1991 GMC Sierra Fuel Pump Relay Location: Your Complete & Actionable Guide (Step-by-Step)

The fuel pump relay on your 1991 GMC Sierra is located in the Underhood Electrical Center (UEC), specifically in position #14. This UEC, often called the engine compartment fuse box or power distribution center, is mounted on the driver's side firewall, near the brake master cylinder.

Understanding the precise location of this critical relay is essential for diagnosing and fixing sudden no-start conditions, engine stalling, or inconsistent fuel pump operation on your GMT400-platform Sierra. Replacing a faulty fuel pump relay is a common and relatively simple DIY repair, saving significant time and money over dealer service. This comprehensive guide provides every detail you need to confidently locate, test, and replace this relay, empowering you to get your classic truck back on the road reliably.

Getting Oriented: The Underhood Electrical Center (UEC)

• Locating the UEC: Pop open the hood of your Sierra. Stand near the driver's front fender. Look towards the back of the engine bay, directly behind the brake master cylinder and brake booster assembly. You will see a rectangular or slightly square-shaped black plastic box bolted vertically to the metal firewall – this is the Underhood Electrical Center. It's the primary hub for fuses and relays protecting and controlling major electrical components under the hood.
• Understanding the UEC Cover: This box is protected by a removable cover. On most '91 Sierras, the cover features two prominent lever-like tabs – one on the top edge and one on the bottom edge, both located towards the driver's side. Squeezing these levers inwards (towards the center of the cover) simultaneously will release their hold on the base, allowing you to lift the cover straight up and off. Set it aside safely.
• Interpreting the Diagram: The inside of the cover you just removed is crucial. It contains a molded or printed diagram clearly labeling the function and position of every fuse and relay housed within the UEC box. This diagram is your primary map. Study it carefully. Look for the legend. You are specifically searching for "Fuel Pump Relay" and its corresponding numbered slot location within the box. Note that different relays might share similar physical sizes and shapes within the box. Rely on the diagram, not guesswork, to avoid pulling the wrong component. Pay close attention to the numbering scheme – whether it lists positions horizontally, vertically, or in some other grid pattern. Identify the position labeled for the fuel pump relay.

Pinpointing Relay Position #14

• Confirming the Label: On all verified 1991 GMC Sierra models, the diagram will identify the Fuel Pump Relay slot position as #14. Double-check this on your specific cover diagram. It might say "Fuel Pmp," "FP Relay," or simply assign "#14" with "Fuel Pump" listed in the legend. Cross-reference the number and description meticulously. This diagram is tailored precisely to your truck's electrical system configuration.
• Locating #14 Inside the Box: With the cover off, peer into the UEC base. You will see slots with inserted relays and fuses. Refer constantly back to the diagram on your cover to establish the orientation and numbering sequence. Position #14 is consistently located in the TOP ROW, towards the DRIVER'S SIDE of the UEC box (closest to the firewall), but NOT in the very corner slot. It is typically the second or third slot from the driver's side end of that top row. Compare its position directly with the diagram to be absolutely certain. If you don't have the cover diagram, this position information remains accurate for the 1991 model year. However, using the cover diagram provides definitive confirmation.
• Visual Identification of the Relay: The fuel pump relay itself is a standard automotive cube relay, approximately 1 inch square in footprint. It's typically black in color. On top, you will usually see a "3" or "4" designation molded into the plastic, indicating the electrical schematic configuration (this isn't crucial for locating it, but confirms the relay type). It plugs into the socket via multiple blade-style metal terminals on its underside. You should see wires connected to it near the base where it plugs into the socket. The relay adjacent to it (often Position #15) might be the same size and color but serves a different function, highlighting the importance of using position #14 as the identifier.

Why Knowing the Location Matters: Symptoms of Failure

The fuel pump relay is an electromagnetic switch commanded by the Powertrain Control Module (PCM). When you turn the ignition key to the "Run" position, the PCM momentarily energizes the relay, activating the fuel pump for 2-3 seconds to pressurize the fuel lines. When you crank the engine, it keeps the relay energized continuously. If the relay fails, power cannot reach the fuel pump, leading to several classic symptoms:

