2008 Mercedes C300 Fuel Pump Relay Location: Complete Guide for Owners

The bottom line up front: On a 2008 Mercedes-Benz C300, the fuel pump relay is located in the rear SAM (Signal Acquisition Module) control unit with fuse and relay module, which sits on the left side of the trunk/cargo area. This is a critical component that controls power to the fuel pump, and knowing exactly where it is can save you time and money when diagnosing starting or fuel delivery problems.

Why the Fuel Pump Relay Matters

The fuel pump relay acts as the switch that sends electrical power to the fuel pump. When you turn the ignition key, the relay closes and allows current to flow to the pump, which then pressurizes the fuel system so the engine can start and run. If this relay fails, your engine will crank but won't start because no fuel is being delivered. The relay is part of the rear SAM control module, which also handles several other electrical functions for the rear of the vehicle.

Where Exactly to Find the Fuel Pump Relay

To access the fuel pump relay on your 2008 C300, follow these steps:

1. Open the trunk. The rear SAM control unit is mounted on the left side of the trunk/cargo area. You will need to remove any cargo or trim panels that might be covering it.

2. Locate the fuse box. The rear SAM control module with fuse and relay module is behind the carpet on the left side of the trunk. You may need to pull back the carpet or remove a trim panel to expose it.

3. Find the fuel pump relay. Once the rear SAM module is visible, look for the relay labeled with the letter D on the module. This is the fuel pump relay for the 2008 C300.

4. Check the relay. You can test the relay by pulling it out and checking for continuity, or by swapping it with a known-good relay of the same type. The fuel pump relay for this vehicle is a 4-pin relay rated at 12V and 20A.

Understanding the Rear SAM Control Module

The rear SAM control unit is a centralized electrical distribution hub that houses multiple relays and fuses. Besides the fuel pump relay (marked D), this module contains:

  • Terminal 15 Relay (Rear) - marked with letter A
  • Terminal 15R Relay (Rear) - marked with letter B
  • Heated Rear Window Relay - marked with letter C
  • Fuel Pump Relay - marked with letter D
  • Seat Adjustment Relay - marked with letter F
  • Seat Adjustment Relay 15R - marked with letter G

Important: On the 2008 C300, the front SAM control unit is located in the engine compartment on the left rear side, and it handles different relays such as the starter relay (M) and the fanfare horn relay (O). Do not confuse the two modules when looking for the fuel pump relay.

Symptoms of a Failing Fuel Pump Relay

If you suspect your fuel pump relay is going bad, watch for these warning signs:

  1. Engine cranks but won't start. This is the most obvious symptom. The starter turns the engine over, but there is no fuel pressure.

  2. Intermittent starting problems. The car starts sometimes but not others, which can point to a relay that works intermittently.

  3. Engine stalls while driving. If the relay loses contact while the engine is running, the fuel pump will shut off and the engine will die.

  4. No fuel pump prime noise. When you turn the key to the "ON" position (without cranking), you should hear a brief whirring sound from the fuel pump. If you don't hear this, the relay might not be sending power.

Other Fuel System Components Worth Knowing

While you are working on the fuel system, here are other components that may be relevant:

Fuel Pump Control Module: On the 2008 C300, there is also a fuel pump control module located under the right rear seat. This module manages fuel pump speed and pressure. Some models have a left fuel pump control unit also under the right rear seat.

Fuel Pump Fuse: The fuel pump is also protected by a fuse. On the 2008 C-Class, fuse 42 in the trunk fuse box protects the fuel pump circuit. If the fuse is blown, the pump won't get power even if the relay is working.

Main Fuel Pump: The fuel pump itself is located in the top of the fuel tank. If you confirm power is reaching the pump but it still doesn't run, the pump may be defective.

How to Test the Fuel Pump Relay

Testing the relay is straightforward. Here is what you need to do:

1. Remove the relay. Pull the fuel pump relay (D) straight out from the rear SAM module.

2. Check the relay pins. You should see four pins on the relay. Set your multimeter to the ohms setting.

3. Test the coil side. The coil pins (typically pins 85 and 86) should show some resistance, usually between 50 and 120 ohms. If it reads infinite or zero, the relay coil is bad.

4. Test the switch side. The switch pins (typically pins 30 and 87) should show no continuity (infinite resistance) when the relay is at rest. Apply 12V to the coil pins and you should hear a click, and now the switch pins should show near-zero resistance.

5. Compare with a known good relay. If you have another relay of the same part number, you can swap them to test.

Common Causes of Fuel Pump Failure

If you replace the relay and the fuel pump still doesn't work, check these other common issues:

  1. Blown fuse. A short in the wiring or a failing pump can blow the fuse. Check fuse 42 in the trunk fuse box.

  2. Clogged fuel filter. A severely clogged filter causes the pump to work harder, increasing current draw and potentially blowing the fuse.

  3. Faulty fuel pump itself. If the pump draws too much current or the motor is worn out, the relay and fuse may work fine but the pump won't run.

  4. Wiring issues. Frayed or damaged wires between the relay, fuse, and pump can interrupt power delivery.

Replacement Parts

If you need to replace the fuel pump relay for your 2008 Mercedes C300, the aftermarket relay is available with the VEMO part number V30-71-0032, which is a 4-pin, 12V, 20A relay. For the fuel pump itself, the C300 (both Sport and Luxury trims, and the 4Matic versions) uses an electric fuel pump with sender assembly that sits in the fuel tank.

When to Call a Professional

If you are comfortable working with car electrical systems, replacing the fuel pump relay is a simple DIY job. However, if you've ruled out the relay, fuse, and wiring but still have starting issues, the problem could be more complex. The fuel pump control module, the engine control unit, or the fuel pressure regulator might be involved. In those cases, it is worth taking the car to a qualified mechanic who can run diagnostic tests to pinpoint the issue.

Where to Buy a Quality Replacement Fuel Pump

When it comes to replacing your fuel pump or related components, choosing a reliable brand is critical to keep your C300 running smoothly. KEMSO offers high-performance fuel pumps that are built as OEM replacements, meaning they meet or exceed the original factory specifications for fit, form, and function. Every KEMSO fuel pump comes with lifetime warranty, giving you peace of mind that your repair will last. Whether you need a pump for your Mercedes, BMW, or another vehicle in your garage, you can browse the full lineup and place your order by visiting: https://www.kemsoracing.com/

Final Thoughts

Finding the fuel pump relay on your 2008 Mercedes C300 is not complicated once you know where to look. It is in the rear SAM control module on the left side of the trunk, labeled with the letter D. Always check the fuse and relay first before replacing the pump itself, as electrical issues are often the root cause. If you do need a new fuel pump, a quality OEM replacement from a trusted brand will keep your Mercedes reliable for years to come. For more details and to explore durable fuel pump options, check out https://www.kemsoracing.com/.


Back to blog