Booster: Identify the large, round brake booster mounted to the firewall (driver's side of the booster faces the engine). Just in front and to the outside (towards the fender) of the brake booster, you will find the Primary Underhood Electrical Center.
  4. Identify the Box: It's a rectangular black plastic box, roughly 8-10 inches long, mounted vertically against the inner fender panel.
  5. Remove the Cover: The cover is usually held on by plastic clips. Squeeze the clips or press down on tabs and lift the cover straight up.
  6. Find the Fuel Pump Relay: Inside the box, you'll see several relays and fuses.
    • The fuel pump relay is typically a brown or tan square plastic relay, about 1.5 inches square. This color is a key identifier.
    • It is usually one of the relays near the top row or centered within the relay positions.
    • Crucially, it will be labeled. Look closely on the underside of the cover you just removed. Molded into the plastic should be a diagram identifying each relay position. Find the one labeled "FUEL PUMP," "F/P," "FP," or sometimes "ECM/FP." Match this label to the relay's position in the box. If the cover diagram is missing or worn, the brown/tan color and square shape are your best visual guides.
    • You may see another relay nearby that might be the same color or shape – common ones are for the EGR valve or A/C compressor. Rely primarily on the label under the cover and the specific color.
  7. Distinguish from Fuses: Relays are larger, usually square plastic components with electrical terminals underneath (visible when pulled). Fuses are smaller, often blade-type, and clearly visible glass or plastic tops.

(Actual photo of a GM underhood electrical center - Fuel Pump Relay typically brown/tan and labeled FP or Fuel Pump.)

Visual Confirmation is Key: What You're Looking For

  • Shape: Square plastic body.
  • Size: Approximately 1.5 inches x 1.5 inches.
  • Color: Brown or Tan. This is the most consistent identifier across GM trucks of this era. While other relays (like the horn relay) might sometimes be similar in color, the location and labeling set it apart.
  • Terminals: Four or five electrical blade terminals protruding from the bottom. You don't see these until you pull it out.
  • Labeling: Position is labeled "FUEL PUMP," "F/P," "FP," or "ECM/FP" on the underside of the electrical center cover.

Why Finding the Correct Relay Matters

The fuel pump relay is the switch commanded by the Powertrain Control Module (PCM) that provides battery power to the electric fuel pump inside the fuel tank. When you turn the ignition key to "ON" (before cranking), the PCM briefly energizes the relay for about 2 seconds to prime the fuel system. When you crank or the engine is running, the PCM keeps the relay energized continuously. If this relay fails, or its circuit is compromised, the pump gets no power, leading to a no-start condition.

Simple Test: Is Your Fuel Pump Relay Working?

Suspect the relay? Before replacing anything, test it quickly:

  1. Listen for the Pump: With the ignition key in the "ON" position (don't crank), have someone listen near the fuel tank filler neck while you turn the key. You should hear a distinct "humming" sound for about 1-2 seconds. If you hear it, the relay and pump circuit are at least working momentarily. Move on to other tests. No sound? The relay is a prime suspect.
  2. Swap Test (The Best Quick Check):
    • Identify another relay in the electrical center with the same shape, size, number of terminals, and (ideally) color – common candidates are the Horn relay or the A/C Compressor relay.
    • Pull out the suspected fuel pump relay.
    • Pull out the "donor" relay (e.g., horn relay).
    • Plug the donor relay into the FUEL PUMP relay socket.
    • Turn the ignition key to "ON." Listen for the fuel pump prime. If you hear the pump run for 2 seconds now, your original fuel pump relay is faulty. If you still don't hear it, the problem is elsewhere (bad pump, wiring, blown fuse, PCM issue).
    • (Safety Note: If using the horn relay, your horn won't work while swapped. Test the horn afterwards by plugging both relays back into their original positions to confirm.)
  3. Check Fuses First! While focusing on the relay, never skip the basics. In the same underhood electrical center, look at the fuses. Identify the fuse labeled "Fuel Pump" or "ECM" (or "PCM/IGN," "IGN-ECM"). Use the diagram on the underside of the cover. Visually inspect this fuse. A blown fuse will show a broken metal strip inside the transparent top. If blown, replace it with an identical amperage fuse (e.g., 20A). Note: A blown fuse often indicates an underlying short circuit issue, not just a simple failure. If a new fuse blows immediately, further diagnosis is needed.
  4. Relay Socket Voltage Check (Requires Multimeter):
    • Carefully pull the fuel pump relay straight up out of its socket.
    • Set your multimeter to measure DC Volts (20V range).
    • Turn the ignition key to "ON" (engine off).
    • Identify the socket terminals. Two large terminals (usually adjacent) are for the power circuit: one should be hot with battery voltage at all times (BAT+), the other goes to the fuel pump (FUEL PUMP). Two smaller terminals are the control coil: one should be grounded (GND), the other should get +12V from the PCM only when the ignition is "ON" or the engine is running (IGN SW/PCM CONTROL).
    • Using the wiring diagram in your factory service manual (or referencing a known online source for 1991 Sierra), or carefully probing while an assistant turns the key:
      • Test one of the large terminals for constant +12V relative to the battery negative (-) terminal.
      • Test one of the small terminals for constant Ground.
      • Test the other small terminal for +12V only when the ignition is in the "ON" or "RUN" position (should come on for 2 secs at "ON" and stay on during cranking/running).
    • If you have constant +12V on one large terminal, ground on one small terminal, and +12V on the other small terminal when commanded, but the pump doesn't run when a known good relay is plugged in, then the problem is downstream (wiring, pump ground, or the pump itself). If any of these key voltages are missing, the problem is in the power feed, ground circuit, or PCM command.

