July 2025

E46 Fuel Pump Relay: Comprehensive Guide to Diagnosis, Repair, and Prevention

The fuel pump relay is a critical, yet often overlooked, component in your BMW E46. When it fails, your car will either not start or stall unexpectedly. Understanding its function, recognizing symptoms of failure, knowing how to test it accurately, and executing a proper replacement are essential skills for any E46 owner seeking reliable operation.

This component acts as a switch, controlled by the Engine Control Unit (DME), supplying high-current power to the electric fuel pump located inside the fuel tank. Without this relay activating correctly, the fuel pump receives no power, resulting in a complete loss of fuel delivery to the engine. Identifying relay failure quickly can save considerable time and diagnostic cost. For persistent engine crank-no-start situations or sudden stalling, the fuel pump relay should be among the first components checked after verifying basic fuses.

Understanding the E46 Fuel Pump Relay's Role and Location

The primary function of the fuel pump relay is straightforward: it provides a robust electrical pathway for the significant current required by the fuel pump motor, triggered by a low-power signal from the DME. This design protects the sensitive electronics within the DME from the pump's electrical load.

In the E46, the main fuel pump relay is integrated into the DME Relay Module, located within the DME box (also known as the ECU box) in the engine compartment. This black plastic box is situated on the passenger side (right-hand side for LHD vehicles, left-hand side for RHD vehicles), mounted near the base of the windshield. Inside this box resides the DME itself (the main engine computer) alongside a carrier holding several critical relays. The fuel pump relay is often specifically identified within this carrier, sometimes labeled with "K6307" or other internal designations, though physical inspection or wiring diagrams are definitive. Other relays housed here typically include the main DME relay and potentially secondary air pump or oxygen sensor heater relays. Accessing the relays requires removing the cover of the DME box, which involves undoing several plastic nuts or screws around its perimeter. Care must be taken to prevent debris or water ingress during this process.

Recognizing the Symptoms of a Faulty Fuel Pump Relay (E46 Specific)

Failure of the fuel pump relay manifests in distinct ways, directly tied to the interruption of fuel pump operation:

Engine Cranks but Does Not Start: This is the most common and definitive symptom. The starter motor turns the engine over normally (you hear the cranking sound), but the engine fails to fire and run. This occurs because no fuel is reaching the fuel injectors due to the inactive pump.
Engine Stalls Unexpectedly (Often While Driving): A relay with an intermittent fault, often due to internal overheating, cracked solder joints, or internal contact corrosion, may function normally when cold but cut out suddenly once warmed up or subjected to vibration. This causes the engine to lose power and stall immediately, as if the ignition was switched off.
No Fuel Pump Prime Sound: When you turn the ignition key to the "ON" position (before cranking), you should hear a distinct humming or whirring sound from the rear of the car (near the fuel tank) for approximately 2-3 seconds. This is the fuel pump pressurizing the fuel rail. No sound at this stage strongly indicates a problem with the fuel pump circuit, very commonly the relay.
Check Engine Light (CEL) with Fuel System Codes: While not every relay failure triggers a CEL, common Diagnostic Trouble Codes (DTCs) associated with fuel pump circuit issues that could originate from a relay fault include:
- P0230: Fuel Pump Primary Circuit Malfunction (This code points directly at the circuit controlling the pump, including the relay, wiring, or pump motor itself).
- P3100: Low Fuel Pressure (This can be caused by pump failure or failure of components delivering power to the pump).
- P0171 / P0174: System Too Lean (Bank 1/Bank 2) - Lack of fuel delivery can cause these lean mixture codes, though they have many other potential causes.

How to Accurately Test the E46 Fuel Pump Relay (Step-by-Step Guide)

Diagnosing the relay requires a systematic approach, preferably using a multimeter and referencing the specific relay socket layout. Always disconnect the negative battery terminal before handling relays or wiring in the DME box to prevent accidental shorts. Identify the fuel pump relay based on its position within the DME Relay Module or verified diagrams.

