1991 Acura Integra Fuel Pump Relay: Your Complete Guide to Diagnosis, Testing, and Replacement

The fuel pump relay is a frequent culprit behind a non-starting or stalling 1991 Acura Integra. If your Integra cranks but won't start, lacks power, or randomly dies, a failing fuel pump relay is highly likely. Diagnosing and replacing this relay is often straightforward and significantly cheaper than fuel pump replacement or complex diagnostics. Understanding its role, location, testing steps, and replacement process empowers you to get your classic Acura back on the road.

Understanding the Fuel Pump Relay's Job

This relay acts as a heavy-duty switch controlled electronically. Its primary function is crucial:

  1. Controlled Power Delivery: The engine control module cannot handle the high electrical current (amperage) required by the fuel pump directly. The relay bridges this gap.
  2. Safety & Efficiency: The relay powers the fuel pump only when needed: when the ignition is turned on (briefly to prime the system) and while the engine is cranking or running. It prevents the pump from running continuously with the ignition off or in accessory mode, enhancing safety and component longevity.
  3. Signal Integration: The ECM sends a small electrical signal to the relay's control coil. When this signal is present (indicating ignition on and ECM receiving key signals like crank position), the relay closes its internal high-current contacts, completing the circuit to the fuel pump.

Common Symptoms of a Failing Fuel Pump Relay (1991 Integra)

Be alert for these signs:

  • Engine Cranks But Won't Start: The most frequent symptom. The engine turns over with the starter but never fires up because no fuel reaches the injectors.
  • Engine Stalls Intermittently: The car runs fine one moment, then suddenly dies without warning, especially when idling, slowing down, or encountering bumps/vibration. It may restart immediately or after cooling off.
  • No Fuel Pump Prime Sound: When you first turn the ignition key to "ON" (before starting), you normally hear a distinct buzzing/humming sound from the rear (fuel tank area) for 1-2 seconds. No sound upon ignition "ON" strongly points towards relay, fuse, or wiring issues.
  • Complete Loss of Power: The car exhibits no signs of life when attempting to start – no crank, no dash lights (often associated with the "Main Relay" function in Hondas/Acuras of this era, which the fuel pump relay is part of).
  • Intermittent Starting Problems: The car starts perfectly sometimes, but refuses to start at other times, especially on hot days or after the engine is heat-soaked. Loose solder joints inside the relay expand with heat.

Locating the Fuel Pump Relay (PGM-FI Main Relay)

Finding it on a 1991 Integra takes careful effort:

  1. Position: Sit in the driver's seat.
  2. Look Up & Under: Look underneath the dashboard on the driver's side.
  3. Target Area: Locate the fuse and relay box mounted high under the dash, above the hood release lever and close to where the steering column meets the dash.
  4. Identify the Relay: It will be one of several plug-in modules (black or gray cubes with wiring harness plugs). It's commonly referred to as the PGM-FI Main Relay in Honda/Acura service manuals. Sometimes it's labeled "F/PMP" or "FUEL PUMP" on the relay housing itself or on the fuse box lid diagram.
  5. Challenge: Access is extremely tight. You will likely need to contort yourself, lying on the driver's floor mat looking upwards with a strong flashlight. A mechanic's telescoping mirror is very helpful.

Tools Needed for Testing and Replacement

Gather these beforehand:

  • Flathead Screwdriver (for trim/pull panels if needed)
  • Replacement Relay (OEM Honda/Acura is ideal, Denso or quality aftermarket like Standard Motor Works, Beck/Arnley also good. Confirm part number! Common ones include Honda/Acura 39400-SR0-003, 39400-SR0-004, 39400-SR0-006. Double-check compatibility for your specific '91 Integra).
  • Socket Wrench Set or Combination Wrenches (likely 8mm & 10mm for any relay bracket bolts)
  • Basic Automotive Fuse Puller or Needle Nose Pliers (for accessing fuses)
  • Multimeter (Digital is preferred) with Voltage and Continuity Test Settings
  • Flashlight
  • Trim Removal Tools (optional, but helps avoid scratching plastic)
  • Penetrating Oil (e.g., PB Blaster) - if mounting screws are corroded/seized.

Step-by-Step: Testing the 1991 Acura Integra Fuel Pump Relay

Crucial Safety Step: Always disconnect the NEGATIVE (-) battery terminal before attempting to remove electrical components like relays. This prevents accidental short circuits and potential damage.