What to Do If the Fuel Pump Relay is Faulty

  1. Replace It: This is the best permanent solution.
    • Get the Correct Part: Use a standard automotive relay meeting GM specifications. The most common and reliable part numbers are AC Delco D1746A or Standard Motor Products RY269. Confirm visually that the new relay matches the shape, terminal configuration (number and layout), and the brown/tan color of the old one. Generic relays usually work fine if specs match. Auto parts stores often list it under "Fuel Pump Relay" for your specific 1991 GMC Sierra.
    • Remove Old Relay: Pull it straight up out of the socket.
    • Install New Relay: Align the terminals and press firmly straight down into the socket until fully seated. Don't force it.
    • Test: Turn the key to "ON" and listen for the pump prime. Attempt to start the engine.
  2. Clean the Relay and Socket (If Relay Tests Good by Swap):
    • Sometimes, corrosion or dirt causes poor contact.
    • Carefully remove the relay.
    • Inspect the relay terminals and the socket contacts. Look for green/white corrosion or dark oxidation.
    • Use electrical contact cleaner spray on both the relay terminals and inside the socket. Spray, let it work for a minute, repeat. Do not use abrasive tools. A fine brush (like a clean toothbrush) or cotton swab dampened only with contact cleaner can be used gently.
    • Reinstall the relay firmly.
    • Test as above.
  3. The Temporary "Bypass" (Emergency Use ONLY):
    • Warning: This FORCES the fuel pump to run whenever the ignition is "ON". ONLY use this to confirm the pump works or to drive the vehicle DIRECTLY to a repair location. IMMEDIATELY remove the jumper once testing/driving is done. Leaving it bypassed creates a serious fire hazard if an accident occurs and the fuel pump keeps running.
    • Locate the Fuel Pump Test Terminal. On 1991 Sierras, this is a single, usually tan wire (sometimes with a black stripe) terminating at a small male spade connector. It's commonly found taped to the wiring harness near the underhood electrical center or firewall on the driver's side. If equipped with TBI, it might also be incorporated into the Assembly Line Diagnostic Link (ALDL) connector inside the cab below the dash, near the driver's knees – look for the connector labeled "G" or "F/P" (Refer to your owner's manual for the ALDL pinout). Important! This bypass method applies constant power directly to the pump circuit.
    • Procedure:
      • Turn the ignition key to "OFF".
      • Pull the Fuel Pump Relay out of its socket. This step is critical for safety.
      • Take a heavy-gauge wire (like a jumper wire or even a short piece of insulated wire with stripped ends, rated for 15-20A) and insert one end into the socket hole that was connected to the relay terminal receiving constant +12V battery power (BAT+). Usually, this terminal connects to the relay via its largest wire.
      • Insert the other end of the jumper wire into the socket hole that goes to the fuel pump (FUEL PUMP output). Note: This creates a direct connection between the constant battery feed and the pump, bypassing the relay.
      • Turn the ignition to "ON". You should immediately hear the fuel pump run continuously. The engine should now start and run as long as the jumper is connected and the key is "ON".
      • Crucial: Remember to remove the jumper wire as soon as you are done testing or get to a safe repair spot, and reinstall the relay socket correctly (even the old one if that's all you have, though replacing it is ideal). Driving with the pump bypassed is hazardous.

Why Do Fuel Pump Relays Fail on 1991 Sierras?