The "Tap" Test (Simple Initial Check): With the ignition in the "ON" position (engine off), locate the suspected relay. Listen carefully for the fuel pump priming sound. Using a screwdriver handle or similar non-conductive object, gently tap the top of the relay. If you suddenly hear the pump prime after tapping, or if tapping causes an intermittent relay to stop working during cranking, it strongly suggests an internal fault within the relay (common with aged relays).
Physical Relay Swap: The DME Relay Module usually contains multiple relays of the exact same type. Common replacements are standardized 4-pin or 5-pin "ISO Micro" relays. Identify a relay performing a less critical function (e.g., the secondary air pump relay, often located nearby). Swap it with the suspected fuel pump relay. If the car now starts, or you hear the pump prime sound (which you didn't before), the original fuel pump relay is faulty. Note: Ensure the relay you swap is identical.
Voltage Testing at the Relay Socket (Ignition ON, Engine OFF):
- Identify Pins: Consult E46-specific wiring diagrams or component location guides (readily available online) to identify the relay socket pins for the fuel pump relay. Standard relay pins are:
  - 85 & 86: Control Coil (Low current from DME)
  - 30: Constant Power Input (Battery Positive, typically +12V)
  - 87: Switched Power Output (To Fuel Pump)
  - (87a): Only present on 5-pin "changeover" relays, usually unused in a simple fuel pump application - typically not connected in the E46 setup.
- Locate Ground (e.g., Pin 85/86): Set multimeter to DC Volts (20V scale). Place the black probe on a known good ground (engine block, battery negative terminal). Touch the red probe to one of the coil terminals (pin 85 or 86). With ignition ON, one of these pins should show battery voltage (~12V), while the other should go momentarily to ground (0V) when ignition is first turned on (during the prime cycle), confirming the DME is sending the activation signal.
- Check Constant Power (Pin 30): With black probe on ground, red probe on Pin 30. Should show ~12V constantly, regardless of ignition position (key off, key on, cranking). This power comes directly from the main battery cable/fusible link.
- Check Switched Output (Pin 87) for Power During Prime: Black probe on ground, red probe on Pin 87. Turn ignition to "ON" (do not crank). Pin 87 should read ~12V only during the brief 2-3 second priming period. Voltage should then drop back to 0V. No voltage at Pin 87 during prime indicates a problem with the relay, its control signal, or the constant power input.
Bench Testing the Relay (Advanced): If still uncertain, carefully remove the relay. Using a multimeter in continuity/resistance mode:
- Check Coil Resistance (Pins 85-86): Should typically read between 60-100 ohms for most ISO Micro relays. An open circuit (infinity) indicates a broken coil.
- Check Switching: Apply 12V directly across pins 85 (+) and 86 (-) using fused jumper wires or a bench power supply. You should hear and feel a distinct click. Use the multimeter in continuity mode to test:
  - Pin 30 to Pin 87: Should have continuity (near 0 ohms) only when the coil is energized.
  - Without coil power, Pin 30 to Pin 87 should be open circuit (infinity). Testing can be done with a multimeter set to continuity/resistance mode. The key points are: 1) Measure resistance between pins 85 and 86 (this tests the coil) - a good relay coil typically shows 60-100 ohms. An open circuit indicates a failed coil. 2) Measure resistance between pin 30 and pin 87 - it should be infinite (open circuit) when the relay coil is not powered. 3) Apply power (12V) to pins 85 (+) and 86 (-). A functioning relay will produce an audible click. While power is applied, measure resistance between pin 30 and pin 87 - it should drop to nearly zero ohms, indicating the contacts have closed. Failure to click or failure to establish continuity between 30 and 87 when powered points to a dead relay. Note that this method requires safely supplying 12V to the relay terminals.

Replacing the E46 Fuel Pump Relay: Detailed Instructions

Replacement is typically straightforward once correctly diagnosed:

Locate the Relay: Identify the fuel pump relay within the DME Relay Module inside the engine bay DME box (as described earlier).
Disconnect Battery: Always disconnect the negative battery terminal using a 10mm wrench to prevent electrical shorts.
Remove Old Relay: Grip the relay firmly and pull it straight up and out of its socket. Avoid excessive rocking or twisting which might damage the socket.
Inspect Socket: Briefly inspect the relay socket terminals for any signs of burning, corrosion, bent pins, or damage. Clean carefully with electrical contact cleaner and a soft brush if necessary.
Install New Relay: Ensure the new relay matches the old relay exactly in pin number and configuration. Align the pins correctly with the socket and press firmly down until it seats completely. A distinct click may be heard.
Reconnect Battery: Reattach the negative battery terminal securely.
Test: Turn the ignition key to "ON" and verify you hear the fuel pump prime for 2-3 seconds. Then attempt to start the engine. A successful start confirms the repair.

