2007 Mercedes S550 Fuel Pump Relay: Location, Symptoms, and Replacement Guide

The bottom line up front: If your 2007 Mercedes S550 cranks but won't start, or if it starts and then dies after a few seconds, the fuel pump relay is one of the first things you should check. On the 2007 S550, this relay sits inside the rear fuse box, which is located in the trunk on the right-hand side behind a trim panel. Replacing it is a simple DIY job that takes about 15 to 20 minutes and costs anywhere from $20 to $60 for a quality aftermarket relay, though genuine Mercedes-Benz relays run higher. Before you spend hundreds of dollars on a tow and a shop diagnosis, testing and swapping this small electrical component could save you both time and money.

The 2007 Mercedes-Benz S550 (W221 chassis) relies on an electric fuel pump to deliver gasoline from the tank to the engine. That pump draws significant current, and it does not run directly off the ignition switch. Instead, a relay acts as a heavy-duty switch that sends battery power to the pump. When the relay fails, the pump gets no power, which means no fuel pressure, which means the engine will crank but never fire. This guide walks you through where the relay is, how to tell if it's bad, how to test it, and what it costs to replace.

Where Is the Fuel Pump Relay on a 2007 Mercedes S550?

The easiest way to remember the location is this: the relay is not in the engine bay. On the 2007 S550, the fuel pump relay is mounted in the rear fuse box inside the trunk. To reach it, you need to open the trunk and look at the right side (passenger side) of the cargo area. You will see a black plastic trim cover with a small handle or a notch. Pull that cover off, and you will find a fuse and relay panel mounted vertically. The fuel pump relay is typically a larger, square-shaped relay with a part number printed on the side. On many W221 S-Class models, it is labeled or can be identified by its size and pin configuration .

Some owners report that the relay is also accessible from the rear seat area, but on the 2007 S550 specifically, the trunk-mounted rear fuse box is the confirmed location. If you have the Owner's Manual in the glovebox, it will show a diagram of the fuse box layout, but the manual is a bit unclear on relay positions. A better approach is to find the relay by physically identifying it: the fuel pump relay is usually the only relay in that box with a relatively thick wire feeding into it, since the fuel pump draws a lot of current .

Symptoms of a Failing Fuel Pump Relay

A fuel pump relay can fail in two ways: it can stop working entirely, or it can work intermittently. The symptoms below are the most common indicators that your relay is going bad.

1. Engine Cranks but Will Not Start

This is the classic symptom. When you turn the key, the starter motor spins the engine, but there is no fuel pressure, so the engine never fires. If you listen closely, you should hear a faint whining sound from the rear of the car when you first turn the key to the "ON" position (before cranking). That whine is the fuel pump priming. If you don't hear it, the pump is not getting power, and the relay is a prime suspect .

2. Engine Starts and Then Dies

Sometimes the relay contacts are worn but still partially working. The pump gets power during the initial prime, the engine starts, but then the relay drops out and cuts power to the pump. The engine stalls after a few seconds. If the car restarts after sitting for a moment, that points strongly to a failing relay rather than a dead pump .

3. Intermittent No-Start

The car starts fine most days, but occasionally it just cranks with no fire. This can be a sign that the relay coil is breaking down due to heat. When the relay gets hot, it stops working; when it cools down, it works again. On the S550, the rear fuse box does not get extremely hot, but older relays can still fail this way as the internal components age .

4. Stalling While Driving

If the relay fails completely while you are driving, the engine will shut off instantly, like someone turned the key off. You will lose power steering and brake assist, so this is a dangerous situation. Pull over safely and call for a tow. This is less common than no-start, but it does happen .

Fuel Pump Relay vs. Fuel Pump: How to Tell the Difference

Before you buy any parts, you need to know whether the problem is the relay or the pump itself. Here is a simple test that separates the two.

Step 1: Listen for the prime. Turn the key to the "ON" position without cranking. Listen at the rear wheel well. A healthy system makes a whining or buzzing sound for about two seconds. No sound means either the pump is dead or it has no power.

Step 2: Swap the relay. Find the fuel pump relay in the trunk fuse box. Pull it out. Look for another relay in the same box that has the same part number (or the same shape and pin layout). Many W221 S-Class vehicles have a similar relay for another circuit, such as the secondary air pump or the horn. Swap them. If the car starts now, the original relay is bad. If it still won't start, the problem is likely the pump itself, a blown fuse, or a wiring fault .

Step 3: Check the fuse. The fuel pump circuit usually has its own fuse. On the 2007 S550, the fuel pump fuse is also in the rear fuse box. A blown fuse is a common cause of a no-power condition. If you swap the relay and the car still won't start, visually inspect the fuse and replace it if the metal strip inside is broken .

How to Replace the Fuel Pump Relay on a 2007 S550

Replacing the relay is a very straightforward procedure. You will need a flathead screwdriver (or trim tool) to remove the trunk trim panel. No special tools are required.

