1976 Mercedes 450SL Fuel Pump Relay Location: The Definitive Guide

The fuel pump relay on a 1976 Mercedes-Benz 450SL is located in the main front relay box, situated behind the battery against the firewall on the driver's side (left-hand side for US models) of the engine compartment. You need to remove the battery to access it safely and properly.

If your classic 1976 Mercedes-Benz 450SL suddenly refuses to start, cranks without firing, or stops running intermittently, the culprit could very well be the fuel pump relay. Knowing its precise location is critical for diagnosis and repair. Unlike some components scattered throughout the vehicle, Mercedes-Benz logically grouped key relays together, but accessing this particular one requires some effort.

Why the Fuel Pump Relay is Crucial to Your 450SL's Operation

The K-Jetronic mechanical fuel injection system used in the 1976 450SL relies on high-pressure fuel delivery. The electric fuel pump, typically mounted near the fuel tank, generates this pressure. However, the pump requires significant electrical current, far more than the ignition switch or a simple fuse can safely handle. This is where the fuel pump relay acts as an indispensable intermediary:

  1. Power Control: The relay serves as a heavy-duty switch. A small electrical signal from the ignition switch activates an electromagnet inside the relay. This electromagnet then pulls internal contacts together, closing a high-amperage circuit that provides direct battery power to the fuel pump.
  2. Safety Function: The relay is integral to the safety system. It is designed to cut power to the fuel pump immediately if the engine stalls or in the event of a crash (via a separate crash switch). This prevents the pump from continuing to run and potentially spraying fuel if there's a ruptured line.
  3. System Integrity: Failure of this relay means the fuel pump receives no power, resulting in zero fuel pressure. Without fuel pressure, the K-Jetronic injection system cannot function, causing a no-start condition or sudden engine shutdown while driving.

The Challenge of Access: Why Location Matters

Mercedes-Benz engineers prioritized protection and centralization, placing the main relay box in a physically robust location against the firewall. This placement protects vital relays from excessive engine heat and potential water spray. However, the battery's large size and position directly in front of this box mean:

  • The battery physically blocks direct access. You simply cannot see, touch, or reliably test the relay box without removing the battery.
  • Attempting access without removal risks shorts. Tools or hands reaching around the battery risk accidental contact with the positive terminal or surrounding brackets, potentially causing dangerous sparks, damage to components, or personal injury.

Step-by-Step Guide: Finding and Accessing the Relay

Here’s exactly how to get to the fuel pump relay on your 1976 450SL:

  1. Prepare the Vehicle:
    • Park on a level surface, engage the parking brake firmly, and place wheel chocks behind the rear wheels.
    • Turn the ignition OFF and remove the key.
    • Pop the hood and secure it safely.
  2. Disconnect the Battery:
    • Identify Terminals: Locate the negative (-) and positive (+) terminals on the battery. The negative terminal is usually marked with a minus sign ("-") or "NEG", often with a black cable. The positive is marked with a plus sign ("+") or "POS", typically with a red cable.
    • Disconnect Negative FIRST: Using the appropriate size wrench (usually 13mm for the clamp bolts), loosen the nut/bolt securing the negative cable clamp. Carefully lift the negative cable completely off the battery terminal and secure it away from any metal. This step is crucial for safety, eliminating the risk of accidental grounding and sparks.
    • Disconnect Positive: Repeat the process to loosen and remove the positive cable clamp from its terminal. Tuck it safely away.
  3. Remove the Battery Hold-Down:
    • The battery is secured by a hold-down bracket, typically at the base near the bottom edge. This usually requires a 13mm or 10mm wrench.
    • Unbolt or loosen the bracket sufficiently to allow the battery to be lifted out.
  4. Remove the Battery:
    • Carefully lift the battery straight up and out of the battery tray. Place it securely on the ground or a sturdy surface away from sparks or flames.
  5. Locate the Main Relay Box:
    • With the battery removed, you will now clearly see a rectangular, usually light gray or beige, plastic box mounted vertically against the engine compartment firewall. It is situated where the battery's rear corner was located.
  6. Access the Relay Box Cover:
    • The relay box has a protective cover secured by metal clips or tabs near its top edge. Carefully unclip or unfasten these clips.
    • Lift the cover upwards and hinge it back out of the way. Some light dusting might be needed to clearly see the relays.
  7. Identify the Fuel Pump Relay:
    • You will see a series of identical (or nearly identical) black plastic cube-shaped relays plugged into sockets within the box. This is the main relay grouping for the engine management and associated systems.
    • Crucial Identification: The fuel pump relay is generally the relay positioned closest to the outer fender (driver's side fender for US cars). In Mercedes terminology of the era, this is often referred to as position "K" within the relay box. Look for markings on the relay itself. It may bear numbers like "001 545 03 02", "001 545 48 25", "0 332 014 107", or "0 332 014 126". Crucially, the label might explicitly say "Fuel Pump Relay". Verify the factory wiring diagrams specific to the 1976 450SL if possible, as relay positions can sometimes be changed or vary slightly.
    • Connector Pin Count: The fuel pump relay typically has a 5-pin connector socket. Many other relays in the box (like the horn relay, headlight relay, etc.) might be 4-pin. Counting the pins on the relay base is a good secondary identifier.

