1991 Chevy S10 Fuel Pump Relay Location: Your Ultimate Guide (Find it Fast & Fix the Problem)

Finding the fuel pump relay in your 1991 Chevy S10 is straightforward: it's located inside the engine compartment, mounted on the firewall (the vertical metal wall separating the engine bay from the passenger cabin), directly behind the brake master cylinder. This assembly, often called the Underhood Electrical Center or Power Center, houses several vital fuses and relays. The fuel pump relay is typically situated within this black, rectangular box, clearly labeled among its components. Knowing this exact spot is essential when diagnosing a no-start condition or a silent fuel pump.

While the core location is consistent across the 1991 S10 models (whether it's the standard S10, the S10 Blazer, or the different cab styles), understanding exactly where to look, what you're seeing, and how to confirm you have the right relay saves valuable time and frustration. This detailed guide walks you through every step, explains the relay's vital role, how to test it, and how to address common pitfalls, ensuring you can tackle this repair with confidence.

Understanding the Fuel Pump Relay: Why its Location Matters

Before digging into the precise spot, grasp what the fuel pump relay does and why knowing where it is becomes crucial during troubleshooting:

  1. The Power Switch: The fuel pump relay acts like a high-power switch. The relatively small electrical signal from your ignition switch (or oil pressure switch/ECM command, depending on startup phase) isn't designed to handle the substantial electrical current the fuel pump motor requires. The relay bridges this gap.
  2. Relay Function Simplified: When you turn the key to "Run" or "Start," a control circuit energizes an electromagnet inside the relay. This magnet pulls internal contacts closed, connecting the large power source (directly from the battery via a fuse) to the fuel pump motor circuit. Power flows, the pump runs.
  3. Failure Points: Like any electrical component, relays can fail. Internal contacts can weld shut (pump runs constantly), burn out (no connection), become corroded, or have loose connections. A failed relay prevents the fuel pump from getting power, stopping the engine from starting.
  4. Symptom Correlation: The primary symptom of a bad fuel pump relay is the fuel pump does not make its characteristic brief humming noise for 2-3 seconds when the ignition is turned to the "Run" position (before cranking). This often leads directly to a "crank but no start" condition. Knowing the relay location lets you quickly verify or rule it out as the culprit.

Pinpointing the 1991 S10 Fuel Pump Relay Location: A Step-by-Step Walkthrough

Now, let's get you directly to the relay. Follow these visual and physical cues:

  1. Locate the Engine Compartment (Hood Release): Pull the hood release lever inside your truck, usually found under the dashboard near the driver's door. Go outside, release the secondary safety latch (usually in the center front of the hood), and lift the hood securely.
  2. Identify the Brake Master Cylinder: Standing at the front of the truck, look just behind and slightly to the driver's side (left side when facing the truck) of the engine compartment. You'll see a metal reservoir with brake fluid, attached to the brake booster (a large, round, black or silver vacuum chamber). This is the brake master cylinder assembly.
  3. Focus on the Firewall Behind the Master Cylinder: Directly behind the brake master cylinder, mounted flat against the vertical metal firewall, you will see a black, rectangular plastic box. This is the Underhood Electrical Center or Power Center.
  4. Identify the Relay/Fuse Block: This box has a large, visible plastic lid that snaps or screws shut. It houses many fuses and relays critical to engine and vehicle operation.
  5. Remove the Cover: Carefully unclip or unscrew (depending on your specific block) the plastic cover to expose the contents inside. Place the cover somewhere safe.
  6. Understand the Layout: Inside, you'll see several cube-shaped relays and various fuses arranged in rows or clusters. These components are typically labeled directly on the plastic housing or on a diagram attached to the underside of the cover you just removed.
  7. Find the Fuel Pump Relay Label: Crucially, look for the relay labeled "FP" (Fuel Pump), "FUEL PUMP," or sometimes "ECM B" (though FP or FUEL PUMP is most common for '91). Common labels printed on the plastic near the relay socket include FP, FUEL P/L, or similar abbreviations clearly indicating the fuel pump. This label might be beside the relay socket, molded into the plastic base nearby, or listed on an overlay diagram.
  8. Visual Confirmation: The fuel pump relay in your 1991 S10 will look identical to the other standard cube relays in the box (like the horn relay or cooling fan relay). It's typically a small, black or dark grey plastic cube approximately 1 inch (25mm) square, with 4 or 5 blade terminals sticking out of its bottom. It plugs into a corresponding socket in the base.
  9. Reference the Underhood Diagram: ALWAYS consult the diagram printed on the underside of the lid you removed. This diagram maps out every fuse and relay position, confirms amperage ratings, and explicitly identifies the fuel pump relay location (FP, FUEL PUMP, etc.). Match the position in the diagram to the physical location in the box to be absolutely certain.

