The Essential Guide to Finding the W221 Fuel Pump Relay Location (2006-2013 S-Class)

Instantly locate the fuel pump relay in your Mercedes-Benz S-Class W221 (2006-2013): Your critical relay is inside one of two major fuse boxes. For models built from 2006 until approximately early 2009, the relay is located in the front passenger footwell fuse box (K40)**. For models built from approximately late 2009 onwards (often corresponding to model year 2010 and later), the relay is located in the rear right fuse box (K92) in the trunk. Identifying your specific model year is crucial. Check the detailed sections below for precise identification steps, exact relay positions, diagrams, and fault-finding procedures.**

If your Mercedes-Benz S-Class W221 (chassis code W221, produced from 2006 to 2013) cranks but stubbornly refuses to start, a faulty fuel pump relay is one of the most likely culprits. This small, inexpensive component controls power to the electric fuel pump. When it fails, your engine gets no fuel, causing a no-start situation that can leave you stranded. Knowing the precise W221 fuel pump relay location is the critical first step in diagnosing and resolving this common problem. This guide provides definitive, workshop-manual verified information to find it quickly on all W221 model years.

Why the Fuel Pump Relay Location Matters in the W221

The fuel pump relay acts as an electronically controlled switch. Receiving a signal from the Engine Control Unit (ECU) when you turn the ignition key (or press the start button), it closes its internal circuit. This action sends battery voltage directly to the fuel pump, priming the system before starting and supplying fuel continuously while the engine runs. A failed relay means this vital power supply is interrupted.

Symptoms directly pointing towards a potential fuel pump relay failure include:

  • Engine Cranks Normally But Does Not Start: The most classic sign. The starter spins the engine, but there's no ignition due to lack of fuel.
  • No Fuel Pump Prime Sound: Listen carefully near the rear of the car (where the fuel tank is) when you first turn the ignition to the "ON" position (without cranking). You should hear a distinct whirring/humming sound for about 2-3 seconds. Complete silence strongly suggests a relay, fuse, or pump issue.
  • Intermittent Starting Issues: The relay might work sometimes and fail others, leading to unpredictable no-start events, especially when hot or after the car has been running.
  • Relay Clicking Audibly: While sometimes normal operation involves a soft click, a loud, rapid, or continuous clicking from the relay area often indicates internal failure.

Locating the relay swiftly allows you to check, swap, or replace it, potentially getting you back on the road in minutes.

Crucial Preliminary Step: Identify Your W221 Model Year Accurately

Mercedes-Benz relocated the primary fuse and relay boxes during the W221 production run. Using the incorrect location information will lead you to the wrong box and waste significant time. Determining your specific model year is essential.

  • 2006 - ~Early 2009 Production (Often Referred to as Pre-Facelift): The fuel pump relay is in the front passenger footwell fuse box (designated K40).
  • ~Late 2009 - 2013 Production (Often Model Year 2010-2013 / Facelift Models): The fuel pump relay is in the rear right fuse box located in the trunk (designated K92).

How to Find Your Model Year:

  1. Check Your Vehicle Registration Documents: The model year is clearly listed.
  2. Look at the Driver's Door Jamb Sticker: Open the driver's door and locate the manufacturer's sticker on the door frame or B-pillar. It lists the month and year of production (e.g., "09/2008" for September 2008).
  3. Identify Key Visual Facelift Differences (General Guide, Not 100% Reliable Alone):
    • Headlights: Early models have more "triangular" outer corners. Facelift models (approx late 2009+) have LED running light strips and a more curved outer contour.
    • Taillights: Facelift models feature distinctive LED light guides.
    • Front Bumper: Facelift models usually have larger air intakes and a different lower grille design.
  • Recommendation: Rely primarily on the production month/year from the door jamb sticker or registration documents. Visual differences can help corroborate but use the documented date for certainty.

