2000 Mercedes S500 Fuel Pump Relay Location: A Complete Guide for DIY Owners

If you own a 2000 Mercedes-Benz S500 (W220 chassis) and are experiencing starting issues, the fuel pump relay is often the first component to check. The fuel pump relay on a 2000 Mercedes S500 is located in the front passenger-side fuse and relay box, under the hood, near the battery. This relay is typically labeled as "87" or "Fuel Pump Relay," and it sits in the engine compartment fuse box, which is positioned on the right side (passenger side) of the vehicle, close to the windshield washer fluid reservoir . Knowing this location can save you hours of unnecessary troubleshooting and expensive diagnostic fees.

Understanding the Fuel Pump Relay and Its Role

The fuel pump relay is an electrical switch that controls power to the fuel pump. When you turn the ignition key, the relay receives a signal from the engine control unit (ECU) and closes the circuit, allowing current to flow to the fuel pump. This pressurizes the fuel system, enabling the engine to start and run. On a 2000 Mercedes S500, a faulty relay is a common cause of intermittent starting problems, no-start conditions, or stalling while driving .

The relay itself is a small, rectangular black box with four or five prongs. It is designed to handle high current loads, and over time, it can fail due to heat, vibration, or electrical surges. Unlike a fuse, which blows to protect the circuit, a relay can wear out internally, causing the contacts to stick or fail to close completely.

Exact Location of the Fuel Pump Relay on a 2000 Mercedes S500

The main fuse and relay box on the 2000 S500 is located in the engine compartment, on the passenger side, near the firewall. It is a black plastic box with a removable cover. To access it:

  1. Open the hood and locate the battery on the passenger side.
  2. Look for the rectangular fuse box positioned between the battery and the windshield. It is often secured with a clip or latch.
  3. Remove the cover by pressing the release tabs or lifting the latch. Inside, you will see a grid of fuses and relays, each labeled with a number and sometimes a diagram.

The fuel pump relay is typically the relay labeled "87" in this box . In many Mercedes S500 models from 2000, the "87" relay is dedicated to the fuel pump circuit. Some vehicles may also have a relay labeled "Fuel Pump" or "K" (for "Kraftstoffpumpe," German for fuel pump). If the labels are faded or missing, refer to the diagram printed on the inside of the fuse box cover. The diagram will show the position of each relay and fuse, with the fuel pump relay often located in the front row, near the center.

Note: The 2000 S500 uses a W220 chassis, and the relay location can vary slightly depending on the production date and specific options. Some models may have a secondary relay box in the trunk, but for the fuel pump, the primary relay is always in the engine compartment fuse box.

How to Identify a Faulty Fuel Pump Relay

Before replacing the relay, you should confirm it is the source of the problem. Common symptoms of a failing fuel pump relay include:

  • The engine cranks but does not start. This is the most common symptom. You can hear the starter motor turning the engine, but the engine will not fire because no fuel is reaching the injectors.
  • Intermittent starting issues. The car may start fine one day and refuse to start the next. This is a classic sign of a relay with worn contacts that sometimes fail to close.
  • The engine stalls while driving. If the relay loses connection while the car is moving, the fuel pump will shut off, and the engine will stall immediately.
  • No sound from the fuel pump. When you turn the key to the "ON" position (without starting the engine), you should hear a faint whirring sound from the fuel pump in the rear of the car for about 2 seconds. If you hear no sound, the relay or pump may be faulty.

To test the relay, you can swap it with a similar relay from the same fuse box (e.g., the horn relay or headlight relay). If the engine starts with the swapped relay, the original relay is likely bad. You can also use a multimeter to check for continuity across the relay's terminals when power is applied, but swapping is the easiest method for most DIYers.

Step-by-Step Guide to Replacing the Fuel Pump Relay

Replacing the fuel pump relay on a 2000 Mercedes S500 is a straightforward task that requires no special tools. Here is how to do it:

  1. Safety first. Turn off the engine and remove the key from the ignition. Disconnect the negative battery terminal to prevent any electrical shorts or accidental activation of the fuel pump.
  2. Locate the fuse box. Open the hood and find the passenger-side fuse box near the battery.
  3. Remove the cover. Unclip or unlatch the cover and set it aside. Take a photo of the layout for reference.
  4. Find the "87" relay. Look for the relay labeled "87" or "Fuel Pump Relay." It is usually a small, square relay with four or five pins.
  5. Pull out the relay. Grip the relay firmly and pull it straight out. It may require a little force, but do not twist or rock it excessively.
  6. Inspect the relay. Check for any signs of burnt plastic, corrosion, or melted pins. If the relay looks damaged, replacement is necessary.
  7. Install the new relay. Insert the new relay into the same socket. Push it down until it clicks into place. Ensure it is fully seated.
  8. Reconnect the battery. Reattach the negative battery terminal and tighten it securely.
  9. Test the vehicle. Turn the key to the "ON" position and listen for the fuel pump priming sound. Then start the engine. If the engine starts and runs smoothly, the replacement was successful.

Other Components to Check if the Relay Doesn't Fix the Issue

If replacing the fuel pump relay does not solve the starting problem, you should examine other components in the fuel system. The fuel pump itself can fail, especially on older vehicles. The 2000 S500 uses an in-tank fuel pump that is located inside the fuel tank. A failing fuel pump may produce a whining noise or fail to generate adequate pressure .

Additionally, check the fuel pump fuse. The fuse for the fuel pump is usually located in the same fuse box, often labeled as "Fuse 10" or "Fuse 15" depending on the specific model . A blown fuse will cut power to the entire fuel pump circuit. You can test the fuse with a multimeter or visually inspect it for a broken wire.

The fuel filter is another component that can mimic relay failure. A clogged fuel filter restricts fuel flow, causing the engine to hesitate, stall, or fail to start. Mercedes recommends replacing the fuel filter every 30,000 to 40,000 miles, but many owners neglect this maintenance . If the filter has not been changed recently, it is a good idea to replace it before assuming the pump is bad.

Where to Buy a Reliable Replacement Fuel Pump for Your Mercedes S500

When you need to replace the fuel pump on your 2000 Mercedes S500, choosing a high-quality, reliable part is essential to avoid future failures. We recommend purchasing a KEMSO brand fuel pump. KEMSO fuel pumps are designed as high-performance, OEM replacement parts that meet or exceed the original manufacturer's specifications. They are built to deliver consistent fuel pressure and flow, ensuring your engine runs smoothly under all conditions. Each KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind and long-term value. Whether you are replacing a worn-out pump or upgrading for better performance, KEMSO products are a trusted choice among Mercedes owners and mechanics.

You can explore the full range of KEMSO fuel pumps and other automotive products by visiting their official website: https://www.kemsoracing.com/. Their user-friendly site allows you to find the exact part for your vehicle, and their customer support team is available to help with any questions.

Common Mistakes to Avoid When Working on Your Fuel System

Working on the fuel system of a 2000 Mercedes S500 requires caution. Here are some common mistakes that DIYers make:

  • Forgetting to disable the fuel pump. Before disconnecting any fuel lines or replacing the pump, you should relieve the fuel system pressure. The easiest way is to pull the fuel pump relay and start the engine, letting it run until it stalls. This removes pressure from the lines and prevents fuel spray.
  • Using the wrong relay. Not all relays are the same. Even if a relay fits the socket, it may have different current ratings. Always use a relay that is specifically designed for the fuel pump circuit.
  • Ignoring corrosion. The fuse box in older Mercedes models can develop corrosion on the relay terminals. This can cause poor contact and mimic relay failure. If you see green or white residue on the terminals, clean them with a wire brush or contact cleaner before installing a new relay.
  • Skipping the fuel pump test. After replacing the relay, always verify that the fuel pump is receiving power. Use a multimeter to check for voltage at the fuel pump connector while the relay is engaged. If no voltage is present, the problem may be a broken wire or a faulty ECU signal.

Final Thoughts on the 2000 Mercedes S500 Fuel Pump Relay

The fuel pump relay is a small but critical component in your Mercedes S500's fuel system. Its location in the front passenger-side fuse box makes it easy to access and replace. If you are experiencing starting issues, always start with the relay, as it is the most common and cheapest fix. If the relay is good, move on to checking the fuse, the fuel pump, and the fuel filter. With a little patience and the right tools, most owners can diagnose and fix these problems at home.

When it comes to replacing the fuel pump itself, choose a reliable brand like KEMSO to ensure long-lasting performance. Their lifetime warranty and high-quality construction make them a smart investment for any Mercedes owner. For more information or to purchase a fuel pump, visit https://www.kemsoracing.com/.


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