Selecting the Correct Replacement Relay

Always use a high-quality relay. The original BMW part number is often 61 36 8 377 103, but these relays are generic automotive components manufactured by major suppliers like Bosch, Hella, Tyco, or Omron. A highly recommended direct-fit, reliable replacement is the Bosch 0 332 019 150. When purchasing, ensure it's a Micro ISO Relay. Key specifications are:

Rating: Typically 20A / 12VDC (meets E46 requirements).
Pin Configuration: ISO 280 (Micro Relay) - 4 or 5 pins, but the E46 uses the switching function (Pins 30 and 87).
Terminal Layout: Standard automotive (Pins 85, 86, 30, 87).
Mounting: Plug-in, designed for a standard automotive relay socket module.

Avoid the cheapest no-name relays, especially from unknown online sellers, as their internal components and contact materials are often inferior and prone to premature failure. Investing in a Bosch, Hella, or genuine BMW relay ensures longevity and reliability.

Crucial Precautions and Safety Information

Working on the E46's electrical system requires attention to safety and precision:

Battery Disconnection Mandatory: Always disconnect the negative battery terminal before touching any relay, fuse, or wiring within the DME box. A short circuit here can damage the DME, a costly component. Reconnect only after installation is complete.
Protect the DME Box: Ensure the area is clean and dry before opening the DME box. Work carefully to prevent dropping tools or debris inside. Close the box securely after completing work to protect against water and contaminants.
Handle Relays Carefully: Pull relays straight out. Do not force them. Ensure pins are straight before insertion. Apply even pressure vertically to seat them fully without bending the pins or damaging the socket.
Diagnostic Accuracy: Avoid throwing parts at the problem. Proper testing as outlined above ensures you replace the correct component. Misdiagnosis wastes time and money. Consider fuel pump failure or wiring issues if testing the relay shows it's functional yet symptoms persist. Common wiring problems include chafed wires near the DME box connectors or corrosion at the fuel pump connector under the rear seat. Pressure testing the fuel system at the Schrader valve on the fuel rail provides definitive confirmation of fuel pump health when combined with electrical tests.
Relay Lifespan: While reliable, these relays do wear out over time due to repeated switching cycles and thermal stress. Consider replacement around the 15-20 year mark as preventative maintenance, especially if the car is exhibiting any minor, occasional electrical quirks. Using a high-quality relay like the Bosch unit minimizes the risk of premature failure.

E46 Fuel Pump Relay Frequently Asked Questions (FAQ)

