1983 Mercedes 380SL Fuel Pump Relay Location – Your Essential Guide

Finding the fuel pump relay in your 1983 Mercedes-Benz 380SL is straightforward: It's located in the primary fuse box, mounted on the rear wall of the engine compartment (on the firewall). Specifically, it occupies position number 4 on the relay rack. This information is crucial for diagnosing fuel delivery problems, as a faulty relay is a common cause of no-start conditions in these classic R107 SL models. Knowing exactly where it is saves valuable time during troubleshooting and repairs.

Understanding the Engine Bay Fuse/Relay Box

The central fuse and relay box in the 1983 380SL is a key component housing vital electrical systems. Unlike some modern cars where components might be scattered, Mercedes consolidated many relays and fuses into this one easily accessible location:

  1. Access: Open the hood. Stand facing the windshield. Look directly ahead at the rear wall separating the engine bay from the passenger compartment – this is the firewall.
  2. Location on Firewall: The rectangular, black plastic fuse box is attached vertically to the firewall, typically slightly offset towards the driver's side (left side for US-spec LHD cars). It's held in place by screws or plastic fasteners.
  3. Opening the Box: The lid usually has press tabs on either end. Squeeze the tabs inward or push them down (depending on design) and lift the lid straight up. Set the lid aside safely.
  4. The Relay Rack: Inside the box, you'll see a vertical column holding multiple relays and possibly some fuses mounted alongside them. This rack often has positions labeled numerically (like 1, 2, 3, 4, 5...) on the box itself or on a diagram inside the lid.
  5. Identifying the Fuel Pump Relay (Position 4):
    • Position: Look specifically for relay position number 4 within this rack.
    • Appearance: The original relay is a Bosch unit, typically a standard black plastic cube measuring approximately 1 inch x 1 inch x 1.5 inches (2.5cm x 2.5cm x 4cm). It has multiple electrical prongs (terminals) extending from its base.
    • Marking: The original relay likely has a Bosch part number like 0 332 014 113, 0 332 014 135, or similar stamped on it. Sometimes the letters "KJ" or "KP" are also visible, designating its function.

Why Knowing Its Location Matters: Symptoms of a Failing Fuel Pump Relay

The fuel pump relay controls power to the primary electric fuel pump(s) located underneath the car near the fuel tank. When this relay fails, the pump(s) receive no power, starving the engine of fuel. Common symptoms include:

  1. Engine Cranks But Won't Start: The starter motor turns the engine over normally, but the engine fails to fire and run. This is the most frequent symptom.
  2. Intermittent Starting: The car starts fine sometimes, and other times refuses to start, particularly when the engine bay is hot ("heat soak"). This happens because heat exacerbates internal faults in the aging relay.
  3. Sudden Engine Stall: The car dies while driving and refuses to restart immediately.
  4. No Fuel Pump Noise: When you turn the ignition key to the "Run" position (before cranking), you should hear a faint humming or buzzing sound from the rear of the car (the fuel pumps priming) for 1-2 seconds. No noise at all is a strong indicator of either a relay failure, fuse blow, or pump failure. Always listen for this prime cycle.

Verifying Relay Position and Troubleshooting Basics

Simply finding the relay isn't always enough. Confirmation and basic checks are prudent:

  1. Consult Lid Diagram: While many fuse box lids have degraded diagrams over 40 years, examine yours carefully. It often clearly labels relay positions, including position 4 as the fuel pump relay.
  2. Compare to Adjacent Relays: If position markings are unclear, temporarily swap the relay from position 4 with an identical relay from another position known to be working correctly (like the Oxygen Sensor relay, position 3, or Flasher relay). Exercise caution: Ensure the relays are truly identical (same Bosch number) before swapping. If the starting problem moves or resolves, you've identified the faulty relay.
  3. Check Related Fuse FIRST: Crucially, before blaming the relay, always check the 8-amp fuse that powers the fuel pump circuit! On the 380SL, this fuse is usually located in the lower row of fuses within the same fuse box, likely Fuse 1 (often labeled in diagram, or fuse slot positions may be numbered). A blown fuse causes identical symptoms to a bad relay. Replace it if blown, but investigate why it blew (potential wiring short) to prevent recurrence.
  4. Inspect the Relay Socket: Remove the relay carefully (it may need a wiggle). Examine the socket connectors inside the fuse box for signs of burning, melting, corrosion, or loose pins. Damaged sockets prevent a new relay from working and require repair.