Test 1: The Listening Test (Quick Check)

  1. Ensure the ignition is OFF.
  2. Have an assistant turn the ignition key to the "ON" position (DO NOT crank the engine).
  3. Listen carefully near the fuel tank filler area or under the rear seat. You should hear the fuel pump buzz/hum for 1-2 seconds. If you hear the pump, the relay and its basic circuit are likely functioning at that moment. If you hear nothing, proceed to further checks below. Note: An intermittent failure might pass this test but still be faulty.

Test 2: Fuse Check

  1. Locate the underhood fuse box (typically in the engine bay near the battery or strut tower).
  2. Identify the fuse labeled "F/PUMP" or "FUEL PUMP" (Often a 15A fuse). Refer to your owner's manual or the fuse box lid diagram.
  3. Pull the fuse. Visually inspect the metal strip inside the fuse. If broken or significantly discolored/melted, replace it with a fuse of the exact same amperage rating. Test again. If the fuse blows again immediately after replacement, you likely have a short circuit in the fuel pump wiring or pump itself – further diagnosis needed.
  4. Also check the Engine Control Module (ECM) fuse(s) in the underhood box, often labeled "ECM," "FI," "ENG CONT," etc. (15A or 20A). Burned ECM fuses can prevent the relay from receiving its control signal.

Test 3: Multimeter Testing Power at the Relay Socket

  1. Disconnect the Battery Negative Terminal.
  2. Remove the PGM-FI Main Relay from its socket (often requires pulling straight down with a firm tug or releasing a small locking tab first).
  3. Identify the relay socket terminals. They are usually labeled with tiny embossed numbers near the socket or on the relay housing itself. Critical terminals for basic power pump function:
    • Terminal #1: Control Signal Output from ECM to activate relay coil. (+12V Signal)
    • Terminal #2: Control Coil Ground (Should have continuity to chassis ground).
    • Terminal #5: Constant Battery Power Input to relay (Should have +12V at all times).
    • Terminal #7: Switched Power Output from relay to Fuel Pump. (Power out only when relay is active).
  4. Check Constant Power Input (Terminal #5):
    • Reconnect the negative battery terminal.
    • Set multimeter to Volts DC (20V scale).
    • Connect the multimeter's BLACK probe to a known good ground (unpainted metal bolt/part of chassis).
    • Connect the multimeter's RED probe to the socket cavity corresponding to Terminal #5.
    • Ignition OFF: You should read approximately 12 volts. If no power, check related fuses (underhood, potentially others - consult wiring diagram for '91 DA chassis) and wiring harness.
  5. Check Control Signal Input (Terminal #1):
    • Leave BLACK probe grounded.
    • Move RED probe to the socket cavity for Terminal #1.
    • Turn ignition key ON (not start). Within 1-2 seconds, you should see approximately 12 volts. If no voltage, the issue could be a blown ECM fuse, faulty ECM, bad wiring to the relay socket, or faulty ignition switch signal. This signal only appears briefly at ignition ON and stays on while cranking/running.
  6. Check Ground Circuit (Terminal #2):
    • Disconnect battery negative terminal again for safety.
    • Set multimeter to Continuity (Ohms - usually indicated by a diode symbol or speaker symbol. It beeps if connection detected).
    • Place one probe on Terminal #2 socket cavity. Place the other probe on a known good ground point. You should see continuity (near 0 Ohms resistance and/or a beep). High resistance or no continuity indicates a bad ground path, possibly corroded terminals or damaged wiring.
  7. Check Relay Output (Terminal #7 - Requires Activating Relay):
    • If power (Terminal #5), control signal (Terminal #1), and ground (Terminal #2) check out, the issue might still be the relay itself or downstream (fuel pump).
    • Alternative method: While harder to access with the relay removed, you can have an assistant turn the ignition to "ON" while you probe the fuel pump electrical connector at the pump itself (located on top of the fuel tank under the rear seat/carpet access panel). You should see +12V briefly at one of the pins when ignition is turned "ON". If no voltage reaches the pump socket and the inputs to the relay socket are good, the relay itself is likely faulty. If voltage does reach the pump socket but the pump doesn't run, the pump or its connector is likely the problem. Proceed to Test 4.