1. Engine Cranks But Won't Start: This is the most common symptom. The starter turns the engine over perfectly, but the engine never fires because no fuel is being delivered to the injectors. A sudden, unexpected no-start condition often points to relay failure.
2. Sudden Engine Stall While Driving: A relay can fail after the engine has started. If it cuts out while driving, the engine will abruptly lose power and stall as fuel pressure drops to zero. This is highly inconvenient and can be dangerous in traffic.
3. No Fuel Pump Prime Sound: With the ignition key turned to "Run" (but not cranked), you should hear a distinct electric motor whine from under the rear of the truck (where the pump resides in the fuel tank) for those 2-3 seconds as the pump primes the system. Absence of this priming sound is a STRONG indicator of a fuel pump circuit issue, with the relay being a primary suspect. (Note: Sometimes the sound can be faint; listening near the fuel filler neck can help).
4. Intermittent Starting Problems: A relay on the verge of failure may work sometimes and not others, leading to frustrating random occurrences where the truck doesn't start, only to start fine later. This unpredictability is a hallmark of relay contacts starting to corrode or burn.
5. Electrical Clicking Noises Near UEC: Sometimes, a failing relay may rapidly click on and off when the ignition is turned on, indicating it's struggling to hold its contacts closed properly. This clicking might be audible from the driver's seat or when standing near the open hood.
6. Completely Dead Ignition (Less Common): In rare instances where other relays also fail or there are severe wiring issues, no ignition power might be present, but this usually involves multiple failures. Lack of fuel pump prime sound is the key fuel relay symptom.

Before You Start: Crucial Safety & Prep

Diagnosing and replacing the relay is safe if done carefully. Follow these steps before touching anything:

1. Park Safely: Ensure the truck is on level ground with the parking brake firmly engaged. Place wheel chocks behind the rear tires for an extra layer of security against unintended rolling.
2. Turn OFF Ignition Completely: Remove the key from the ignition switch and ensure all accessories are turned off. This removes power from the relay sockets.
3. Disconnect the Battery NEGATIVE Terminal: This is the single most critical safety step. Using the appropriate size wrench (usually 8mm, 10mm, or 13mm), loosen the bolt clamping the black negative (-) battery cable to the battery negative terminal post. Lift the cable clamp completely off the post and place it safely to the side away from the terminal. This prevents accidental electrical shorts and potential sparks near flammable battery gases. Wrap the end of the cable with a clean cloth to further minimize accidental contact. Leave the positive (+) cable connected.
4. Gather Tools: You will likely only need:
   • A flat-head screwdriver (small or medium) for prying the relay if needed, or potentially the fuse puller.
   • Your hands (relays are designed for hand removal).
   • Optional but Recommended: A multimeter for testing.
   • Optional: Needle-nose pliers for handling fuses/small parts, or accessing stubborn relays.
   • Your replacement relay (GM Part # 12020848, Standard Motor Products RY-199, or equivalent).
5. Organize Parts: Relays and fuses are small and easily lost. Have a small container or magnetic tray ready to hold the old relay and any fuses or covers you remove. Avoid working on gravel or grass where parts can disappear.
6. Have a Light Source: A bright flashlight or shop light is essential for clearly seeing the labels and components within the UEC, especially under the lip of the box.

Step-by-Step: Locating, Testing & Replacing the Fuel Pump Relay (1991 Sierra)

