HOW TO WIRE AN S13 FUEL PUMP: COMPLETE 240SX DIAGRAM & GUIDE

Struggling to start your Nissan 240SX? A failing fuel pump or wiring issue is a common culprit. This definitive guide provides the complete S13 fuel pump wiring diagram along with practical instructions to diagnose and repair or rewire your fuel delivery system correctly and safely. Understanding this circuit is essential for troubleshooting no-start conditions, upgrading to a high-performance pump, or installing aftermarket engine management.

The S13 fuel pump wiring diagram reveals a relatively simple but critical circuit. Power originates from the battery, passes through the Engine Control Unit (ECU) controlled fuel pump relay, travels via specific color-coded wires in the chassis harness, connects through the fuel pump hanger assembly plug, and finally reaches the pump motor itself. A dedicated ground wire completes the circuit. Common failure points include the relay, corroded connectors (especially within the fuel tank access cover area), damaged wires, and the pump motor. Using the correct wire colors and gauge, ensuring secure connections, and protecting wires from heat and chafing are paramount for reliable operation. This guide will break down each component and step.

Understanding the Core S13 Fuel Pump Circuit

The fuel pump's job is simple: deliver pressurized fuel from the tank to the fuel rail. The wiring makes this happen only when needed, controlled by the ECU. Here's the essential flow:

  1. Power Source: 12-volt power starts at the vehicle's battery.
  2. Main Fuse: Protection is provided by the EFI Main Relay fuse, typically a 15A or 20A fuse located in the main fuse box under the hood.
  3. Fuel Pump Relay: This is the command center. The relay acts as a heavy-duty switch. One side receives constant power from the EFI Main Relay fuse via a Black/White (B/W) wire. The other side sends power to the fuel pump when activated.
  4. ECU Control: The ECU triggers the relay. When you turn the ignition to "ON," the ECU provides a brief ground signal to the relay's coil (via a Black/Red (B/R) wire) to prime the system. During cranking and while the engine is running (receiving RPM signal), the ECU holds this ground signal, keeping the relay closed and the pump running. If the engine stalls, the ECU cuts the ground signal after a few seconds, turning the pump off for safety. The relay coil gets power from another ignition-switched source via a Blue/Black (L/B) wire.
  5. Power Delivery to Pump: When the relay is activated (closed), power flows from the relay's output terminal (connected to the B/W feed) through the relay contacts and out on the critical Black/Red (B/R) wire. This is the main power wire heading towards the fuel pump.
  6. Chassis Harness to Pump: The B/R power wire travels through the vehicle's main wiring harness along the floor pan, down the driver's side (LHD vehicles), towards the rear of the car.
  7. Fuel Pump Hanger Plug: Near the fuel tank access cover, the chassis harness terminates in a specific connector (often a 3-pin or 4-pin plug) that mates with the connector on the fuel pump hanger assembly. The B/R power wire connects through this plug.
  8. Pump Hanger Assembly: Inside the hanger assembly, the B/R power wire connects directly to one terminal of the fuel pump motor.
  9. Ground Completion: The other terminal of the fuel pump motor connects to a dedicated Black (B) ground wire. This ground wire travels back through the hanger assembly plug, into the chassis harness, and eventually connects to a solid chassis ground point, typically near the fuel tank or rear strut tower. A good ground is absolutely essential.

Standard S13 Fuel Pump Wiring Diagram (Colors based on common USDM LHD models)

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[Simplified Schematic - Illustrative Purposes]

+12V BATTERY  -------- [EFI MAIN RELAY FUSE (15/20A)] --------+
                                                               |
                                                          [FUEL PUMP RELAY]
                                                               |
       (IGN SW Power) BLUE/BLACK (L/B) ------------>| Coil      |
                                                      | Relay |----- (To ECU) BLACK/RED (B/R) - ECU Ground Control
       (Batt Power) BLACK/WHITE (B/W) --------|>------| Switch |----- (To Fuel Pump) BLACK/RED (B/R)
                                                               |
                                                          CHASSIS GROUND
                                                               |
                                                               |
FUEL PUMP MOTOR (Inside Tank) <----- BLACK/RED (B/R)  <---------+
                       |              (POWER)
                       |
                       +---------------- BLACK (B) -------------------- CHASSIS GROUND (Near Tank)

Critical Wire Colors Explained

  • Black/Red (B/R): This is the key player. It carries switched 12V power FROM the fuel pump relay TO the fuel pump itself. Identify this wire in both the engine bay relay area and at the rear harness connector. Its continuity is vital.
  • Black/White (B/W): Provides constant battery power TO the fuel pump relay's input terminal (from the EFI Main fuse). It's hot all the time.
  • Black (B): The ground wire for the fuel pump motor. Must connect securely to clean, bare metal.
  • Blue/Black (L/B): Provides switched ignition power (ON/RUN/START) to the coil side of the fuel pump relay. Powers the relay's control circuit.
  • Black/Red (B/R) (ECU Pin - e.g., Pin 18 or 104): The ECU uses this wire to ground the fuel pump relay coil, activating the relay. This is a control wire, not the high-power pump wire.

