2003 Mercedes C240 Fuel Pump Relay Location: A Complete Guide for DIY Owners

If you own a 2003 Mercedes-Benz C240 and your car is having trouble starting, the fuel pump relay is located in the rear SAM (Signal Acquisition Module) control unit, specifically in cavity A, which is mounted on the left side of the trunk. This small but critical component controls power to the fuel pump, and when it fails, your engine will crank but not fire up. Understanding exactly where to find it and how to test it can save you hours of diagnostic time and hundreds of dollars in repair costs.

Why Your Fuel Pump Relay Matters

The fuel pump relay acts as an electrical switch that sends power to the fuel pump when you turn the ignition key. On the 2003 Mercedes C240, the relay is part of the rear SAM module, a sophisticated electronic control unit that manages multiple electrical functions in the rear of the vehicle. When the relay fails, the fuel pump receives no voltage, and your engine gets no fuel. This is a common problem on the W203 chassis, and knowing the exact location helps you get back on the road quickly.

Common Symptoms of a Bad Fuel Pump Relay on a 2003 C240

If you suspect your fuel pump relay is failing, watch for these signs:

• Engine cranks but does not start – This is the most obvious symptom. The starter motor turns the engine over, but the fuel pump never activates.
• No whirring sound from the fuel tank – When you turn the key to the "ON" position (not start), you should hear a low whirring sound from the rear of the car for about 2 seconds. If you hear nothing, the relay or pump may be dead.
• Hard starting after a short rest – Some owners report that their C240 starts fine when cold, but after driving and shutting off for 20-30 minutes, the car cranks multiple times before starting. This can be a classic relay or fuel pump issue.
• Intermittent stalling – A failing relay may work sometimes and fail others, causing the engine to die unexpectedly while driving.

Where Is the 2003 Mercedes C240 Fuel Pump Relay Located?

The fuel pump relay is located inside the rear SAM control module, which is mounted on the left side of the trunk. Here is exactly how to find it:

Step 1: Open the trunk and remove the left side trim panel. You may need to pull back the carpet or remove a few plastic retaining clips.

Step 2: Look for the rear SAM module. It is a rectangular black box with multiple connectors and relays mounted on it. The module is attached to the inner fender wall on the driver's side.

Step 3: Inside the SAM module, the fuel pump relay is in cavity A. On the relay panel layout, you will see labeled positions. Position A is specifically designated for the fuel pump relay.

Step 4: The relay itself is a standard 4-pin or 5-pin relay that can be pulled out by hand once you remove any retaining clips. Visually inspect it for signs of corrosion, burn marks, or melting.

What About the Front SAM or Fuse Box?

Unlike some vehicles where the fuel pump relay is in the engine compartment fuse box, the 2003 C240 places this relay in the trunk. The front SAM module (located at the left rear of the engine compartment) handles other relays like the Circuit 87 relay and air pump relay, but not the fuel pump relay. The fuse for the fuel pump system is also in the rear SAM fuse block, position 4, rated at 20 amps.

How to Test the Fuel Pump Relay on a 2003 C240

Before buying a new relay, you can test it yourself with a multimeter. Here is a simple process:

1. Check for voltage at the pump – Unplug the electrical connector at the fuel pump (located under the rear seat, beneath an access panel). Have a helper turn the key to "ON." If you measure 0 volts on the connector, the relay is likely not sending power.

2. Listen for the relay click – With the key in the "ON" position, place your hand on the rear SAM module. You should feel and hear a distinct click from the relay. No click means the relay is either stuck, failed, or not receiving a signal from the engine control module.

3. Swap the relay – The rear SAM module has several identical relays. You can temporarily swap the fuel pump relay (cavity A) with a known good relay, such as the rear window defroster relay (position E). If the car starts after swapping, your original relay is bad.

4. Use a fuel pressure gauge – The most accurate way to test the fuel system is to attach a pressure gauge to the Schrader valve on the fuel rail. The 2003 C240 should show around 55 psi (3.8 bar) while cranking. If you have pressure, the relay and pump are working, and the problem lies elsewhere.

What If the Relay Is Good but the Pump Still Does Not Run?