Testing the Relay Without Removal (Initial Checks)

Before pulling out the relay for bench testing, perform a basic operational check:

  1. Listen During Cranking: Have an assistant turn the ignition key to the "Start" position while you listen near the relay box. A distinct, solid "click" sound when the key is turned to "Start" and potentially another when released back to "Run" usually indicates the relay is at least activating electromagnetically. No click is a strong sign of a potential relay problem.
  2. Check for Voltage:
    • Carefully plug the relay back into its socket if removed for identification.
    • Set a multimeter to DC Volts (20V scale or similar).
    • Signal Input: With the ignition key turned to the "Run" (ON) position (do not crank), carefully probe the connection at the relay socket designated for the "signal" wire (this triggers the relay). Using diagrams is best, but often this pin has a wire color related to the starter circuit (like yellow/black or black/yellow depending on model/year). You should see battery voltage (approximately 12V) at this pin only when the key is in the "Start" or "Run" position. If this signal is missing, the relay cannot activate, pointing to a fuse, ignition switch, or wiring issue upstream.
    • Power Output: With the key turned to "Run" or while cranking, probe the socket pin designated for the main power output to the fuel pump. Again, diagrams are ideal, but it connects to a thick wire running towards the rear of the car. If the relay is activating properly, you should read battery voltage here during cranking/Run.

Removing the Relay for Further Testing or Replacement

If initial checks point to a faulty relay, carefully remove it:

  1. The relay snaps into the socket. Firmly grasp it by its sides (avoid pulling on wires if any are attached) and gently rock it back and forth while pulling straight out.
  2. Before inserting a new relay, visually inspect the socket pins for any corrosion, damage, or bent contacts. Clean them gently if necessary with contact cleaner and a brush.

Important Considerations and Variations

  • Safety Relay (Older Models/Early Production): Some very early 107 SLs (or modified setups) might have an additional "safety relay" or "crash relay" associated with the fuel pump circuit. This relay is not located behind the battery. It is often found mounted on the driver's side inner fender wall near the brake booster or within the cabin behind the glove box. Its primary function is to cut power in an accident. If your car cranks but won't start, and the main fuel pump relay is clicking and providing power during cranking (tested at its output pin), then a faulty safety relay could be interrupting power further down the line. Locating and testing this relay requires separate steps.
  • Wiring Issues: Corroded, damaged, or loose wiring connections at the relay socket, along the fuel pump power line, or at the fuel pump itself are common failure points, especially on 50-year-old vehicles. Visual inspection and continuity/resistance checks are vital.
  • Ignition Switch Problems: A failing ignition switch may not provide the crucial signal voltage to trigger the fuel pump relay, mimicking relay failure. Check the signal input at the relay socket as described.
  • Quality Parts: Use high-quality replacement relays. Inferior parts are prone to premature failure. Genuine Mercedes-Benz or reputable OEM suppliers (Hella, Bosch) are recommended. Old relays often suffer from internal contact burning or coil degradation.

Symptoms Confirming Relay Location Importance

Understanding the symptoms that pinpoint the fuel pump relay (and thus necessitate finding its location) reinforces why knowing where to look is the first step:

  • Engine Cranks But Won't Start: The most common symptom of a failed fuel pump relay or a related open circuit. No fuel pressure means the injectors have nothing to spray.
  • Sudden Engine Stalling: The engine dies unexpectedly while driving or idling. This happens if the relay loses its signal input (ignition switch intermittent) or suffers an internal failure (contacts opening).
  • No Fuel Pump Noise: When you turn the ignition to "Run" (before cranking), you normally hear the electric fuel pump run for a couple of seconds to build pressure, then stop. You also hear it continuously while cranking and running. Complete silence from the fuel pump area during the "Run" position or cranking strongly indicates a power interruption, with the relay being a prime suspect.
  • Failure to Prime: If the initial 2-second pump run doesn't happen when turning the key to "Run," check the signal input to the relay and the relay itself.

Conclusion: Location Leads to Resolution

For any owner or mechanic troubleshooting a 1976 Mercedes-Benz 450SL that cranks but refuses to start or dies unexpectedly, verifying the condition of the fuel pump relay is fundamental. As established, its fixed location in the main relay box behind the battery on the driver's side firewall is the key starting point. While access requires battery removal for safety and visibility, the process is straightforward with the correct preparation. Confirming relay operation through audible clicks and voltage checks at the socket allows for quick isolation of this potential failure point. Whether confirming its function, testing it thoroughly, or replacing a faulty unit, knowing the 1976 Mercedes 450SL fuel pump relay location empowers you to get this classic roadster running reliably once again. Always prioritize safety when working on the electrical system by disconnecting the battery negative terminal first. For complex wiring issues or if the problem persists, consult a specialist familiar with classic Mercedes-Benz fuel injection systems.

Back to blog