Locating Key Components

  1. Fuel Pump Relay: Typically found in the engine bay fuse/relay box. Consult your owner's manual or box diagram. It's often identical to the ECCS (ECU) relay. Testing or swapping with a known good relay (like the horn relay, if same type) is a common diagnostic step.
  2. Chassis Harness Connector: Located near the fuel tank access cover under the rear trunk carpet (coupe) or rear seat cushion (hatchback). Lifting this cover reveals the fuel pump hanger and the connector linking the chassis harness to the pump assembly.
  3. Fuel Pump Hanger Assembly: The unit that holds the pump, strainer, fuel level sender, and attaches to the top of the fuel tank. The pump motor wires terminate here.
  4. ECU: Located on the passenger side floor behind the kick panel (LHD). Accessing the ECU connector allows for testing ECU control signals.

Step-by-Step: Testing the S13 Fuel Pump Circuit

SAFETY FIRST: Avoid sparks! Disconnect the battery negative terminal before working near fuel lines or the pump assembly. Work in a well-ventilated area. Have a fire extinguisher nearby.

Tools Needed: Multimeter, Test Light (or Power Probe), Basic Hand Tools.

  1. Listen for Initial Prime: Turn the ignition key to "ON" (do not start). You should hear a faint hum/whine from the rear fuel tank area for 1-3 seconds. If heard, the circuit is likely activating correctly to that point. If not heard, proceed to testing.
  2. Check Voltage at the Pump Connector:
    • Disconnect the chassis harness plug from the fuel pump hanger assembly connector (rear access area).
    • Turn Ignition to "ON."
    • Set multimeter to DC Volts (~20V range).
    • Place the meter's Red probe on the terminal for the B/R wire in the chassis harness side of the connector. Place the Black probe on a KNOWN GOOD GROUND (bare metal nearby).
    • You should see battery voltage (~12V) for the 1-3 seconds the prime cycle runs.
    • If YES: Power is reaching the connector. The fault is likely the fuel pump, its internal wiring, or the ground within the tank (hanger assembly). Proceed to step 4.
    • If NO: Power is not reaching the pump connector. Proceed to relay testing.
  3. Testing the Fuel Pump Relay & Control Circuit:
    • Locate Relay: Identify the Fuel Pump Relay in the engine bay fuse box.
    • Test for Power Input (B/W): With the relay removed, turn Ignition to "ON." Check for constant ~12V between the relay socket terminal corresponding to the B/W wire and ground (Black probe to ground, Red probe to terminal). Should be constant power.
    • Test for Control Power (L/B): Check the relay socket terminal corresponding to the L/B wire. Should have ~12V only when Ignition is ON/RUN/START.
    • Test ECU Control Signal (B/R): Reinstall the relay. Backprobe the wire from the relay socket going to the ECU (B/R control wire). Connect multimeter Black probe to ground, Red probe to backprobed wire. Turn Ignition ON. Should see near 0V for 1-3 secs (ECU grounding), then battery voltage (~12V). While cranking, should see near 0V. If voltage doesn't drop low during prime/crank, ECU may not be grounding the relay. Check ECU grounds/power or consult a specialist.
    • Test Relay Output (B/R from Relay): Locate the B/R wire coming from the Fuel Pump Relay output terminal (follow the wire a short distance from the relay socket). With Ignition ON, test voltage between this B/R wire and ground. Should get ~12V for the 1-3 sec prime cycle. If YES at relay but NO at rear connector, there's a break in the B/R wire running rearward. If NO voltage here, but relay control checks out, the relay itself is likely bad.
  4. Testing the Fuel Pump Motor & Ground:
    • If power reaches the rear connector (Step 2, YES) but the pump doesn't run:
      • Check Pump Ground (B): Disconnect pump assembly plug. Set multimeter to Ohms (Ω). Place one probe on the B terminal of the pump hanger assembly connector. Place the other probe on a KNOWN GOOD GROUND (bare metal). Should read very low resistance (less than 1 ohm). Higher resistance indicates a bad ground path inside the tank or on the assembly.
      • Direct Power Test: EXTREME CAUTION - NO SPARKS NEAR OPEN FUEL TANK! Ensure area is well ventilated, no ignitions sources present. Battery still disconnected. Temporarily connect fused jumper wires: Connect one jumper from Battery POSITIVE (+) to the B/R terminal on the pump hanger assembly side. Connect the other jumper from Battery NEGATIVE (-) to the B terminal on the pump hanger assembly side. The pump should run immediately. If it runs, the pump and internal wiring are good, confirming the external circuit fault. If it doesn't run, the pump or internal connection is dead.
  5. Visually Inspect: Throughout testing, check for:
    • Corroded or loose terminals at the fuel pump hanger plug and relay socket.
    • Damaged, pinched, or burnt wires, especially near the fuel tank access cover opening (common wear point).
    • Signs of previous poor repairs (splices, wrong wire colors, electrical tape).