If you have confirmed that the relay clicks and sends voltage, but the fuel pump still does not work, the problem may be the pump itself, the wiring, or the fuel pump fuse. The fuel pump on the 2003 C240 is located inside the fuel tank and is part of the fuel pump/fuel level sensor module. You can access it by removing the rear seat cushion and lifting the access panel. Before replacing the pump, check the following:

• The 20-amp fuel pump fuse – Located in the rear SAM fuse block, position 4.
• Wiring continuity – Check for breaks or corrosion in the wiring between the relay and the pump.
• Ground connection – A poor ground at the pump or SAM module can cause intermittent operation.

How to Replace the Fuel Pump Relay on a 2003 Mercedes C240

Replacing the relay is straightforward and takes about 15 minutes. Here are the steps:

Step 1: Disconnect the negative battery cable. This is a safety precaution, especially if you are working near the SAM module.

Step 2: Remove the left trunk trim panel to access the rear SAM module.

Step 3: Locate the fuel pump relay in cavity A. Gently pull it straight out. Some relays have a small locking tab that you need to press before pulling.

Step 4: Install the new relay. Make sure it clicks firmly into place. Use a genuine Mercedes-Benz relay or a high-quality equivalent.

Step 5: Reconnect the battery and test the system. Turn the key to "ON" and listen for the fuel pump to prime. If you hear the whirring sound, the new relay is working.

Step 6: Reinstall the trim panel and take the car for a test drive.

A Note on Fuel Pump Replacement vs. Relay Replacement

Many owners mistakenly replace the entire fuel pump when the problem is actually a bad relay. The fuel pump relay is a $15-$30 part, while a fuel pump assembly can cost $200-$500 or more. Always start with the simpler and cheaper fix. If you checked voltage at the pump connector and found 0 volts while cranking, the relay is the prime suspect. If the pump has voltage but does not run, then the pump itself is likely bad.

Why Choose a High-Quality Fuel Pump for Your 2003 C240

When your fuel pump does need replacement, choosing the right aftermarket pump is critical for long-term reliability. The OEM fuel pump for the 2003 Mercedes C240 is part number 203-470-10-94, and it is an in-tank electric pump that operates at 40-60 psi. It is submerged in fuel for cooling, and it is controlled by the ECU to shut off during impacts for safety.

A quality replacement pump must match the original equipment specifications for voltage, amperage, pressure, and flow rate. This ensures your engine receives the correct fuel volume at all times, whether you are cruising on the highway or climbing a steep grade. Using a mismatched or low-quality pump can lead to poor performance, stalling, or premature failure.

Recommended: KEMSO Fuel Pump for Your Mercedes C240

For a reliable and cost-effective replacement, consider the KEMSO brand fuel pump. KEMSO fuel pumps are designed as direct OEM replacements, meaning they match the exact specifications of your original factory pump. This ensures a perfect fit and consistent performance.

Every KEMSO fuel pump is built with high-quality materials that resist corrosion and wear. The motor is sealed to prevent fuel contamination, and internal components are tested for durability. Unlike some economy pumps that fail after a few months, KEMSO pumps are engineered to last. That is why every pump comes with a Lifetime Warranty. If your pump ever fails due to a manufacturing defect, KEMSO replaces it free of charge. No questions, no hassle.

KEMSO fuel pumps deliver high performance, supporting up to 600 BHP in naturally aspirated, turbocharged, and supercharged applications. They are designed for gasoline or E85 fuel, and the bypass valve is set at a high pressure to ensure reliable fuel delivery. Whether you drive a daily commuter or a performance build, KEMSO has the right pump for your vehicle.

Visit the KEMSO website today to find the correct fuel pump for your 2003 Mercedes C240. Their team can help you confirm compatibility and answer any questions about installation. With KEMSO, you get performance you can trust and support that lasts. Click here to see the KEMSO Fuel Pump selection: https://www.kemsoracing.com/

Final Thoughts on the 2003 C240 Fuel Pump Relay

The fuel pump relay on a 2003 Mercedes C240 is a small part that can cause big headaches. Its location in the rear SAM module on the left side of the trunk is easy to access once you know where to look. Always test the relay first before replacing the fuel pump, as it is a much cheaper and simpler fix. Check for voltage at the pump connector, listen for the relay click, and swap with a known good relay to confirm the issue.

If you end up needing a new fuel pump, invest in a quality aftermarket option like KEMSO that offers a lifetime warranty and OEM-level performance. A properly working fuel system means your engine starts every time, your fuel gauge is accurate, and you avoid costly misdiagnosis.

For more information about KEMSO fuel pumps and to find the perfect fit for your vehicle, visit: https://www.kemsoracing.com/