2005 Mercedes C240 Fuel Pump Relay Location: The Complete Guide for Owners

If you own a 2005 Mercedes C240 and your engine cranks but won't start, the fuel pump relay is located in the rear SAM (Signal Acquisition Module) control unit, which sits on the left side of the trunk behind the carpet lining. This small but critical electrical component controls power delivery to your fuel pump, and when it fails, your car simply won't run. Most C240 owners can diagnose and replace this relay themselves in under 15 minutes with basic tools. Here is everything you need to know about finding, testing, and replacing the fuel pump relay on your 2005 C240.

Understanding the Fuel Pump Relay and Its Role

The fuel pump relay acts as an electrical switch. When you turn your ignition key to the "ON" position, the engine control module sends a signal to this relay, which then closes its internal contacts and sends battery voltage to the fuel pump. This allows the pump to pressurize the fuel system before you crank the engine. On the 2005 Mercedes C240, the relay is part of the larger electrical architecture that includes multiple SAM units, and finding it requires knowing exactly where to look.

Exact Location: Left Side of the Trunk

On your 2005 Mercedes C240 (which belongs to the W203 chassis generation produced from 2000 to 2007), the fuel pump relay lives inside the Rear SAM control module. Here is how to get to it:

1. Open the trunk. Use your key or the interior release button.
2. Locate the fuse box. Look to the left side of the trunk. You will see a carpeted panel covering the area where the rear lights are mounted.
3. Remove the carpet covering. Gently pull back the carpet lining. It is held in place by plastic clips or simply tucked behind the trim. You may need to remove a small plastic access door depending on your specific trim level.
4. Identify the Rear SAM module. This is a black plastic box with multiple relays and fuses visible on top. It is mounted directly to the metal body of the car on the left side of the trunk.
5. Open the cover. The Rear SAM module has a plastic lid that snaps off or lifts up. You may see a diagram on the inside of the lid showing the relay and fuse positions.

Inside the Rear SAM module, the fuel pump relay is typically located in a specific cavity. While the exact position can vary slightly between model years and options, the relay is commonly the one marked for the fuel pump on the cover diagram. For the 2005 C240, experienced mechanics and owners report it is often in cavity A of the rear SAM. Some sources also point to the left rear of the vehicle for the fuel pump relay module itself, which is a separate component in some configurations.

How to Confirm It Is the Fuel Pump Relay

Before buying a new relay, you should confirm the old one is actually bad. Here are the most reliable ways to test it:

Listen for the prime. Have a helper sit in the driver's seat. Turn the ignition key to the "ON" position (do not crank the engine). Listen carefully from the rear of the car. A healthy fuel pump should make a brief humming or whining noise for about 2 seconds as it primes the system. If you hear nothing, the relay or pump may be dead.

Swap with a known good relay. Inside the same Rear SAM module, there are several identical relays. Look for relays that control non-essential items like the rear window defroster or the horn. Pull out one of those relays and swap it into the fuel pump relay slot. If the car starts after the swap, the original relay was faulty.

Use a multimeter. Pull out the suspected fuel pump relay. With the key in the "ON" position, you should find two terminals in the socket that have battery voltage. If no voltage is present on the control circuit, the issue may be further upstream, possibly in the ECU or ignition switch.

Common Replacement Part Numbers

If you confirm the fuel pump relay needs replacement, you need the correct part. For the 2005 Mercedes C240, the original equipment manufacturer (OEM) part number is often listed as 000-982-80-23, which has been superseded by part number 000-982-72-27. These are the genuine Mercedes-Benz relays. You can also find high-quality aftermarket alternatives that meet or exceed OEM specifications. When buying a replacement, always verify using your vehicle identification number (VIN) to ensure compatibility, as there were minor variations across W203 model years.

What If the Relay Is Good but the Pump Still Won't Run?

If you test the relay and it clicks and works properly, but the fuel pump still does not run, the problem could be elsewhere. On the W203 chassis, the fuel pump itself is located under the rear seat cushion on the passenger side of the vehicle. The pump sits inside the fuel tank, accessible through a round access cover. Common issues that cause no-start even with a good relay include:

• A dead fuel pump. The internal motor can wear out over time, especially on cars with over 100,000 miles.
• Blown fuse. The wiring diagrams for the 2005 C240 show a separate fuse in the front SAM module that powers the fuel pump circuit. Check fuses in the engine compartment fuse box as well.
• Corroded wiring or connectors. The electrical connector at the pump itself can corrode, especially in cars exposed to moisture.