Detailed Visual Aids & Diagrams for Absolute Clarity

Finding a relay in a busy fuse box requires matching what you see physically to an authoritative guide. Here's how to ensure you're looking at the right component:

  • Referencing the Service Manual: The definitive source is the 1991 Chevrolet S10 Pickup/Blazer Service Manual. Electrical section schematics and component location views explicitly show the Underhood Electrical Center on the firewall behind the brake master cylinder, indicating the FP or FUEL PUMP relay position.
  • Common Labeling Convention: While minor variations can occur in early vs. late '91 production, FP is overwhelmingly the standard label used by GM for this relay slot in the S10 power center during this model year.
  • Using Known Relays for Comparison: Notice that relays like the horn or cooling fan might be similarly sized. If unsure, pulling the horn relay (often labeled HORN) and comparing it visually confirms the relay type. Crucially, you can often safely swap the horn relay (if identical) with the suspected fuel pump relay temporarily for testing. If the horn works when plugged into the fuel pump socket (and you hear the pump run!), it confirms a bad fuel pump relay. If the horn doesn't work in its own socket or the fuel pump socket, it points to wiring issues elsewhere. If the pump runs with the horn relay in its socket, your original fuel pump relay is faulty.

Confirming You Have the Right Relay: Testing & Verification

Simply finding the component labeled FP isn't always the end of the story. Here's how to be sure it's the fuel pump relay and how to check if it's functioning:

  1. The Ignition Key Test:
    • Have a helper turn the ignition key to the "Run" position (do not crank the engine).
    • Place your fingers lightly on the relay body.
    • You should feel and often hear a distinct "click" within 1-3 seconds as the relay energizes. This is the electromagnet pulling the contacts closed.
    • After 2-3 seconds, you should feel/hear another "click" as the relay de-energizes (unless oil pressure is sensed, keeping it closed on some older systems).
    • If you DO NOT feel/hear this click: It strongly suggests a problem with the relay or the control circuit telling it to switch on. Proceed to test the relay itself or swap it.
  2. The Swap Test (Safest & Most Practical):
    • Identify another relay in the box known to be working and visually identical to the fuel pump relay. The Horn Relay is almost always identical and an excellent candidate.
    • Carefully pull both relays straight out of their sockets (you may need to wiggle slightly if tight).
    • Insert the known good horn relay into the fuel pump relay socket.
    • Turn the ignition key to "Run." Listen carefully near the gas tank filler neck or under the rear bed. You should now hear the fuel pump prime for 2-3 seconds. If it does, your original fuel pump relay is faulty. If it does not:
      • Try swapping another known good relay (like the A/C relay if identical).
      • Check the large fuse powering the fuel pump circuit (often labeled FP or EFI/PUMP, usually 20A - verify on your lid diagram!). It's located in the same underhood power center.
      • If fuse is good and swapping known good relays gets no pump action, the problem likely lies deeper (fuel pump ground, wiring harness issue, bad pump, ECM command problem).
    • If the fuel pump ran with the swapped relay, replace the original fuel pump relay.
  3. Multimeter Testing (For Confirmation):
    • Requires a basic digital multimeter.
    • Set meter to measure DC Volts (usually 20V scale).
    • Check Power Feed: With the relay removed from its socket, turn the ignition key to "Run." Carefully probe socket terminal 30 (common designation - often one of the larger terminals; check diagram!) with the meter's red lead. Ground the black lead to the battery negative or clean metal. You should see battery voltage (approx. 12V). If not, the fuse feeding the relay is blown or there's a wiring break.
    • Check Control Circuit: With ignition "Run," probe socket terminal 86 (often a smaller terminal; check diagram!) with the red lead (black to ground). Should read approx. 12V. If not, the problem is in the switch/ECM/oil pressure circuit activating the relay.
    • Test Relay Itself (Bench Test): Remove the relay. Using your meter in resistance (Ohms) mode. Identify the control coil terminals (usually 85 and 86). You should measure some resistance (typically 50-100 ohms; an open circuit indicates a bad coil). Identify the switch terminals (usually 30 and 87). With no power, resistance should be infinite (open circuit). Apply 12V across 85 (-) and 86 (+); you should hear a click, and 30 to 87 should now show near zero ohms (closed circuit). Remove power, it should click open again. Failure at any point means a bad relay.

Understanding Common Symptoms & Related Issues

Finding the relay is step one; understanding why you need to find it is crucial:

  • Primary Symptom: No Fuel Pump Prime Sound: Turning the key to "Run" yields no audible "hum" from the fuel pump located in the gas tank for 2-3 seconds. Action: Start by checking the relay and fuse.
  • Secondary Symptom: Cranks But No Start: The engine cranks over with the starter motor but fails to fire or run. This can have many causes, but a silent fuel pump is a top suspect. Action: Check fuel pump prime sound. If missing, proceed to relay/fuse checks.
  • Intermittent No-Start: Especially when hot. A failing relay can work intermittently. Action: When the failure occurs, check immediately for the fuel pump prime sound at key-on. If missing, suspect relay or thermal failure.
  • Fuel Pump Runs Constantly: Less common, but if relay contacts weld shut. Action: Verify pump runs with key off/out. Locate relay and disconnect it. If pump stops, replace relay.
  • Not Just the Relay: A blown FP/EFI/PUMP fuse (located in the same underhood box) will also cause a no-pump condition. Always visually inspect and test the fuse. A completely dead fuel pump, faulty oil pressure switch (backup signal), broken wire, bad ground connection, or faulty ignition switch can mimic a bad relay.

