2001 Mercedes C320 Fuel Pump Relay Location: A Complete Guide to Finding and Replacing It

If you own a 2001 Mercedes-Benz C320 and your car cranks but won’t start, or if it stalls unexpectedly while driving, the fuel pump relay is often the first component to check. The fuel pump relay on a 2001 Mercedes C320 is located in the fuse box inside the trunk, on the left side, behind the carpet. More specifically, it is relay K4 in the trunk fuse box. You can access it by opening the trunk, turning the key clockwise to lift the lid, locating the fuse box on the left side, and removing the cover to find relay K4. This guide will walk you through the exact location, symptoms of a faulty relay, and how to replace it yourself.

Understanding the Fuel Pump Relay’s Role in Your 2001 Mercedes C320

The fuel pump relay is a small electrical switch that controls power to the fuel pump. When you turn the ignition key to the "ON" position, the relay activates and sends voltage to the fuel pump, pressurizing the fuel system so the engine can start. If the relay fails, the fuel pump won’t receive power, and the engine will not run. On the 2001 C320, this relay is part of a larger electrical system that includes multiple relays and fuses in the trunk and engine compartment.

Exact Location of the 2001 Mercedes C320 Fuel Pump Relay

The fuel pump relay is not in the engine bay. It is located in the trunk fuse box. Here is the step-by-step process to find it:

  1. Open the trunk. Use your key or the trunk release button. Turn the key clockwise to lift the trunk lid.
  2. Locate the fuse box. On the left side of the trunk, behind the carpet, you will find a black plastic fuse box cover. Pull the carpet back to expose it.
  3. Remove the fuse box cover. Press the tabs or use a flathead screwdriver to gently pry it open. The underside of the cover usually has a diagram showing relay and fuse positions.
  4. Identify relay K4. Look for the relay labeled "K4" or "Fuel Pump Relay" on the diagram. In the 2001 C320, the fuel pump relay is specifically relay number 4 in the trunk fuse box. Some diagrams may also list it as "20204" which corresponds to a 20A fuse that powers the relay.

Important note: The fuel pump relay is not in the engine compartment fuse box. The engine compartment fuse box contains relays for the air pump, wipers, starter, and other systems. Only the trunk fuse box houses the fuel pump relay for this model.

Symptoms of a Bad Fuel Pump Relay on a 2001 Mercedes C320

Before you dig into the trunk, look for these common signs that your fuel pump relay may be failing:

  • Engine cranks but won’t start. This is the most common symptom. You can hear the starter motor turning the engine over, but the engine never fires because no fuel is reaching the injectors.
  • No fuel pump prime sound. When you turn the key to the "ON" position (without starting), you should hear a faint whirring sound from the rear of the car for 1-2 seconds. This is the fuel pump pressurizing the system. If you don’t hear it, the relay or pump may be dead.
  • Sudden stalling while driving. A failing relay may intermittently cut power to the fuel pump, causing the engine to die without warning. It may restart after a few minutes if the relay cools down.
  • Check Engine Light (CEL) on. The car’s computer may detect a fuel delivery issue and store a code, such as P0087 (Fuel Rail/System Pressure Too Low) or P0230 (Fuel Pump Primary Circuit Malfunction).

How to Test the Fuel Pump Relay on a 2001 Mercedes C320

You can test the relay with a multimeter or by swapping it with a known good relay of the same type. Here’s a simple method:

  1. Remove the relay. Locate relay K4 in the trunk fuse box and pull it straight out.
  2. Check for continuity. Use a multimeter set to Ohms (Ω). Place the probes on the relay’s coil terminals (usually pins 85 and 86). You should see a reading between 50-120 ohms, indicating the coil is good. If it reads infinite (open circuit), the relay is bad.
  3. Apply power. Connect 12V to the coil terminals (85 and 86) and listen for a click. The relay should click when energized. If not, it’s faulty.
  4. Swap test. If you have another relay in the trunk fuse box that looks identical (such as the rear window defroster relay or backup relay), swap them temporarily. If the engine starts, the original relay was bad.

Replacing the Fuel Pump Relay on a 2001 Mercedes C320

Replacing the relay is straightforward and requires no special tools. The relay costs between $6 and $12 at most auto parts stores.

