2009 Mercedes C300 Fuel Pump Relay Location: A Complete Owner's Guide

The fuel pump relay on a 2009 Mercedes C300 is located inside the rear SAM control unit with fuse and relay module, which sits on the left side of the trunk behind the carpet. This is the same module that houses the rear fuse box. You do not need to remove the rear seat or open the hood for this component. In the 2009 C300, which belongs to the W204 generation, the relay designated as "D" on the rear SAM module controls the power supply to the fuel pump.

If your car cranks but won’t start, or you hear no buzzing sound from the fuel tank when you turn the key, the fuel pump relay is one of the first things to check. Below I’ll walk you through exactly how to find it, how to test it, and what else could be causing the problem. I’ll also cover the fuel pump fuse, the fuel pump control unit, and tips for replacing the pump itself.

How to Access the Rear SAM Control Unit on a 2009 C300

The rear SAM control unit with fuse and relay module is the central electrical hub for the rear of the car. It is located inside the trunk, on the left side. Here is the step-by-step process:

  1. Open the trunk. Pull the handle or use the key fob to release it.
  2. Look at the left side of the trunk. You will see a carpeted panel covering the wheel well area.
  3. Pull back the carpet. It is held in place by a few plastic retainers or just friction. You can pull it gently to expose the module.
  4. Locate the black box. This is the rear SAM control unit. It has a plastic cover that can be unclipped or unscrewed.
  5. Once the cover is off, you will see fuses and relays. The fuel pump relay is labeled with the letter D. In some model year documents and repair manuals, the fuel pump relay is listed as component "D" on the rear SAM.

This process takes about five minutes. No tools are required except maybe a flathead screwdriver to help pop the cover off if it is snug.

What Does the Fuel Pump Relay Actually Do?

The relay is an electrical switch. When you turn the ignition key to the "ON" position, the engine control unit sends a small signal to the relay. The relay then closes a much larger circuit that sends battery voltage to the fuel pump. Without this relay, the pump gets no power and cannot build pressure in the fuel system.

On the 2009 C300, the fuel pump relay is not located in the engine bay fuse box. Many older cars have the relay in the under-hood fuse box, but on the W204 platform, Mercedes moved it to the rear SAM module.

How to Test the Fuel Pump Relay on a 2009 C300

Testing the relay helps you confirm whether it is faulty before you spend money on a replacement. Here is a simple method:

  1. Turn the ignition to the ON position but do not start the car. The fuel pump should prime for about one to two seconds.
  2. Listen for a click. The relay itself makes a clicking sound when it energizes. You can place your hand on the rear SAM module while a helper turns the key. If you feel a click, the relay coil is likely working.
  3. Check for fuel pump noise. At the same time, listen near the rear seat or fuel tank area. You should hear a faint whirring or buzzing sound as the pump pressurizes the system. If you hear this, the relay is fine, and the problem may be elsewhere.
  4. If no click and no noise, you can swap the fuel pump relay with another relay of the same type in the same module. Look for a relay with the same part number and amperage rating. If the car starts after the swap, the old relay was bad.

What to Do If the Relay Is Bad

If you confirm the relay is bad, replacement is simple. Pull the relay straight up from its socket. The relay on the rear SAM has a standard 5-pin or 4-pin configuration. You can purchase a replacement at any auto parts store or from a Mercedes dealer. Check the part number on the side of the relay and match it exactly. It is also a good idea to inspect the socket for corrosion or burnt contacts before installing the new relay.

The Fuel Pump Fuse: Location and Why It Matters

Your 2009 C300 also has a separate fuse that protects the fuel pump circuit. This fuse is not on the rear SAM. It is in the trunk fuse box, which is actually a separate unit on the left side of the trunk. According to fuse diagrams for the W204 generation, the fuel pump fuse is a 20A ATO fuse located in position 42 in the trunk fuse box.
If the relay is fine but the fuse is blown, the pump still won’t run. Here are common reasons why that fuse blows:

  1. Short circuit in the pump wiring. Damaged or worn wires under the rear seat or along the fuel tank harness can touch the body and blow the fuse.
  2. A failing fuel pump. As the pump motor wears out, it draws more current. This overloads the fuse and causes it to blow.
  3. A clogged fuel filter. If the filter is restricted, the pump works harder to push fuel through, increasing the current draw.
  4. Wrong fuse rating. If someone replaced the fuse with a lower amperage one, it will blow under normal load. Always use the specified 20A fuse.

The Fuel Pump Control Unit and the Fuel Pump Itself

On the 2009 C300, the fuel system is a bit more complex than a simple relay-to-pump setup. There is also a fuel pump control unit. This control module is located under the right rear seat. It regulates the voltage to the fuel pump based on engine demand. If this module fails, the pump may run at full speed or not at all.