Finding the Relay in Early W221 Models (2006 - ~Early 2009 / K40 Fuse Box)

If your production date falls before approximately late 2009 (check door jamb sticker!), follow these steps:

  1. Locate the Front Fuse Box (K40): Open the front passenger door. Directly below the glove compartment, against the side of the center transmission tunnel, you will see a long, black plastic trim panel running front-to-back. This is the cover for the fuse box.
  2. Remove the Fuse Box Cover:
    • Carefully pry off the plastic cover (sometimes labeled "FUSES"). It is held on by plastic clips. Start at the front or rear edge and gently work your way along. A small plastic trim tool helps avoid damaging the clips.
  3. Identify the Relay Position: Look inside the exposed fuse box. The fuel pump relay in these early models is typically position "KJ" (German for "Klappe J" or "Relay J"). Refer to the diagram printed inside the fuse box cover (or see description below). It will clearly label relays and fuses.
    • Common Visual Description: Relay KJ is usually a standard black or gray cube-shaped relay (about 1 inch square). Its designation will be printed on the circuit board next to its socket. Look for "KJ", "Relay J", "Pumpe", "Kraftstoffpumpe" (German for fuel pump), or similar labeling. If the cover diagram is missing, count sockets – it's often located centrally within the relay section.
  4. Optional Check: Relay Identification by Color/Part Number (If Visible): The original relay part number for many early W221s was A 004 545 82 25 (Mercedes number). Visually, it might have a blue base or be all black/grey. Don't rely solely on appearance; the position (KJ/K40) is key.
  5. Important Note on Early Models: In some very early W221 builds (launch period), reports suggest Relay H (KH) might have been temporarily used. However, Relay J (KJ) is overwhelmingly standard for the fuel pump in the documented K40 location for this period. Always check the cover diagram first.

Finding the Relay in Later W221 Models (~Late 2009 - 2013 / K92 Fuse Box in Trunk)

For vehicles produced from approximately late 2009 onward (Model Year 2010-2013), the relay moved:

  1. Locate the Rear Right Fuse Box (K92): Open the trunk. Look on the right-hand side (passenger side in LHD vehicles, driver side in RHD). Behind the trunk liner wall (next to the trunk latch mechanism and often near the battery compartment access), you'll find a vertical or slightly angled fuse box cover.
  2. Remove the Fuse Box Cover: The cover is usually secured by one or more screws (commonly Torx T20 size). Carefully unscrew them and set aside. Gently pull the cover off, revealing the fuse and relay panel inside. Some covers are clipped; gently pry around the edges.
  3. Identify the Relay Position: Look inside the box and on the cover diagram. In the K92 box, the fuel pump relay is consistently known as Relay "N" (KN).
    • Visual Confirmation: Relay N is typically a larger, "taller" relay compared to others (sometimes referred to as a "midi" relay), often white or gray. Crucially, it will be labeled "N", "KN", or "Relay N" on the circuit board next to its socket. This is absolutely the most reliable identifier. Common part number for these later models is A 222 540 17 24.
  4. Location Within K92: Relay N (KN) is frequently positioned in the lower section of the relay block. Match the location precisely to the "N" marking on the board.

Summary Table: W221 Fuel Pump Relay Location by Model Year

Model Year/Production Period Fuse Box Name Fuse Box Location Relay Position ID Common Relay Part Number Key Identification Marker
2006 - ~Early 2009 K40 Front Passenger Footwell KJ (Relay J) A 004 545 82 25 Labeled KJ/J near socket
~Late 2009 - 2013 K92 Rear Right Trunk Compartment KN (Relay N) A 222 540 17 24 Labeled KN/N near socket

How to Test or Replace the W221 Fuel Pump Relay

Safety First:

  • Park the vehicle on a level surface and engage the parking brake firmly.
  • Turn the ignition OFF and remove the key (or ensure start button is inactive).
  • Disconnect the Negative (-) Battery Terminal. This is crucial to prevent short circuits and potential airbag deployment when working near fuse boxes. Wait 10-15 minutes for systems to power down.

Testing the Relay:

  1. Locate: Follow the precise steps above for your model year to find the correct relay.
  2. Remove: Relays pull straight out of their sockets. Grasp firmly and pull vertically. Avoid excessive wiggling. Note its orientation.
  3. Visual Inspection: Check for any signs of overheating: melted plastic, cracks, dark burn marks, or a burnt smell. Severe visual damage means immediate replacement.
  4. Swap Test (Best Practical Test):
    • Find another relay in the same fuse box with the exact same part number and physical shape. A common candidate is the auxiliary fuel pump relay (often K5/KV in K40, K7/KD in K92) or rear window defogger relay. Always check the cover diagram to identify a compatible relay with the same part number.
    • Swap the suspect fuel pump relay with this known good relay.
    • Reconnect the battery negative terminal (ignition still OFF).
    • Turn the ignition to the "ON" position (do not crank). Listen carefully near the rear of the car for the 2-3 second fuel pump priming sound. If the sound returns after the swap, the original relay is faulty. If there's still no sound, proceed to fuse testing below.
  5. Electrical Testing (Multimeter - Optional): This requires understanding relay pin functions (Power In, Ground Control, Control Signal, Power Out to Pump). Pin diagrams can be found online (search relay part number + pinout). Testing involves verifying coil resistance and switch operation with a meter. While accurate, the swap test is usually sufficient for most owners.