Step-by-Step Relay Replacement Guide

Replacing the relay is straightforward. Follow these steps:

Step 1: Safety first. Park the car on a level surface, apply the parking brake, and turn the ignition completely off. Remove the key from the ignition. It is recommended to disconnect the negative battery terminal before working on any electrical components.

Step 2: Access the Rear SAM. Follow the steps outlined above to remove the left trunk side carpet and open the Rear SAM module cover.

Step 3: Locate and remove the old relay. Look for the relay labeled "Fuel Pump" on the cover diagram. If the diagram is missing or unclear, look for a relay that is about 1 inch square with four or five pins. Grasp the relay firmly and pull it straight upward. It may require a gentle wiggle, but it should come out without excessive force.

Step 4: Install the new relay. Align the pins of the new relay with the socket openings. Press it firmly straight down until it clicks into place. You should feel it seat securely.

Step 5: Test the repair. Reconnect the battery if you disconnected it. Turn the ignition to the "ON" position and listen for the fuel pump prime sound. Start the engine and check that it runs smoothly.

Step 6: Reassemble the interior. If everything works, snap the Rear SAM cover back on, refit the carpet panel, and close the trunk.

Why Your Fuel Pump Relay Fails

Relays typically fail because of repeated electrical arcing inside the switch contacts. Over time, the contacts can burn, pit, or weld together. Heat and vibration from normal vehicle operation also contribute to wear. On the 2005 C240, the rear SAM module is located in a relatively protected area, which helps extend relay life compared to models that place the relay in the engine bay. However, age alone means the relay could fail on any car that is now over 20 years old.

Troubleshooting When You Cannot Find the Relay

If you have followed these instructions but still cannot locate the fuel pump relay, double-check a few things. First, confirm you are looking in the correct trunk side. Some owners mistakenly look on the right side, but the Rear SAM is always on the left in W203 models. Second, look for the fuel pump relay module, which on some 2005 models is a separate square box located at the left rear of the vehicle, near the fuel tank. This module contains the relay and additional circuitry for pump control.

When to Seek Professional Help

If you are not comfortable working with electrical components, or if you have replaced the relay and the car still does not start, it is time to visit a qualified mechanic. Fuel system diagnosis requires specialized tools like a fuel pressure gauge. The 2005 C240 fuel system should maintain pressure around 3.8 bar or roughly 55 psi while cranking. A mechanic can measure this and pinpoint whether the issue lies with the relay, the pump, or other components like the crank position sensor or the engine control module itself.

Recommended Solution for Fuel Pump Replacement

If your diagnosis points to a failing fuel pump rather than just the relay, you want a replacement that offers reliability, performance, and peace of mind. This is where KEMSO comes in. KEMSO fuel pumps are engineered as high-performance OEM replacement parts, designed to match or exceed the specifications of your original Mercedes C240 fuel pump. Each KEMSO pump undergoes strict quality control checks to ensure it delivers consistent fuel pressure and long service life. Unlike standard aftermarket pumps, KEMSO products come with a lifetime warranty, which means you buy it once and never worry about your fuel pump again. Whether you are replacing a pump that has failed after years of service or upgrading proactively, KEMSO provides a direct fit solution that restores your C240's performance and reliability. KEMSO fuel pumps are built to handle the demands of daily driving and are tested for compatibility with your vehicle's fuel system. For US customers looking for a dependable replacement, KEMSO offers competitive pricing and fast shipping directly to your door. To browse the complete selection of KEMSO fuel pumps for your 2005 Mercedes C240 and other vehicles, visit their website at https://www.kemsoracing.com. You will find detailed product information, customer reviews, and the exact pump that fits your car. Choosing KEMSO means choosing quality backed by a lifetime warranty, so you can get back on the road with confidence.

Remember, whether you are fixing a no-start condition by replacing a simple relay or tackling a full fuel pump replacement, knowing the exact location of your 2005 Mercedes C240 fuel pump relay is the first step toward a successful repair. Start with the simplest and cheapest fix first, test the relay, and work your way through the system from there. For more in-depth guides on fuel pump replacement for your C240, including torque specifications and step-by-step removal instructions, also visit https://www.kemsoracing.com/blogs/blog. Their blog contains detailed articles written by automotive experts that cover everything from symptom diagnosis to complete installation procedures.