Safety First: Crucial Precautions Before You Start

Working around electricity and fuel systems requires caution:

  1. Disconnect the Battery: ALWAYS disconnect the negative (-) battery terminal before touching relays, fuses, or wiring in the underhood box. This prevents shorts, sparks, and potential damage to sensitive electronics (like the ECM). Wrap the cable end to prevent accidental contact.
  2. Relieve Fuel System Pressure: Although disconnecting the battery stops the pump, residual pressure remains in the fuel lines. Before working on any fuel line connection (test port, filter, rail), relieve pressure safely. Consult a manual for the exact Schrader valve location (on the fuel rail) and procedure, typically involving careful rag placement while slowly depressing the valve core.
  3. No Smoking/Open Flames: Obvious, but critical near gasoline vapors. Work in a well-ventilated area.
  4. Use Correct Replacement: Ensure the replacement relay matches the original specifications (contact rating, terminal configuration). An auto parts store can match yours visually or via the OE part number (common GM relay types like AC Delco/Denso are widely available).
  5. Secure Wiring: When reinstalling a relay or fuse, ensure it is fully seated in its socket. Loose connections cause heat and failure.
  6. Check Terminals: While the relay is out, inspect the metal terminals inside the power center socket for any signs of corrosion, burning, melting, or bent pins. Clean gently with contact cleaner and a small brush if needed. Severe damage may require socket replacement.

Beyond the Relay: When the Problem Persists

If you've located, tested, and potentially replaced the fuel pump relay and fuse, and you're still not hearing the fuel pump run, the issue lies elsewhere. Here's where to look next:

  1. Fuel Pump Itself: The most common outcome after confirming relay and fuse is good. The pump motor can fail due to age, contamination, or running the tank very low frequently. Access is typically via an access panel under the truck bed or inside the cab behind the seat (Blazer/Jimmy). Requires specialized tools and dropping the tank. Testing involves verifying power and ground at the pump connector near the tank.
  2. Fuel Pump Ground: A critical and often overlooked component. The fuel pump relies on a solid ground connection. Common ground points are under the truck near the frame or near the fuel tank. Locate the ground strap/wire, disconnect it, clean the connection points (wire terminal and chassis) thoroughly to bare metal, reattach securely, and coat with dielectric grease. A poor ground prevents the pump from running even with good power.
  3. Ignition Switch: Provides the initial "Run" signal to energize the fuel pump relay. Failure in the ignition switch can prevent this signal. Symptoms often include other accessories losing power or acting strangely.
  4. Engine Control Module (ECM) / Fuel Pump Control Circuit: The ECM typically controls the relay ground path on modern EFI systems after the initial prime. While the initial prime is often triggered by the ignition switch, sustained running relies on the ECM. Faulty ECM, bad wiring connections at the ECM, or damaged wiring along the signal path from the ECM to the relay socket terminal 85 can prevent the relay from energizing or staying energized. Requires more advanced diagnostics.
  5. Oil Pressure Switch (Backup Signal): Some older GM systems use an oil pressure switch as a backup/secondary method to command the fuel pump relay on if the primary circuit fails. A faulty oil pressure switch won't usually prevent the initial prime but can cause the engine to die shortly after starting if it's the only path left. Often has a wire going to the relay socket terminal 85 along with the ECM signal.
  6. Wiring Harness Damage: Inspect the wiring harness from the underhood power center towards the fuel tank. Look for any signs of chafing, pinching (especially where it passes through the frame near the driver's side door area), rodent damage, or corrosion. A broken wire anywhere in the power feed (30 to pump) or control circuit (86, 85) will stop the pump.

Conclusion: Mastering the Core of Your S10's Fuel Delivery

The heart of diagnosing many common starting problems in your 1991 Chevy S10 begins at the firewall-mounted Underhood Electrical Center, just behind the brake master cylinder. The fuel pump relay, typically labeled FP or FUEL PUMP, is a critical switch residing there. By methodically locating, testing (using the click test, swap test, and/or multimeter), and replacing this inexpensive relay when necessary, you've addressed a frequent failure point.

Always prioritize safety - disconnect the battery first - and double-check the fuse in the same box. If resolving the relay doesn't restore the crucial fuel pump prime sound, you're equipped with the knowledge of where to look next: the fuel pump itself, its vital ground connection, the ignition switch, the wiring harness integrity, or related ECM controls. Remember, the simple act of precisely locating and verifying the operation of the relay behind the master cylinder is often the quickest path to getting your reliable 1991 S10 Pickup or Blazer back on the road.

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