Rewiring Considerations (Relocation, Upgrades, Aftermarket ECU)

  1. Upgrading the Fuel Pump: Installing a higher-flow pump (Walbro 255lph is popular) often requires replacing the small factory connector pigtail on the pump itself with a more robust connector or soldering/heavy-duty crimping. Always use the correct gauge wire (typically 12-14 AWG) and proper connectors.
  2. Relay Rewiring (Common Performance Mod): The factory wiring, especially the ground path inside the fuel tank hanger, can be a bottleneck for high-demand setups. A popular and highly recommended upgrade is a "rewire kit" or custom relay setup:
    • Run a new heavy gauge (10-12 AWG) power wire directly from the battery positive (with an appropriate inline fuse, 20-30A) to a new, high-amperage relay mounted near the fuel tank.
    • Trigger this new relay using the original B/R signal wire from the chassis harness at the rear connector (providing switched ground control).
    • Run a new heavy gauge (10-12 AWG) wire from the new relay's output to the B/R input on the pump hanger assembly connector.
    • CRITICAL: Run a new heavy gauge (10-12 AWG) ground wire from the B terminal on the pump hanger assembly connector directly to a pristine bare metal ground point on the chassis (drill/tap, remove paint, use star washer). This dedicated ground often makes the biggest difference in voltage delivery.
    • This bypasses the potentially weak factory wiring, ensuring maximum voltage reaches the pump for optimal performance.
  3. Aftermarket ECU: Standalone ECUs (e.g., Haltech, Link, AEM) will have their own Fuel Pump Output pin. This output typically needs to connect to the trigger input (coil control side - usually 85 or 86 terminal) of the fuel pump relay (either original or a new relay in a rewire setup). Connect this ECU output to one relay coil terminal; connect the other coil terminal to switched +12V (L/B wire or equivalent). The ECU output acts as the ground switch, replacing the factory ECU control signal. Consult your specific ECU manual.
  4. Relocating the Fuel Pump (Surge Tank Setup): If using an external pump or surge tank setup, the wiring principles remain the same:
    • The pump needs power, ground, and relay control.
    • Use appropriately sized wiring based on pump amperage requirements.
    • Ensure the relay control is connected to the original ECU signal (or standalone ECU output) to maintain key-on prime and engine-running operation.
    • Incorporate an inertia safety switch in the main power line if not already present in your factory setup or if relocating components significantly.

1994-1995 S13 (KA24DE) Wire Color Note

Some late-model USDM S13s (mainly 1994-1995 with the KA24DE engine) exhibit a wire color variation:

  • The power wire running from the fuel pump relay to the rear pump connector may be *Black/Yellow (B/Y)* instead of Black/Red (B/R).
  • Functionally, it performs the identical role. Verify colors at your specific relay and rear connector.

Safety and Best Practices

  • Disconnect Battery: Always disconnect the negative terminal before starting electrical work.
  • No Sparks: Never create sparks near an open fuel system. If testing fuel pump directly, ensure excellent ventilation, no sparks, before connecting battery power. Ideally, lower the fuel level first.
  • Quality Connections:
    • Use heat shrink butt connectors or solder and heat shrink for permanent splices. Avoid electrical tape alone.
    • Ensure crimps are tight and secure.
    • Protect wires from heat sources (exhaust) and abrasion (use conduit, grommets).
  • Fuse Protection: Any new power wire run directly from the battery MUST have an appropriately sized inline fuse within 6-12 inches of the battery terminal. Never bypass fuses.
  • Proper Grounds: Grounds are half the circuit! Clean attachment points to bare metal. Use star washers. Ground directly to the chassis or engine block, not to painted surfaces or small brackets.

Conclusion: Mastering the S13 Fuel Pump Circuit

Diagnosing and repairing your S13 fuel pump wiring is a manageable task with the right information, tools, and safety precautions. The core components – battery, fuse, relay (ECU controlled), power wire (B/R or B/Y), pump motor, and ground – form a straightforward loop. By methodically testing voltage at key points (relay output, rear connector, pump terminals) and understanding the ECU's control role, you can pinpoint failures like bad relays, blown fuses, broken wires, poor grounds, or a dead pump. For stock applications, repairing factory wiring is often sufficient. However, for upgraded pumps or performance builds, implementing a dedicated fuel pump relay rewire with a thick ground wire offers significant reliability and performance benefits by ensuring consistent voltage delivery. Keep the detailed wiring diagram provided here as a reference, focus on safety, and ensure every connection is solid and protected for long-term reliability.

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