Q: Why does my E46 have two fuel pump relays? I found conflicting information.
- A: Standard E46 models (petrol) typically have only one primary fuel pump relay located within the DME Relay Module in the engine bay. However, some variations exist:
  - Later E46 models (M54 engines) might integrate a fuel pump control module relay within a secondary module, but the primary switching power still comes from the DME box relay.
  - E46 M3: The S54 engine M3 does have a secondary relay specifically for the low-pressure transfer fuel pump inside the fuel tank, alongside the main high-pressure pump relay. However, for the vast majority of E46 owners (non-M3), there is only one main fuel pump relay.
  - Replacement Part Confusion: Relay modules often contain identical-looking relays. Mistaking another relay (like the DME main relay, secondary air pump relay, or O2 heater relay) for a second fuel pump relay is common.
- Conclusion: For troubleshooting a crank-no-start or no prime sound on a standard E46, focus on the relay identified in the DME Relay Module (often using the swap test method).
Q: My relay tests fine, but I still have no prime/no start. What else should I check?
- A: If the relay tests good on the bench and you have confirmed voltage at the output pin (87) during ignition ON prime, the problem lies elsewhere. Critical checks include:
  - Fuel Pump Fuse: Locate the specific fuse for the fuel pump circuit in the E46's glove compartment fuse box (often Fuse #54, typically 20A - verify your specific model year owner's manual or fuse chart). Test it visually and with a multimeter.
  - Fuel Pump Itself: Test the pump connector (located under the rear seat bench) for power and ground during the prime cycle. If power is present at the connector during prime, but the pump doesn't run, the pump itself is likely faulty. You can also attempt the temporary bypass method (see next question).
  - DME Signal: Was the control signal (at the relay socket, pins 85 or 86 going to ground during prime) confirmed during testing? If not, the DME might not be sending the trigger signal due to internal fault, immobilizer issues (check EWS), ignition switch problems, or a fault detected by the DME inhibiting pump activation.
  - Wiring Harness: Inspect wiring between the DME box and the fuel pump for damage, especially where it passes through bulkheads or flexes near connectors. Check for broken wires or corrosion.
Q: How do I temporarily jump/bypass the fuel pump relay to test the pump?
- A: Proceed with extreme caution. This is a diagnostic aid only, not a permanent solution. It requires understanding relay pin function. Find the relay socket in the DME module:
  - Identify Pin 30 (Constant Battery Power, ~12V) and Pin 87 (Output to Fuel Pump). Consult diagrams to be absolutely certain.
  - With the key in the OFF position, use a short piece of heavy gauge wire (or a specialized fuse bypass jumper) to create a secure connection only between Pin 30 and Pin 87 in the empty relay socket.
  - DO NOT connect to any other pins. Ensure the jumper cannot slip and touch adjacent pins or metal.
  - Turn the ignition key to the ON position (do not crank). If the fuel pump is functional and wiring downstream is intact, you should hear the pump run continuously until the jumper is removed or the key is turned off.
  - Warning: Bypassing the relay means the pump runs whenever the key is ON, bypassing all safety controls. This test should be brief, only to confirm pump operation and downstream wiring. Remove the jumper immediately after testing. Never drive the car with the relay bypassed.
Q: I replaced the relay, but the pump still doesn't prime on ignition ON. The car won't start. What now?
- A: Re-diagnose systematically:
  1. Fuse: Double-check the fuel pump fuse in the glovebox (Fuse #54, typically 20A).
  2. Relay Swap: Try swapping the new relay into another socket of the same relay type in the module (e.g., secondary air pump relay). Test if the secondary air pump activates. If not, the new relay might be DOA (Dead on Arrival) - rare but possible, especially with non-genuine parts. Put the original known-good relay back in its original socket and verify function.
  3. Power at Relay Socket:
    - Is constant +12V present at Pin 30 (relay socket, key off/on)?
    - Is the DME control signal present? (One coil pin should go to ground when ignition is turned ON). If not, the DME isn't commanding the relay on. Check DME fuses, ignition signal to DME, immobilizer status (EWS sync).
  4. Jumper Test (Carefully!): Perform the temporary Pin 30 to Pin 87 jumper test as described above. If the pump doesn't run during this test (with key ON), the problem is downstream of the relay socket (fuse again, wiring damage under the car or to the fuel tank, or a completely dead fuel pump). Measure voltage at the fuel pump connector under the rear seat while the jumper is installed and key ON. If power reaches the connector but the pump doesn't run, the pump is bad. If no power reaches the connector, the wiring harness has a break.
Q: Can a bad fuel pump relay cause misfires or hesitation?
- A: Generally, no. A fuel pump relay failure is binary: it either works correctly (pump receives power) or it fails completely (pump gets no power). Intermittent failures cause complete loss of power and engine stall. Hesitation or misfires during driving are more likely caused by a failing fuel pump (decreasing flow/pressure), clogged fuel filter (less common on E46 due to in-tank filter/pump assembly), fuel pressure regulator issues, ignition coil problems, vacuum leaks, faulty MAF sensor, or other sensor failures.

Conclusion

The E46 fuel pump relay is a small, inexpensive, yet indispensable component for engine operation. Its failure manifests as a definitive crank-no-start condition or sudden stalling, often preceded or accompanied by the absence of the characteristic fuel pump priming hum. Located within the DME box under the hood, diagnosing it through visual inspection, the swap test with identical relays in the module, and precise voltage measurement at its socket is crucial before replacement. Selecting a high-quality replacement relay, such as the Bosch 0 332 019 150, and installing it correctly after ensuring the socket and wiring are intact ensures reliable performance for years to come. Understanding how to test and replace this relay empowers E46 owners to address a common failure point quickly and efficiently, restoring their vehicle's drivability. Always prioritize safety, particularly by disconnecting the battery when working within the DME box and double-checking fuse integrity before delving deeper into the fuel delivery system.

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