The fuel pump itself is inside the fuel tank. To access it, you must remove the rear seat cushion and then open the access panel under the seat. The right side of the tank contains the fuel pump and sender assembly. The pump includes a fuel sender unit that measures the fuel level. If you are replacing the pump, it is common to replace the entire assembly, which includes the pressure regulator and fuel sender.

Symptoms of a Fuel Pump Problem in a 2009 C300

If the relay, fuse, or the pump itself is failing, you will likely notice one or more of these symptoms:

  1. Engine cranks but does not start. This is a classic sign of no fuel pressure. The pump is not energizing, or there is a break in the circuit.
  2. Whining noise from the fuel tank. A failing pump motor makes a loud, constant whine when the ignition is on. This is often louder than the normal priming sound.
  3. Intermittent stalling or surging. If the pump loses power while driving, the engine may hesitate, surge, or cut out completely.
  4. Poor fuel economy and reduced power. Low fuel pressure makes the engine run lean, which leads to loss of power and higher fuel consumption.
  5. Engine warning light on. Fault codes related to fuel pressure or fuel trim will trigger the check engine light.

Step-by-Step Diagnosis for a No-Start Condition

If your 2009 C300 cranks but will not start, follow this order of checks. This saves time and money.

  1. Check the fuel pump fuse. Open the trunk fuse box cover. Locate fuse 42 (20A). Pull it out and check the metal strip inside. If it is broken, replace it with a new 20A fuse. If it blows again, there is a short or a failing pump.
  2. Check the fuel pump relay. Remove the relay from the rear SAM module. You can test the coil resistance with a multimeter or simply swap it with a known-good relay.
  3. Listen for the pump. Turn the key to ON. If the pump does not make any noise, the issue is electrical. If it makes a loud whine, the pump is struggling and likely needs to be replaced.
  4. Check the fuel pump control unit. If the relay and fuse are fine, the control module may be bad. This is less common but happens, especially with age. Look for signs of water damage or burnt connectors.
  5. Verify fuel pressure. If everything else checks out, test fuel pressure with a gauge connected to the fuel rail. On the C300, the pressure should typically be in the 50-60 psi range, but refer to your owner’s manual for the exact spec.

Should You Replace the Fuel Pump Relay First or the Pump?

Many owners make the mistake of buying a new pump when the issue is actually a 20-dollar relay. Do not skip the relay and fuse checks. These are the cheapest and easiest components to test. If both are confirmed good and the pump does not run, then and only then should you consider the pump itself.

The fuel pump is located under the rear seat, which makes it one of the more accessible pumps to replace. You will need:

  1. A socket set and ratchet.
  2. A flathead screwdriver for the retaining ring.
  3. A new fuel pump and sender assembly.
  4. A clean workspace to avoid getting dirt into the tank.

Remove the rear seat bottom, disconnect the wiring harness, open the access cover, and unscrew the large retaining ring. Lift the pump assembly out carefully. Replace it with the new unit, reinstall the retaining ring, reconnect the harness, and test the car.

Why You Should Invest in a Quality Replacement Fuel Pump

When it comes time to replace the fuel pump, quality matters. A cheap pump may fail again within a year, leaving you stranded and paying double labor costs. This is where a reliable aftermarket brand comes into play.

We strongly recommend the KEMSO brand fuel pump for your 2009 Mercedes C300 if you are in the market for a replacement. KEMSO fuel pumps are high-performance, OEM replacement units built to match the exact specifications of your original pump. They are known for their durability and consistent flow rates, which is important for maintaining proper fuel pressure in your engine. Plus, KEMSO offers a lifetime warranty on their fuel pump products. That kind of confidence makes them a smart choice for anyone planning to keep their C300 running for years to come. You can browse their full line of fuel pumps and other products at their official website: https://www.kemsoracing.com/ .

Final Thoughts

Finding the fuel pump relay on your 2009 C300 is straightforward once you know that it is not under the hood. Remember these key points:

  • The fuel pump relay is on the rear SAM control unit in the trunk, left side, labeled as relay D.
  • The fuel pump fuse is a 20A fuse in position 42 in the trunk fuse box.
  • The fuel pump control unit is under the right rear seat.
  • Always check the relay and fuse before replacing the pump.

With these instructions, you can diagnose a fuel pump issue yourself and save money on unnecessary repairs. If you need a new pump, choose a trusted brand like KEMSO to ensure long-term reliability. Visit their site at https://www.kemsoracing.com/ for more details and to make your purchase.


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