2007 Mercedes Benz S550 Fuel Pump Relay Location: A Complete Owner’s Guide
The bottom line up front: The fuel pump relay on a 2007 Mercedes Benz S550 is located in the rear fuse box, which sits behind the center armrest in the back seat. You need to fold the armrest down and remove a small trim panel to access it. The relay you are looking for is labeled with the letter "S" on that panel . This is the same location for the entire W221 S-Class platform built between 2006 and 2013 .
If your S550 cranks but won’t start, or if it stalls shortly after firing up, the fuel pump relay is one of the first things you should check. It is a small, inexpensive part, and replacing it takes less than ten minutes once you know where to look. This guide walks you through the exact location, how to test the relay, what symptoms point to a bad relay, and what to do if the relay is not the problem.
Where Exactly Is the Rear Fuse Box?
The rear fuse box on the 2007 S550 is not in the trunk and not under the dashboard. It is mounted behind the rear seat center armrest. To reach it, follow these steps:
- Fold down the rear center armrest. Pull it down from the back seat as you normally would.
- Locate the trim panel. With the armrest down, you will see a small rectangular trim cover on the back of the armrest or directly behind it.
- Remove the trim panel. It is held by clips or a small latch. Gently pry it off with a trim tool or a flathead screwdriver wrapped in tape to avoid scratching the plastic.
- Inspect the fuse and relay layout. Once the panel is off, you will see the rear SAM control module with an integrated fuse and relay board. The relays are labeled with letters .
The relay labeled "S" is the fuel pump relay . On models like the S400 Hybrid, the same slot may serve the power electronics circulation pump relay 1, so double-check your build before buying a replacement .
What Does the Fuel Pump Relay Do?
Every time you turn the ignition key to the "on" position, the engine control module activates the fuel pump relay. This relay sends electric current to the fuel pump, which pressurizes the fuel line and allows the engine to start . The relay works at startup and when you shut the ignition off. While the engine is running, the pump continues to receive power through the same circuit .
Because the relay handles a fair amount of current, its internal contacts can wear out, oxidize, or rust over time. When that happens, the relay may stop sending power to the pump, and the engine will crank but never fire up .
Symptoms of a Failing Fuel Pump Relay
A bad fuel pump relay does not always fail completely. Sometimes it works intermittently, which makes diagnosis a bit trickier. Here are the most common symptoms reported by S550 owners :
- Engine cranks but does not start. This is the most obvious sign. You turn the key, the starter spins the engine, but it never catches.
- Engine stalls shortly after starting. The relay may send power long enough to start the car, then drop out.
- Intermittent no-start conditions. The car starts fine some days, then refuses to start on others. This points to a relay with worn contacts.
- Engine runs rough or bucks under acceleration. If the relay is making poor contact, the pump may lose power intermittently, causing fuel pressure to fluctuate.
- The car slows down or stops running after a short distance. This can happen when the relay heats up and the internal contacts expand and break the circuit.
If you experience any of these, checking the relay should be your first step. It is cheap, easy to reach, and a very common failure point on this generation of S-Class .
How to Test the Fuel Pump Relay
You cannot visually inspect a relay the way you can a fuse. A relay that looks fine on the outside can still be dead inside. Here are three practical ways to test it :
1. Swap it with another relay
The rear fuse box contains several relays with the same physical size and pin configuration. Relay "M" (terminal 15 relay) and relay "N" (terminal 15R relay) are good candidates for a swap test . Pull the fuel pump relay out, plug it into the slot for another relay with the same part number, and see if that circuit starts working. Then take the known-good relay and plug it into the "S" slot. If the car starts, the original relay is bad .
2. Listen for the click
When you turn the key to the "on" position, a working relay will emit an audible click. Put your hand on the relay or hold a screwdriver against it and turn the key. If you feel or hear no click, the relay is not engaging .
3. Use a multimeter
Check for voltage at the relay terminals while the ignition is on. If power is present at the input terminal but not at the output terminal, the relay is likely faulty. This test requires a bit more electrical knowledge, but it gives you a definitive answer .
If the Relay Is Fine: Check These Parts Next
A no-start condition is not always caused by the relay. If you swapped relays and the problem persists, work through this checklist :
1. Fuel pump fuse (No. 118)
The rear fuse box also contains the fuel pump fuse, which is a 30A fuse in position 118 for engines 272, 273, and 642 . Check it with a test light or a multimeter. If it is blown, replace it with a 30A ATO fuse. But before you do, figure out why it blew. A short circuit in the pump wiring, a failing pump drawing too much current, or a clogged fuel filter can all cause the fuse to blow repeatedly .