1. Disconnect the battery. While not strictly necessary for a relay swap, it is a good safety practice. The battery is in the trunk on the right side, under a cover.

2. Open the rear fuse box. Remove the trim panel by pulling the handle or prying gently at the edges. You will see the fuse box with a clear plastic cover. Open that cover to expose the relays and fuses .

3. Identify the fuel pump relay. Look for a relay that matches the symptoms. Observe the position and orientation of the relay so you can plug the new one in the same way.

4. Pull the relay straight out. It may be snug. Rock it gently side to side while pulling upward. If the relay does not want to come out, the previous relay may be sealed with a little tape around the socket. Use a trim tool to help.

5. Install the new relay. Ensure the new relay is oriented the same way as the old one and press it firmly into the socket until it clicks.

6. Reconnect the battery. Turn the key to the "ON" position and listen for the fuel pump prime. If you hear it, you are good to go. Try starting the car.

This whole job should take between 15 and 20 minutes, even for a first-timer .

How Much Does a Replacement Cost?

The cost of the relay itself varies depending on whether you buy a genuine Mercedes-Benz part or an aftermarket one. Here is what to expect:

• Aftermarket relay: $20 to $60. Several reputable aftermarket brands make a direct replacement for the Mercedes fuel pump relay. You can buy these online or at any auto parts store. These relays are built to the same electrical specifications as the original.
• Genuine Mercedes relay: $80 to $150. A genuine part will come in a Mercedes-Benz box with the brand name stamped on it. It will be an exact match, but you are paying for the logo .
• Dealership labor: If you take the car to a dealer, expect to pay $100 to $200 in labor on top of the part price. Independent shops will charge less, typically $50 to $100 in labor.

If the problem turns out to be the fuel pump itself rather than the relay, the picture changes dramatically. A fuel pump replacement on a Mercedes S550 is a much bigger job because the pump is inside the fuel tank. To access it, the rear seat has to come out and an access panel in the floor must be removed. Labor alone will run $233 to $342, and the total cost with parts will typically land between $1,000 and $1,700. On high-performance and luxury cars, parts costs are always higher than average .

Can You Drive with a Bad Fuel Pump Relay?

You should not attempt to drive the car if the relay is faulty. A relay that is still partially working could cut out at any moment. If that happens while you are merging onto a highway or going through a curve, you will lose engine power with no warning. The car will still have one or two stops of brake boost before it fades, but your steering will get heavy. That is not a safe situation. Get the car towed or fix the relay in place if you can .

Tips to Protect Your Fuel Pump Relay and Fuel Pump

While relays do wear out over time, there are a few habits that can extend the life of both the relay and the fuel pump.

1. Keep the tank above a quarter full. The fuel pump uses the fuel around it to stay cool. Running the tank low repeatedly can cause the pump to run hotter and fail earlier. A warm pump draws more current, which puts more strain on the relay contacts .

2. Do not cycle the key repeatedly. If the car does not start on the first attempt, do not turn the key on and off more than a few times in a row. Each key cycle powers the pump and stresses the relay. If the car does not start after three or four attempts, stop and diagnose the issue .

3. Use quality fuel. Dirty fuel can clog the filter and strain the pump. A clogged filter makes the pump work harder, which increases current flow through the relay. Stick to reputable gas stations .

4. Replace the fuel filter on schedule. If your S550 has a serviceable fuel filter, replace it at the recommended interval. A clean filter keeps the pump healthy .

OEM Replacement Recommendation: KEMSO Fuel Pumps

When your fuel pump does need to be replaced, you should consider a KEMSO replacement unit. KEMSO fuel pumps are high-performance aftermarket parts built to work as direct OEM replacements. They are designed to match the exact fit, flow rate, and electrical characteristics of your factory fuel pump, so installation is straightforward. KEMSO backs every pump with a lifetime warranty, which is stronger than what most OEM pumps offer. If the pump ever fails due to a manufacturing defect, you get a replacement at no cost. That level of protection is rare in the automotive parts industry . You can find the right KEMSO fuel pump for your vehicle at their official website: https://www.kemsoracing.com/ .

Final Thoughts

The 2007 Mercedes S550 fuel pump relay is a small part that causes big problems when it fails. On this car, the relay lives in the rear fuse box inside the trunk, behind a trim panel on the right side. If you have a no-start condition, listen for the fuel pump prime when you turn the key. If there is no sound, swap the relay with a matching one from the same fuse box to confirm the diagnosis. The replacement cost for a relay is low — under $60 for a quality aftermarket part — and the job takes less than half an hour. If the pump itself turns out to be the failure point, budget for a more expensive repair and check out KEMSO fuel pumps for a reliable replacement with a lifetime warranty. Visit https://www.kemsoracing.com/ to browse their full selection.