How the Fuel Pump Relay Works on the 380SL (Bosch K-Jetronic System)

The 1983 380SL uses Bosch's continuous mechanical fuel injection (K-Jetronic) with at least one electric fuel pump, sometimes two (a transfer pump in the tank and a main pump underneath). The relay's job is critical:

  1. Signal Source: When you turn the ignition key to "Start," power is sent via the ignition switch to one coil inside the fuel pump relay. This coil is also energized when the engine is running, via a signal from the alternator's D+ terminal or an oil pressure switch backup circuit.
  2. Relay Activation: This electrical signal energizes the relay coil, creating a magnetic field.
  3. Power Switch: The magnetic field pulls internal switch contacts closed. This connects battery voltage (from Fuse 1 and a larger constant power feed) directly to the output terminal leading to the fuel pump(s).
  4. Pump Operation: With power flowing from the relay output, the fuel pump(s) turn on, supplying pressurized fuel to the fuel distributor and injectors.
  5. Safety Cutoff: When you turn the ignition off, the signal to the relay is removed. The magnetic field collapses, opening the switch contacts and cutting power to the pumps, stopping fuel flow.

Testing the Relay (Advanced Verification)

If you suspect the relay after checks above:

  1. Listen & Feel: Have an assistant turn the ignition key to "Run" (don't crank). You should hear and feel a distinct click from the relay in position 4 as its internal switch activates. A healthy relay clicks reliably; a failing one might click weakly, intermittently, or not at all.
  2. Bench Test (Requires Basic Tools & Knowledge): Safely remove the relay.
    • Identify the control coil pins (typically terminals 85 and 86). Apply the correct voltage (usually 12V from a small battery or power supply with fused leads) across these pins.
    • You should hear and feel it click.
    • Use a multimeter set to continuity/Ohms to test the main switch contacts (typically terminals 30 and 87). With no power applied, it should be open circuit (infinite resistance). With power applied to coil pins 85/86, it should switch to closed circuit (near zero resistance). Failure to click or switch properly confirms a bad relay.
  3. Circuit Testing (Recommended if Unsure): This involves using a multimeter or test light to check for voltage at key points in the circuit (at fuse 1, at the relay socket pins when ignition is in "Run" and during cranking, and at the fuel pump connector) to isolate where the power is lost. This is more accurate than simple relay testing alone.

Replacing the Fuel Pump Relay on a 1983 380SL

If confirmed faulty, replacement is simple:

  1. Purchase Correct Relay: Source a new Bosch fuel pump relay. As mentioned, common original part numbers were 0 332 014 113, 0 332 014 135, or 0 332 019 333. Modern equivalent replacements are widely available (e.g., Bosch 332906051) – consult a reliable parts supplier specializing in classic Mercedes or Bosch parts. Verify compatibility using the OEM number or VIN. "Universal" relays can cause problems.
  2. Disconnect Battery: Always disconnect the negative (-) terminal of the battery before working on electrical components to prevent shorts or sparks.
  3. Locate Fuse Box & Position 4: Open the fuse box as described.
  4. Remove Old Relay: Firmly grasp the suspect relay and pull it straight out from its socket. It shouldn't require excessive force but might need a slight rocking motion if corroded.
  5. Socket Inspection: Visually inspect the relay socket terminals for corrosion, dirt, or damage. Clean if necessary with electrical contact cleaner and a small brush.
  6. Install New Relay: Align the new relay carefully with its socket. Ensure the prongs align correctly with the slots. Push it firmly and squarely into place until it seats fully.
  7. Reconnect Battery.
  8. Test: Turn the ignition key to "Run." You should hear the fuel pumps prime (brief hum from rear). Attempt to start the engine.

Important Precautions and Considerations

  • Battery Disconnect: Emphasized again – always disconnect the battery before touching fuses or relays.
  • Heat Sensitive: Older relays are highly susceptible to heat-related failure. Replacing a relay that fails only when hot is common.
  • Socket Integrity: A corroded or melted relay socket is a fire hazard and must be repaired properly (soldering new terminals or replacing the entire socket/harness section) before installing a new relay. Do not ignore socket damage.
  • Ground Connection: Ensure the ground connection for the relay/fuse box is clean and secure. A poor ground path can mimic relay failure symptoms. The ground strap is usually bolted to the firewall near the box.
  • Fuel Pressure: While this guide focuses on the relay, remember that a failed fuel pump, clogged fuel filter, or faulty fuel accumulator (common on K-Jet) can also cause fuel starvation. The relay is step one.
  • O-Ring Seal: When closing the fuse box lid, ensure the rubber o-ring or gasket is seated correctly to maintain weather resistance.
  • Use Genuine or Quality Replacement: Cheap, off-brand relays are notoriously unreliable. Bosch or OEM is strongly recommended.

Conclusion

Knowing the 1983 Mercedes 380SL fuel pump relay location – specifically position 4 in the engine compartment fuse box on the firewall – is fundamental knowledge for any owner or mechanic. Symptoms like cranking without starting, especially if accompanied by the absence of the tell-tale fuel pump priming buzz when the ignition is turned on, point directly to checking Fuse 1 and then this relay. Access is simple, inspection and replacement are manageable DIY tasks if proper safety precautions are followed, and using a quality Bosch replacement relay is key to reliable operation. This relay is a known wear item on these classic cars, so locating and testing it should be your first diagnostic step when facing fuel delivery issues. Keeping a spare relay in the glovebox is wise preventative maintenance for trouble-free cruising.

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