1. Access the UEC: Open the hood. Squeeze the cover release levers firmly and lift the cover off as described earlier. Place it where it won't get damaged.
2. Identify Relay #14: Using the diagram inside the cover as your reference, visually locate slot position #14 within the actual UEC box. Remember, top row, driver's side end but not the very corner. Confirm it corresponds to the "Fuel Pump Relay" label on the diagram.
3. Inspect the Existing Relay: Visually check the relay in position #14. Does it look identical to others? Is there any obvious damage (cracks, melted plastic, severe corrosion)? Note its orientation – it plugs in only one way due to the terminal configuration.
4. Manual Confirmation Test: Carefully remove the relay from its socket. Use a slight rocking motion if necessary. Do not yank on the wires! Grip the relay body firmly and pull straight out. Listen for the fuel pump prime sound test:
   • With the relay removed, turn the ignition key to the "Run" position. Listen carefully: You should hear NO fuel pump prime sound. This confirms the circuit is broken as expected.
   • Re-insert the relay firmly back into its socket until you feel it seat completely.
   • Turn the ignition key to "Run" again. You SHOULD hear the 2-3 second fuel pump prime whine.
   • If the prime sound returns when you re-insert the relay, the relay is likely faulty. If the sound never occurs, even with the relay seated properly, the problem could be elsewhere (e.g., fuse, wiring, pump itself, PCM command, ignition switch). Proceed to testing or substitution.
5. Electrical Testing with Multimeter (Pin 87 & Ground): For a more definitive diagnosis:
   • Set your multimeter to DC Volts (20V range).
   • Turn the ignition key to the "Run" position (do not crank).
   • Identify the terminals on the relay socket now that the relay is removed. Refer to the top of the relay itself for the standard terminal number diagram:
     • Pin 30: Usually connected to constant Battery Positive (B+).
     • Pin 87: The "Normally Open" contact. This sends power OUT to the fuel pump when the relay is energized.
     • Pin 86: Receives the "Switch" signal from the PCM (commonly labeled "IGN").
     • Pin 85: Ground connection controlled by the PCM completing the relay coil circuit.
   • Back-probe or touch the multimeter's red probe lead firmly against the metal terminal inside socket slot corresponding to Pin 87.
   • Touch the multimeter's black probe lead firmly to a clean, unpainted metal grounding point nearby on the engine or firewall (like a bolt head).
   • With the ignition in "Run," Pin 87 should show battery voltage (~12V) for approximately 2-3 seconds and then drop back to zero. This confirms the relay is being commanded on by the PCM and switching power through to Pin 87 correctly during prime. If you get constant 12V or no voltage, it might indicate a relay problem (stuck closed or open), but the manual test is usually sufficient in practice. Pin 30 can also be checked for constant B+.
6. Substitution Test (Easiest & Most Reliable):
   • Locate another relay in the UEC that shares the exact same physical size, shape, and terminal markings (e.g., the same "3" or "4"). Commonly, the Horn Relay (often Position #16) or the Cooling Fan Relay (if equipped) use the same standard relay type.
   • CAREFULLY note the exact position and orientation of this substitute relay.
   • Swap the fuel pump relay (#14) with this known-good substitute relay.
   • Re-attempt the fuel pump prime sound test: Turn ignition to "Run." If you hear the prime sound with the substitute relay in place, your original #14 relay is confirmed faulty.
   • If the prime sound is still absent with the known-good relay installed in position #14, the problem lies elsewhere in the fuel pump circuit (fuse, wiring, pump, PCM, ignition switch).
7. Replacing the Faulty Relay: Once you've confirmed the #14 relay is bad:
   • Remove the bad relay from position #14.
   • Take your new fuel pump relay. Ensure it matches EXACTLY in size, terminal configuration (e.g., "3" or "4"), and ratings as the original relay you removed. GM Part # 12020848 is the original specification. High-quality replacements include Standard Motor Products RY-199 or BWD / NAPA RY199.
   • Align the terminals correctly using the markings or keying on the relay base. It will only fit one way. Do not force it.
   • Push the new relay firmly and evenly into the #14 socket until it seats fully. You should feel it click or lock into place.
8. Final Verification:
   • Double-check that the new relay is fully seated in the #14 position.
   • Double-check that the UEC cover is not yet installed.
   • Reconnect the battery: Place the NEGATIVE (-) battery cable clamp back onto the battery NEGATIVE terminal post. Tighten the bolt securely with your wrench.
   • Perform the ignition key "Run" prime sound test again. You SHOULD hear the fuel pump energize for 2-3 seconds.
   • Attempt to start the engine. If the problem was solely the relay, the engine should crank and start normally.
9. Reassembly: Place the UEC cover back onto the box base. Align it correctly and press down firmly over the entire cover area until you hear both securing levers snap back into place securely against the base. Close the hood.

If the Problem Persists After Relay Replacement:

If you've replaced the fuel pump relay in position #14 with a known-good unit and still have no prime sound or engine start, the issue lies elsewhere in the fuel pump circuit. Common next steps:

1. Check the FUEL PUMP FUSE: In the UEC, locate the fuse labeled "Fuel Pmp" or similar in the diagram. For the 1991 Sierra, this is typically a 20 Amp fuse. Its position number might be around #21 or similar - CHECK YOUR COVER DIAGRAM. Physically remove it and visually inspect the small metal wire element inside the fuse's clear plastic body. If the wire is broken or the element shows signs of melting or scorching, the fuse is blown. Replace it with the correct amperage fuse (20A). Even if it looks intact, swap in a new 20A fuse temporarily to rule out an invisible hairline break.
2. Check Fuel Pump Power Directly:
   • Locate the fuel pump test connector. On the 1991 Sierra, it's usually a single-wire connector with a protective cap, located near the UEC on the firewall itself (sometimes near the cowl area on the driver's side). Look for a wire (often Dark Green with a White stripe) coming from the harness to a connector with no mating part, just hanging loose with a rubber cap. DO NOT confuse this with oxygen sensor connectors or other similar plugs. Refer to a service manual diagram if unsure.
   • With the ignition OFF and battery connected, remove the protective cap.
   • Apply 12-volt power DIRECTLY to this terminal. Use fused jumper leads from the battery POSITIVE terminal or a known good 12V source under the hood. Touching 12V to this terminal should immediately energize the fuel pump. Listen carefully for the pump running. If you hear the pump running consistently, this proves the pump motor itself and its wiring from this point back to the tank are functional, pointing the problem back upstream (wiring, relay, fuse, PCM). If you get no pump sound when jumping this terminal, the problem is very likely the pump itself, the wiring from this connector to the pump, the ground for the pump, or the in-tank pump connector. (Safety Note: Work carefully. Avoid sparks near the battery or fuel lines. Keep jumpers away from moving belts/fans).
3. Inspect Wiring and Connectors: Look for damaged wires (chafed insulation, cuts, rodent damage) around the UEC, at the firewall pass-throughs, or running down the frame towards the fuel tank. Disconnect and reconnect wiring harness connectors (at the UEC base, PCM, ignition switch, near the pump) looking for corrosion, bent pins, or loose terminals. Wiggle test wires while trying the pump.
4. Verify Ignition Switch Signal to Relay: The PCM commands the relay coil ON when it receives the proper ignition signal. Checking signals to relay pins 85/86 generally requires a multimeter or test light and referencing a wiring diagram.
5. Consider the Fuel Pump Itself: If the pump receives power via the test connector jump and doesn't run, the pump itself has likely failed. Access involves dropping the fuel tank. Fuel pump failure is extremely common on these vehicles due to age, heat cycles, and potential contamination.
6. Potential PCM Issue (Less Likely): While PCM failure isn't the most common cause of a sudden no-pump issue (relays, fuses, pumps are more typical failure points), it's possible if other commands are failing. Diagnosing this requires specific tools and wiring knowledge. Consult a professional technician with experience in GM trucks if you suspect deeper PCM control issues.

Part Numbers & Compatibility for the 1991 GMC Sierra Fuel Pump Relay

Knowing the exact relay you need saves hassle:

• Original GM Part Number: 12020848. This is the definitive number specified by General Motors for this application. If buying OEM, ask for this number.
• Key Aftermarket Equivalent Numbers:
  • Standard Motor Products RY-199 (Recommended, high quality)
  • ACDelco D1742A (GM's replacement aftermarket brand)
  • BWD (Borg Warner) R199 (Often sold at parts stores under house brands)
  • Echlin (NAPA) RY199
• Compatibility: This relay (GM #12020848 or equivalent) was used on countless GM vehicles across the late '80s, '90s, and early 2000s. It is shared on all GMT400 trucks (C/K 1500/2500/3500, Suburban, Tahoe/Yukon) and many F-body cars (Camaro/Firebird), H-body (Roadmaster/Impala SS), and other platforms during that era. Always physically compare any used relay to ensure exact matching dimensions and terminal layout ("3" or "4").

Cost & Buying Tips

• Cost Range: New fuel pump relays for the 1991 Sierra are very inexpensive. Expect to pay:
  • $10-$20 USD for a quality Standard or BWD/Napa relay.
  • $20-$35 USD for an OEM replacement (ACDelco D1742A).
• Buying:
  • Auto Parts Stores: Your fastest option. Walk in and ask for a fuel pump relay for a 1991 GMC Sierra 1500 (or your specific trim). Give them the part number if possible (RY-199, R199). Always open the box and physically check the relay before leaving. Ensure it looks identical to your old one. Ask for a reputable brand like Standard Motor Products.
  • Online Retailers: Amazon, RockAuto, etc. Offer a wider selection and often lower prices, but factor in shipping time. Search using "1991 GMC Sierra fuel pump relay," "GM 12020848," or "Standard RY-199." READ REVIEWS to ensure you get a reliable part, especially online. Avoid suspiciously cheap no-name relays.
  • Dealer Parts Counter: Most likely the most expensive option but guarantees an OEM relay if that's critical for you. Ask for part #12020848. They may sell it as the ACDelco D1742A.
• Quality Matters: While cheap relays exist, spending a few extra dollars on a Standard Motor Products, Bosch (if they offer compatible), or reputable brand relay is worthwhile. Cheap relays can fail quickly or intermittently. The cost difference is small compared to the hassle of another breakdown.