Test 4: Bench Testing the Relay Itself

  1. Identify Terminals: Look at the metal prongs on the removed relay. Identify pins for Terminals 1, 2, 5, and 7 based on their position matching the socket (use any labels on the relay).
  2. Test Control Coil:
    • Set multimeter to Ohms (Resistance, Ω). Choose low range (e.g., 200Ω).
    • Place probes on pins for Terminals #1 (+ Signal Input) and Terminal #2 (Ground). A good relay coil typically shows resistance between 70 and 100 Ohms. An open circuit (OL/infinite resistance) indicates a burned-out coil inside the relay.
  3. Test High-Current Contacts:
    • Set multimeter to Continuity mode.
    • Place one probe on Terminal #5 (Constant Power Input).
    • Place the other probe on Terminal #7 (Power Output to Pump).
    • Without any power applied to the control coil, there should be NO continuity (open circuit) between #5 and #7. Your meter should show OL (Over Limit) or not beep.
    • Activate the Coil: You need a method to simulate the ECM signal to close the relay. Obtain two short lengths of wire. Connect one wire from Terminal #1 to the POSITIVE (+) terminal of a 9-volt battery. Connect the other wire from Terminal #2 to the NEGATIVE (-) terminal of the 9-volt battery. You should hear a distinct CLICK from inside the relay.
    • While holding this connection (relay activated), check continuity between Terminal #5 and Terminal #7 again. There SHOULD NOW be continuity (meter beeps or shows near 0Ω resistance). If continuity doesn't occur when coil is energized, the internal high-current contacts are burned out or stuck open, confirming relay failure.
  4. Test Results:
    • No Coil Resistance: Relay is bad.
    • No Click When Coil Energized: Relay is bad.
    • No Continuity when Coil Energized: Relay contacts are bad.

Removing the Old Fuel Pump Relay (PGM-FI Main Relay)

  1. Disconnect Negative Battery Terminal: Essential safety step.
  2. Locate Relay: As described above.
  3. Access: Prepare for tight space. Lying on your back in the driver's footwell, head near the pedals, looking up is often required. Ensure good lighting.
  4. Identify Locking Tab: Inspect the relay socket. There might be a small plastic locking tab holding the relay into the socket. Gently pry it up or sideways with a small flathead screwdriver if present. Don't force it; understand its design first.
  5. Remove Relay: Firmly grasp the relay body. Pull it STRAIGHT DOWN towards the floor. Do not rock side-to-side forcefully. A good tug should release it. If it feels stuck, double-check for a locking tab. Avoid pulling only by wires!
  6. Inspect Socket: Once the relay is out, visually inspect the socket. Look for any melted plastic, corrosion (white/green powder), severely bent pins, or debris. Clean corrosion carefully with electrical contact cleaner and a soft brush if found. Address significant damage before installing the new relay.
  7. Consider the Mounting Bracket: Sometimes the entire relay and its bracket are removed as a unit. Look for small bolts (often 8mm or 10mm) securing a metal bracket holding the relay socket. You may need to remove these 1-2 bolts to pull the entire relay-and-socket assembly down for easier access to unplug the wiring harness connector(s) from the socket unit. Taking a picture before disconnecting harness connectors is wise.

Installing the New Fuel Pump Relay

  1. Part Confirmation: Double-check the new relay part number matches the old one or the recommended part number (e.g., Honda 39400-SR0-003, etc.) for a 1991 Integra. Compare pin configuration visually before inserting.
  2. Prepare Socket: Ensure the socket is clean and free of debris.
  3. Align & Insert: Orient the new relay exactly like the old one came out (pin layout must match socket). Align the pins carefully and press firmly and evenly STRAIGHT UP into the socket until it seats completely and audibly clicks (if there's a locking tab). Avoid bending pins.
  4. Reattach Harness (If Removed): If you unplugged the wiring harness connector(s) from the relay socket assembly, plug them back in securely now.
  5. Reinstall Bracket: If you removed a bracket assembly with the relay socket attached, carefully maneuver it back into position and tighten the mounting bolts securely. Don't overtighten into plastic.
  6. Reconnect Battery: Reattach the NEGATIVE (-) battery terminal.
  7. Test for Operation: Turn the ignition key to "ON". Listen for the fuel pump to prime for 1-2 seconds. Attempt to start the engine. It should crank and start if the relay was the sole issue.
  8. Final Check: Verify the engine runs smoothly at idle and under gentle acceleration before assuming full repair.