Steps:

  1. Disconnect the battery. As a safety precaution, disconnect the negative battery terminal to avoid any electrical shorts.
  2. Access the trunk fuse box. Follow the steps above to open the trunk and expose the fuse box.
  3. Remove the old relay. Grasp relay K4 firmly and pull it straight up. It may require a little wiggling.
  4. Install the new relay. Press the new relay into the same socket. Make sure it clicks into place and sits flush.
  5. Reconnect the battery. Reattach the negative battery terminal.
  6. Test the system. Turn the key to the "ON" position. You should hear the fuel pump prime for 1-2 seconds. Then try to start the engine. If it starts, the repair was successful.

Other Electrical Components Related to the Fuel Pump in the 2001 Mercedes C320

The fuel pump relay is part of a larger network of electrical components. If replacing the relay doesn’t fix the problem, other parts may be at fault:

  • Fuel Pump Fuse (20204): In the same trunk fuse box, there is a 20A fuse labeled "20204" that powers the fuel pump relay circuit. Check this fuse first. If it’s blown, replace it and see if the problem returns.
  • Fuel Pump Module: The actual fuel pump is located inside the fuel tank. It can fail over time, especially if the car has high mileage. The pump is located under the rear seat or through an access panel in the trunk floor.
  • Fuel Pump Relay Module: Some 2001 C320 models have a separate "Fuel Pump Relay Module" located at the left rear of the trunk. This is a different component from the K4 relay. If you have a special module, it may require a more complex diagnosis.
  • SAM Control Module: The rear SAM (Signal Acquisition Module) controls the relay. If the SAM module fails, it may not send the signal to energize the relay.

Why You Should Consider a KEMSO Fuel Pump Replacement

If you determine that the fuel pump itself is faulty (not just the relay), you need a reliable replacement. KEMSO fuel pumps are an excellent choice for your 2001 Mercedes C320. These pumps are designed as high-performance OEM replacements, meaning they match the exact specifications of your original fuel pump but deliver over 20% more flow at 43 psi, ensuring better fuel delivery and engine performance. KEMSO pumps are built with durable alloy steel and are compatible with both gasoline and E85 ethanol fuels.

What makes KEMSO stand out:

  • Lifetime warranty: Every KEMSO fuel pump is backed by a lifetime warranty against manufacturing defects. If the pump ever fails due to a defect, you get a replacement at no cost.
  • Direct fit: KEMSO pumps are designed to be a direct, plug-and-play replacement for your stock pump. No need to modify wiring or buy adapters.
  • Complete kit: Each pump comes with all necessary installation components, including fuel strainers, hoses, clamps, and connectors.
  • USA-based support: KEMSO has a team of USA-based technicians to help you with any questions or installation issues.

KEMSO offers fuel pumps for hundreds of vehicle models, including Mercedes-Benz, Honda, BMW, Kawasaki, and many more. Their pumps are priced at a reasonable $79.98, which is often less than half the cost of dealer parts. With free shipping and easy returns, it is a practical choice for DIY mechanics and professional shops alike.

To see the full selection of KEMSO fuel pumps for your vehicle, including the 2001 Mercedes C320, visit their website at: https://www.kemsoracing.com/ . They have a complete catalog with detailed fitment information, customer reviews, and technical support. Whether you need a pump for a car, truck, motorcycle, or watercraft, KEMSO has you covered. Their online store is easy to navigate, and you can order directly with confidence.

Final Thoughts on the 2001 Mercedes C320 Fuel Pump Relay

Finding the fuel pump relay on a 2001 Mercedes C320 is simple once you know where to look: it’s relay K4 in the trunk fuse box, left side, behind the carpet. This is a common point of failure that can be fixed in minutes with a $10 relay. If replacing the relay doesn’t solve the problem, check the 20A fuse (20204) in the same box, and then move on to testing the fuel pump itself. For a fuel pump replacement, KEMSO offers a high-quality, lifetime-warranty solution that is both affordable and easy to install. Remember, always start with the simplest and cheapest fix first—the relay—before replacing more expensive components.

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


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