Common Misconceptions & Pitfalls (Avoid These!)

• "Tapping the Relay Makes it Work" Myth: While a very temporary phenomenon where mechanical jarring might momentarily close failing contacts, this is never a repair. It only delays the inevitable complete failure. Replace the relay promptly.
• "Just Swap Any Relay" Risk: While finding a same-type relay for substitution testing is sound, permanently using a relay designed for a lower current device (like a horn) in the fuel pump circuit is dangerous. The fuel pump draws significant current. A mismatched relay can overheat, melt, or fail catastrophically, potentially causing a fire or leaving you stranded again. Always use the correct relay (GM #12020848 or equivalent RY-199/R199).
• "The Relay Must be Under the Dash" Confusion: Earlier GM vehicles often had the fuel pump relay inside the cabin. The GMT400 platform (1988-1998 for trucks) consolidated engine-related relays under the hood. Looking inside the cab will only lead to frustration for your '91 Sierra.
• Ignoring Other Circuit Components: A blown fuse (especially the 20A fuel pump fuse) or broken pump ground wire causes identical symptoms to a bad relay. Always check the simple things first (fuse) and the pump directly via the test terminal if the relay isn't the culprit.
• Skipping the Battery Disconnect: Accidental tool slippage while probing the UEC with the battery connected can cause devastating short circuits, damaging components or starting a fire. The 1 minute it takes to disconnect the negative terminal is cheap insurance.
• Assuming the Pump is Bad Prematurely: Relays fail far more frequently than pumps. Thoroughly test the relay circuit before undertaking the much more labor-intensive (and expensive) task of dropping the fuel tank to replace the pump. The prime sound test and test connector jump are critical diagnostics.
• Faulty "New" Relays: While rare, even new parts can sometimes be defective out of the box. If you have absolutely verified the circuit up to the relay socket and replaced the fuse, and the new relay doesn't work, try a different replacement relay or a substitution test with a known-good borrowed unit before going further. Keep your receipt.
• Ignoring the Cover Diagram: The printed diagram inside the UEC cover is specific to your truck and is the ultimate authority. Relying solely on generalized position descriptions without confirming it matches your diagram risks pulling the wrong component, potentially disabling something crucial like headlights or ECM power.

Long-Term Reliability & Proactive Maintenance

• Keep it Covered: Always ensure the UEC cover is properly reinstalled and latched after any work. This protects the relays and fuses from water intrusion, road grime, dust, and accidental damage, significantly extending their lifespan.
• Avoid Pressure Washing the Engine Bay: High-pressure water jets can force moisture past seals and into electrical connectors, potentially corroding terminals inside relays and fuses. Clean around the UEC carefully.
• Consider Carrying a Spare: Given the relay's critical function and relatively low cost and size, keeping a known-good spare fuel pump relay and a spare 20A fuse in your glove compartment provides inexpensive insurance against being stranded by a sudden relay failure. This is especially recommended for older vehicles.
• Periodic Visual Inspection: Every time you check your oil or perform routine underhood maintenance, quickly pop the UEC cover (with battery disconnected if doing more than looking) and visually scan for obvious signs of trouble – melted fuse holders, heavily corroded terminals, damaged wires near the box. Look specifically at relay #14. Feel the cover - if it's very hot to the touch after driving, it might indicate an underlying wiring issue or failing component drawing excessive current.

The Bottom Line for Your 1991 GMC Sierra

If your truck cranks but won't start and you hear no fuel pump prime sound when you turn the key to "Run," the first and most likely culprit is the fuel pump relay. You will find it securely mounted in position #14 within the Underhood Electrical Center located on the driver's side firewall. Confirming its location using the diagram inside the UEC cover, performing a simple swap test with another relay (like the horn relay in #16), and replacing it with a quality part (GM #12020848 or Standard Motor Products RY-199) is a straightforward DIY repair that solves the vast majority of these sudden no-start situations. Armed with the precise location, detailed testing steps, replacement procedure, and troubleshooting guide provided here, you can confidently tackle this issue yourself, ensuring your 1991 Sierra provides reliable transportation for years to come.