OEM vs. Aftermarket Relays: What to Choose

  • OEM (Honda/Acura): Highest quality and reliability assurance. Uses higher quality materials and construction. Least likely to fail prematurely or cause intermittent issues. Strongly recommended, given the critical nature of the component and the difficulty of access on the 1991 Integra. Price is higher but justified for longevity.
  • Quality Aftermarket (Denso, Standard Motor Works, Beck/Arnley, TYC): Generally reliable and significantly cheaper than OEM. Good budget option. Quality can vary slightly. Reputable brands are usually fine. Denso is often the supplier to the original equipment.
  • Cheap Generic Aftermarket: Avoid these. Prone to very short lifespans, intermittent operation, and can sometimes fail in a way that leaves you stranded again shortly after replacement. False economy for a crucial part like this relay.

Cost Considerations

  • OEM Relay: Expect to pay between 150, depending on the source (dealer list price vs. online discount parts).
  • Quality Aftermarket: Typically 60.
  • Mechanic Labor: Even if you only buy the part, diagnosis and replacement require significant labor time due to the awkward location. Expect to pay at least 1.0 to 2.0 hours of shop labor (likely 250) on top of the part cost if you have a professional replace it.
  • DIY Cost: The cost of the relay itself only. This is why DIY diagnosis and replacement represent substantial savings if you can manage the task.

DIY vs. Professional Replacement

  • DIY Feasible:
    • Pros: Significant cost savings (only relay cost). Satisfaction of self-repair. You control part quality (OEM selection).
    • Cons: Awkward, cramped access under the dash requires patience and flexibility. Requires basic mechanical aptitude and understanding of electrical tests. Risk of misdiagnosis if other underlying issues exist. Requires tools (especially multimeter) and a safe workspace. Safety risk if battery is not disconnected properly (potential for shorts or blown fuses).
  • Professional Replacement:
    • Pros: Technician experience and equipment can ensure accurate diagnosis and efficient replacement. Access to shop tools and lifts makes physical access easier. Warranty on labor. Saves your time and physical effort.
    • Cons: Significantly more expensive cost (parts + labor). Potential for upselling on unnecessary services. Finding a mechanic familiar with classic imports like the '91 Integra might take extra effort. You relinquish control over part selection unless specified.

Troubleshooting Beyond the Relay

If replacing the fuel pump relay does not solve the starting issue, further investigation is needed:

  1. Fuel Pump: The pump itself could be dead. Listen at the tank when ignition is turned "ON". Test for voltage at the fuel pump connector under the rear seat during ignition "ON".
  2. Wiring: Damaged or corroded wiring between the relay, fuse box, ECM, or fuel pump harness. This requires systematic tracing and repair.
  3. Ignition Switch: Internal failure preventing signal from reaching the ECM and/or relay.
  4. Engine Control Module (ECM): Failure to send the activation signal (Terminal #1) to the relay. Uncommon, but possible, especially if related fuses are good.
  5. Main Grounds: Check critical engine/chassis ground points for corrosion or looseness (battery negative cable terminal, engine ground strap, ground connections near relay/fuse boxes).
  6. Fuel Filter: A severely clogged filter can sometimes mimic pump failure symptoms (low pressure). Relates to engine running poorly or lacking power, not always crank/no-start.
  7. Fuel Pressure Regulator: Failure can cause poor performance or flooding, but usually not a primary cause of crank/no-start unless it leaks pressure catastrophically (rare). Check fuel pressure at the rail with a gauge if pump runs.

Preventative Measures & Longevity

  • Use Quality Parts: Invest in an OEM or high-quality aftermarket relay initially.
  • Clean Connections: Occasionally cleaning the relay socket and relay pins with electrical contact cleaner can prevent minor resistance issues causing heat buildup.
  • Address Underlying Heat Issues: While the under-dash location is unavoidable, ensure no nearby components (like aftermarket wiring/modules) are generating excessive heat near the relay box.
  • Recognize Early Symptoms: Pay attention to stalling or intermittent no-start episodes – don't ignore them. Early replacement is cheaper and less frustrating than being stranded.

Conclusion

A faulty fuel pump relay (PGM-FI Main Relay) is a leading cause of crank-no-start and stalling problems in the 1991 Acura Integra. Located in a challenging spot under the driver's dash, its diagnosis involves listening for the fuel pump prime sound, checking fuses, and using a multimeter to verify power and signal integrity at the relay socket. Bench testing the relay confirms internal failure. Replacement, while physically awkward in the tight space, is manageable DIY with care, saving significant money over professional repair. Opting for an OEM or quality aftermarket relay enhances long-term reliability. Addressing this relay is often the most efficient and economical first step to resolving classic Integra fuel delivery woes.

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