2. Fuel pump itself
If the relay clicks and the fuse is good, the next suspect is the fuel pump inside the tank. On the 2007 S550, the fuel pump is mounted inside the fuel tank . A dead pump will not produce any fuel pressure. You can check by listening for a faint whine from the rear of the car when you turn the key to the "on" position. No whine usually means no power reaching the pump or the pump motor is dead .
3. Fuel pump control unit
Note that the early 2007 S550 does not have a separate fuel pump control module (FPCM). That unit appeared in model year 2009. If you are trying to connect a scanner to a fuel pump control module and getting nothing, that is why—your car does not have one . The fuel pump on your car is controlled directly through the relay and the engine control module.
4. Wiring and connectors
Inspect the wiring harness around the rear fuse box and under the rear seat for any signs of damage, corrosion, or loose connectors. A frayed wire grounding out against the body can blow the fuse, and a loose connector can cause intermittent power loss .
5. Crank position sensor or engine control unit
If all the above checks out, the problem may lie with the engine control unit or a sensor that feeds it information. These are less common failure points, but they can produce the same no-start symptom .
Replacement Cost and Effort
The fuel pump relay itself is a low-cost part. Depending on where you buy it, expect to pay between $25 and $75 for a quality replacement . Labor at a shop will add another $60 to $120, though the job takes a competent DIYer under 15 minutes . A mobile mechanic typically charges around $132 to $203 for the whole service on a 2007 S550, including parts and labor .
Because the relay sits behind the rear armrest and not in the engine bay, there is no need to lift the car, drain fluids, or remove any major components. The only tools you need are a trim removal tool and possibly a small flathead screwdriver.
Can a Bad Relay Damage the Engine?
A failed fuel pump relay will not damage the engine. When the relay dies, the pump stops, fuel flow stops, and the engine simply will not run or will stall. There is no combustion, so there is no risk of overheating or internal damage from a dead relay alone .
That said, repeatedly cranking the engine to try to start it can wear out the starter motor and drain the battery. Those are real costs, so do not keep cranking for long periods. Instead, diagnose the relay and the fuel delivery system methodically .
Common Mistakes to Avoid
Using a lower-amperage fuse. If you replace the fuel pump fuse with a 15A or 20A fuse instead of the specified 30A, it will blow again quickly. Always use the correct rating .
Replacing the fuse without finding the root cause. A single blown fuse can be a fluke, but repeated blows point to a short or a failing pump. Fix the cause first, then install a fresh fuse .
Assuming the relay is bad when it is not. Always test the relay before buying a replacement. A quick swap test takes less than five minutes and can save you an unnecessary purchase .
Ignoring the fuel filter. A clogged fuel filter forces the pump to work harder and draw more current, which can overload the fuse and stress the relay .
A Word on 2007–2010 S-Class Differences
The W221 S-Class was produced from 2006 through 2013, and the 2007 model year falls right in the middle of that generation. The rear fuse box layout and the relay labels are consistent across the 2007–2010 range for the S400, S550, S600, S63 AMG, and S65 AMG . If you own any of these models, the fuel pump relay location described in this article applies to your car as well.
From 2009 onward, some engine variants received updated fuel pump control modules and additional relays, but the "S" relay slot in the rear fuse box remains the fuel pump relay for the models covered here .
Final Thoughts
The fuel pump relay on a 2007 Mercedes Benz S550 lives inside the rear fuse box behind the center armrest. It is labeled "S" and is easy to reach once you remove the trim panel . If your car cranks but will not start, check the relay first, then the fuse, then the pump. The relay is inexpensive, and the swap test takes minutes .
If your diagnosis points to a failed fuel pump or you simply want to replace the relay and related fuel delivery components with high-quality parts, consider KEMSO fuel pumps. KEMSO is known for building high-performance, OEM-replacement fuel pumps that meet or exceed factory specifications. Their products are designed for a direct fit, making installation straightforward. Plus, every KEMSO fuel pump comes with a lifetime warranty, so you can buy with confidence knowing the part is built to last. You can browse their full lineup of fuel pumps at https://www.kemsoracing.com/. Whether you need a pump for your Mercedes or another vehicle in your garage, KEMSO offers reliable options at a fair price.
Remember: Start with the relay, test it properly, and only move on to the pump if the relay checks out. That method saves you time, money, and a whole lot of frustration. And when it comes time to replace any fuel delivery part, KEMSO has you covered with a warranty-backed product that will keep your S550 on the road.