Replacing a Faulty Relay:

  1. Purchase: Obtain a new relay matching the exact Mercedes part number found on your old relay (e.g., A 222 540 17 24 for KN). Genuine Mercedes (OEM) or high-quality brands like Bosch, Hella, or Beru are recommended. Avoid cheap no-name relays. Ensure the terminal layout matches.
  2. Install: With the battery still disconnected, insert the new relay into the correct socket, matching the orientation of the old one. It should push firmly into place. Relays only fit one way.
  3. Reconnect: Reattach the negative battery terminal.
  4. Test: Turn ignition to "ON" and listen for the pump prime. Then attempt to start the engine.

Don't Forget the Fuel Pump Fuse!

A blown fuel pump fuse will cause identical symptoms to a bad relay. Always check the fuse first! It's easier and faster.

  1. Locate: Fuses for the fuel pump are usually in the same fuse box as the relay for your model year. Refer to the fuse box cover diagram.
    • K40 (Early Models): Common fuel pump fuse locations include F32 (15A) or F36 (15A). Check the diagram - look for "FP", "Pump", "Kraftstoffpumpe".
    • K92 (Later Models): Common fuse locations include F44 (15A) or F53 (25A). Always check your specific cover diagram! Designations vary.
  2. Check: Using a fuse puller tool (often in the fuse box or tool kit), remove the suspected fuse. Hold it up to a light. The thin metal strip inside should be intact. If it's broken or blackened, the fuse is blown.
  3. Replace: Replace a blown fuse with one of the exact same amperage rating (e.g., 15A, 25A). Never use a higher amp fuse. If the new fuse blows immediately, a serious short circuit exists (faulty pump, damaged wiring) requiring professional diagnosis.

When the Relay Isn't the Problem: Next Steps in Diagnosis

If replacing the relay and fuse doesn't solve the problem, further investigation is needed:

  1. Fuel Pump Itself: This is the next most likely culprit. Testing involves checking for power and ground directly at the pump connector (located on the top of the fuel tank, accessible often under the rear seat base cushion or through a trunk floor panel). This requires specialized knowledge and safety precautions due to fuel vapors. Professional help is strongly recommended here.
  2. Fuel Pump Control Module: Some W221 models, particularly later ones, have a dedicated control module near the fuel tank. This can fail or suffer from connection/corrosion issues.
  3. Engine Control Unit (ECU) Signal: The ECU sends the signal to activate the relay. If this signal is missing due to an ECU fault, immobilizer issue (incorrect key recognition), or wiring problem, the relay won't engage.
  4. Wiring Harness Faults: Damage, corrosion, or breaks in the wiring between the relay, fuse box, ECU, or fuel pump can cause failure. This requires detailed circuit testing.
  5. Immobilizer System: If the car doesn't recognize your key, it will prevent both fuel and spark. Symptoms include no cranking, or cranking with no start. Try a spare key.
  6. Crankshaft Position Sensor: While more often causing crank-no start with spark issues, a failed CPS can sometimes disrupt the ECU sequence.

When to Call a Professional: If testing the relay and fuse doesn't solve the problem, or if you are uncomfortable performing electrical diagnostics near the fuel system or complex fuse boxes, consult a qualified Mercedes-Benz specialist or dealership technician. They have the tools, wiring diagrams (WIS), and expertise to diagnose efficiently.

Finding Your W221 Fuel Pump Relay: The Key to Resolution

Knowing the exact W221 fuel pump relay location based on your specific model year – whether it's Relay J (KJ) in the front passenger footwell (K40) for early models (2006-~2009) or Relay N (KN) in the rear trunk fuse box (K92) for later models (~2010-2013) – is the crucial first step in diagnosing and fixing a common cause of S-Class no-start problems. Always identify your production year from the door jamb sticker first. Checking the associated fuse is simple and always wise. Testing or swapping the relay is a manageable task. By following the detailed, model-specific information above, you can accurately locate the component and often resolve the issue yourself quickly and inexpensively. For persistent problems after addressing the relay and fuse, seek the diagnostic skills of a qualified Mercedes technician.

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