  • Age and Heat: Located under the hood, relays endure decades of temperature cycling. Internal solder joints weaken or crack over time.
  • Contact Wear & Pitting: Every time the relay clicks "on" or "off," tiny arcs occur at the internal electrical contacts. After millions of cycles, these contacts become pitted, eroded, and fail to conduct current properly. This is the most common failure mode.
  • Coil Failure: The electromagnetic coil that physically moves the contacts can develop an open circuit internally due to overheating or manufacturing flaws, preventing the relay from energizing.
  • Corrosion: Moisture ingress or exposure to elements can corrode internal components or external terminals.
  • Voltage Spikes: Electrical surges from bad alternators, jump-starts, or other faulty components can damage the relay coil or electronics.
  • Poor Quality: Cheap, non-OEM relays often have inferior materials and fail prematurely.

Troubleshooting Beyond the Relay: Why You Might Still Have No Fuel

Even with a new relay, your Sierra might not start. The relay is just one link in the chain. Other common culprits include:

  1. Bad Fuel Pump: The pump itself is a wear item. Common failure symptoms are loss of power, surging, whining noise from the tank, and ultimately, no fuel pressure. Confirmation requires testing fuel pressure at the TBI unit test port or using a multimeter at the pump connector.
  2. Blown Fuel Pump Fuse: As mentioned, always check this first in the underhood center. A blown fuse needs replacement but signals an underlying cause like a short circuit.
  3. Faulty Inertia Switch: Designed as a safety feature to cut fuel pump power in a collision, this switch is usually located behind the passenger-side kick panel, above the carpet line on the firewall inside the cab. It can be triggered by a significant bump. Check it! The switch has a reset button on top – press it firmly. If it clicks, it was tripped.
  4. Fuel Pump Wiring Problems: Wires chafing through their insulation (often near the frame where the harness goes from the cab to the tank, or in the engine compartment near sharp edges), corroded connectors (especially common at the pump tank connector itself, prone to road salt and moisture), or a broken wire inside its insulation.
  5. Failing Oil Pressure Switch (Backup Circuit): Older GM TBI systems have a backup fuel pump circuit via the oil pressure switch. If the primary relay circuit fails, the oil pressure switch should close once the engine builds sufficient pressure (around 4-8 psi) and power the pump. However, this circuit is notoriously unreliable as a primary starting method. Low oil pressure can prevent it from engaging. Its main purpose is engine stall prevention if the relay fails while running. Never rely solely on the oil pressure switch to diagnose a no-start. Focus first on the relay and primary power circuit.
  6. Bad Ground Connection: A critical ground point for the fuel pump is often located under the driver's side dash near the kick panel area or at the engine block. A loose, corroded, or broken ground prevents the pump circuit from completing, even with good power and relay. Inspect and clean ground points.
  7. Faulty Ignition Switch: Problems inside the ignition switch (not just the key cylinder) can prevent the PCM from receiving the "run" signal or providing power to the relay's control coil circuit. This often affects other accessories or lights.
  8. Bad PCM (Powertrain Control Module) or PCM Connections: While less common than the relay, the PCM controls the relay's ground signal. A PCM failure, or more likely, poor connections or corrosion at the PCM connectors, can prevent it from commanding the relay to energize. Reseating PCM connectors is a simple check.

(Diagram showing the 1991 GMC Sierra Fuel System Electrical Circuit: Battery -> Fuse -> Relay -> Inertia Switch -> Oil Pressure Switch -> Fuel Pump -> Ground.)

Pro Tips for Preventing Relay Problems & Enhancing Reliability

  • Use Quality Parts: Stick with reputable brands like AC Delco, Standard Motor Products, Bosch, or Genuine GM/Delphi relays. Cheap relays fail quickly.
  • Inspect Connectors: Periodically check the relay socket terminals for dirt or corrosion. Spray with electrical contact cleaner as part of preventive maintenance.
  • Address Underlying Issues: If you experience repeated relay failures or blown fuses, investigate for wiring shorts, excessive resistance in the pump circuit, or a failing fuel pump drawing too much current.
  • Check Grounds: Clean and tighten critical ground points (battery-to-engine, engine-to-frame, frame-to-body, pump ground) during routine maintenance.
  • Know Your Test Terminal: Understand where the fuel pump test terminal is (usually tan wire) for quick diagnostics without probing the ALDL.
  • Own a Multimeter: Essential for accurate electrical diagnostics beyond simple checks.

Conclusion

Locating the fuel pump relay in your 1991 GMC Sierra – under the hood, passenger side inner fender, brown/tan square relay in the primary electrical center – is the crucial first step in diagnosing or fixing a no-start situation caused by lack of fuel. Combining the relay swap test, fuse check, and listening for the pump primes provides a rapid, accurate diagnosis. Replacing a faulty relay is straightforward, but remember it's part of a larger system. If replacing the relay doesn't solve the problem, methodically check the fuse, inertia switch, oil pressure switch, wiring harness integrity (especially near the tank and frame), pump ground, and finally fuel pump operation to get your trusty Sierra back on the road.

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