2007 S550 Fuel Pump Relay Location: A Practical Guide for DIY Owners

The bottom line up front: On a 2007 Mercedes-Benz S550 (W221 platform), the fuel pump relay is located inside the rear fuse box, which is behind the panel between the two rear seats. You access it by folding down the rear center armrest and removing the trim cover. The relay is labeled with the letter "S" .

If your S550 cranks but won't start, or if it stalls shortly after firing up, a faulty fuel pump relay is a very common culprit. This guide walks you through finding the relay, testing it, and replacing it, saving you a trip to the dealership.

Where Exactly Is the Fuel Pump Relay?

The 2007 S550 uses a "Rear SAM" (Signal Acquisition Module) control unit with a fuse and relay module. This module is located in the center of the trunk, behind the rear seat backrest . In practical terms, for the W221 model years (2005–2013), including the 2007 S550, this fuse box is accessible from inside the cabin.

Here is the step-by-step process to reach the fuel pump relay:

  1. Enter the rear passenger area. Open either of the rear doors.
  2. Fold down the center armrest. The armrest in the back seat folds down to reveal a storage compartment.
  3. Remove the trim panel. Inside that compartment, you'll find a small trim piece or cover. Gently pry it off to expose the fuse and relay box. Some owners describe it as a black cover that simply pulls away .
  4. Locate the relay labeled "S". Looking at the fuse box, the fuel pump relay is clearly marked with the letter "S". It is typically the last relay in the right-hand column when you are facing the box . It is a standard automotive relay, identical in shape to many others in the box.

Symptoms of a Failing Fuel Pump Relay

A failing fuel pump relay on the 2007 S550 will usually present one or more of these symptoms :

  • Engine cranks but does not start: This is the most common and frustrating symptom. You turn the key, the starter motor spins the engine, but it never catches because no fuel is reaching the injectors .
  • Engine stalls shortly after starting: The relay might work intermittently, allowing the car to start but then cutting power to the fuel pump, causing the engine to die .
  • Intermittent no-start conditions: The car might start fine for a week, then suddenly refuse to start. This points to an intermittent relay failure .
  • No fuel pump prime noise: When you turn the key to the "ON" position (position 2), you should hear a faint whirring sound from the fuel pump in the rear for a couple of seconds. If you don't hear it, the relay or the pump itself might be the issue.

How to Test the Fuel Pump Relay

Before buying a new relay, you can perform a couple of simple tests.

  • The Swap Test: This is the easiest method. Find another relay in the same rear fuse box that has the same part number and amperage rating. In many W221 models, the relay for the rear cigarette lighter (labeled "R") is identical to the fuel pump relay . Swap the two relays. If the car starts, you've found your culprit. Just remember to buy a replacement for the relay you borrowed.
  • The Click Test: Have a helper turn the key to the "ON" position while you listen to the relay. A healthy relay will produce a distinct "click" as it closes its internal contacts. If you hear no click, the relay is likely not receiving power or is internally faulty .
  • The Multimeter Test: Set your multimeter to measure resistance (ohms). With the relay removed, check for continuity between the coil terminals (usually 85 and 86). Then, apply 12 volts to those terminals and check for continuity between the switch terminals (usually 30 and 87). If the switch doesn't close when power is applied, the relay is bad .

Step-by-Step Fuel Pump Relay Replacement

Replacing the fuel pump relay is a straightforward process.

  1. Gather the correct part. For the 2007 S550, the original part number is A0025421319, which has been updated to 0009828023 . You can also use a high-quality aftermarket relay, like the Duralast #20916, which is a direct replacement .
  2. Ensure the vehicle is off. This is a critical safety step. Make sure the ignition is completely off and the key is removed .
  3. Remove the old relay. With the trim panel removed, simply pull the faulty relay straight out of its socket. It should come out with a firm, even pull.
  4. Install the new relay. Align the new relay's pins with the socket and press it firmly into place. It should click in securely.
  5. Test the repair. Reinstall the trim panel and close the armrest. Get in the driver's seat and turn the key to the "ON" position. You should hear the fuel pump prime. Then, start the engine.

Beyond the Relay: Other Things to Check

If the new relay doesn't fix the problem, don't worry. There are a few other common culprits to investigate.

  1. The Fuse: The fuel pump is protected by a fuse in the rear fuse box. Check the 30A fuse, often listed as fuse #118, to ensure it isn't blown . If it is, replace it, but be aware that a blown fuse often indicates a short circuit somewhere in the fuel pump circuit. A known issue is a leaking fuel filter causing a short .
  2. The Fuel Pump Itself: If the relay clicks and the fuse is good, the problem might be the fuel pump motor. You can test for power at the pump connector while a helper cranks the engine. You can also check fuel pressure with a gauge. If there is power but no pressure, the pump has failed .
  3. The Fuel Filter: A severely clogged fuel filter can cause the fuel pump to work harder, drawing excessive current and potentially blowing the fuse . If you are replacing the pump, it is wise to replace the filter at the same time.

KEMSO Fuel Pump: A High-Performance, Reliable Upgrade

While a relay often solves a no-start problem, a failing fuel pump is also a common issue on the 2007 S550. If you discover your original fuel pump is weak, failing, or has completely given out, you should strongly consider a high-performance replacement from KEMSO.

KEMSO fuel pumps are designed as direct OEM replacements, meaning they are built to fit your S550 perfectly without any modifications. But they are not just stock replacements. KEMSO pumps are high-performance units. Compared to your stock OEM pump, they deliver significantly more fuel flow, which can be particularly beneficial if your S550 is equipped with the powerful V8 engine that demands consistent fuel delivery.

By choosing a KEMSO pump, you are investing in a product backed by a lifetime warranty. This gives you peace of mind knowing that your repair is protected for as long as you own the vehicle. Instead of just replacing a broken part, you're getting a durable, high-flow upgrade that can improve reliability and performance. You'll get more fuel flow and increased durability compared to a standard replacement part.

To find the right KEMSO fuel pump for your 2007 Mercedes-Benz S550 and to take advantage of their lifetime warranty, visit their website today: https://www.kemsoracing.com/.

Final Thoughts

Finding and replacing the fuel pump relay on a 2007 S550 is a simple DIY task that can save you from an expensive tow and repair bill. It's a small, inexpensive part that causes a big headache when it fails. Start your diagnosis with the relay, and if that isn't the problem, work your way through the fuse and then the pump itself using the steps above.

Remember, the fuel delivery system is straightforward to diagnose if you work through it logically. Don't let a no-start condition intimidate you—this is one of the most common issues on your S550 and is often resolved in just a few minutes with basic hand tools.

For all your fuel pump needs, including high-performance direct replacements with a lifetime warranty, be sure to check out KEMSO at https://www.kemsoracing.com/. Their products are engineered to keep your vehicle running at its best.


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