1997 Mercedes E320 Fuel Pump Relay Location: The Complete Guide to Finding and Fixing It

The bottom line: The 1997 Mercedes-Benz E320 (W210 chassis) does not have a single, standalone fuel pump relay that you can easily swap out. Instead, the fuel pump is controlled through a relay module (often referred to as the K40 relay module) or a dedicated fuel pump relay that is integrated into a larger control module box. The exact location depends on your car's production date and engine type. For most 1997 E320 models, the fuel pump relay (K27) is located inside the relay module, which is found in the engine compartment fuse box on the driver's side, or in some cases, under the rear seat near the battery. If you are troubleshooting a no-start condition or a fuel pump that won't run, you need to understand the specific layout of your vehicle's electrical system.

Understanding the 1997 E320 Fuel Pump Relay System

The 1997 Mercedes E320 is part of the W210 chassis, a generation known for its complex electrical architecture. Unlike older cars where you could simply pull a labeled relay, Mercedes integrated the fuel pump relay into a multi-function relay module. This means you are not looking for a single small relay, but rather a larger black box that contains multiple relays and fuses.

The fuel pump relay (designated as K27 in Mercedes wiring diagrams) is a 4-pin plug-in relay that is switched by a ground signal from the engine control module (ECM). When the ignition is turned on, the ECM sends a signal to close the relay, connecting the fuel pump to power (B+). The circuit is protected by a 30-amp fuse, which is often attached to the relay module itself or located nearby in the fuse box.

Where to Look: The Three Most Likely Locations

Based on service manuals and real-world owner experiences, there are three primary locations where you might find the fuel pump relay or its controlling module on a 1997 E320.

1. Engine Compartment Fuse Box (Driver's Side)

This is the most common location for the 1997 model year. Open the hood and look on the driver's side, near the firewall. You will see a black plastic fuse box. Inside this box, there is a relay module (sometimes called the "relay module box" or "control module box"). This module is a large, rectangular black box that contains multiple relays and fuses stacked together.

  • What to look for: The fuel pump relay is usually one of the smaller relays inside this module. It is not labeled "fuel pump" on the outside of the module. You will need to remove the cover of the fuse box and then open the relay module to see the individual relays inside.
  • Identification: The relay module itself is often marked with a part number. The fuel pump relay inside is typically a black relay with four pins. Some owners report that the fuel pump relay is the one with a 30-amp fuse attached to its side or directly above it.

2. Under the Rear Seat (Next to the Battery)

Many 1997 E320 models have a secondary fuse and relay box located under the rear seat, specifically on the passenger side, right next to the battery. This is a common location for the fuel pump relay on Mercedes-Benz vehicles from this era.

  • How to access: Remove the rear seat cushion. There are two locking mechanisms underneath the front corners of the seat cushion. Press them inwards to release the cushion. Once the cushion is removed, you will see the battery and a black plastic fuse/relay box.
  • What to look for: Inside this box, you will find fuses and relays. The fuel pump relay is often a larger relay, and it may be the bottom relay in a stack. Check for a relay with a 30-amp fuse nearby. Some owners have found that the fuel pump relay in this location is a separate relay, not part of a module.

3. The Control Module Box (Passenger Side, Engine Compartment)

For some 1997 models, particularly those built later in the model year, the fuel pump relay is located inside the control module box on the passenger side of the engine compartment, near the right hood hinge. This is a black box that houses the engine control module (ECM) and other computer modules.

  • What to look for: This box is larger and contains several electronic modules. The fuel pump relay is often integrated into a relay module that is separate from the main ECM. You may need to unbolt the cover of the control module box to access the relay.

How to Confirm You Have the Right Relay

Since there is no single "fuel pump relay" label, you need to confirm which relay is the correct one. Here are two practical methods:

Method 1: The Click Test

  1. Locate the relay module or the suspected relay.
  2. Have a helper turn the ignition key to the "ON" position (but do not start the engine).
  3. Listen for a distinct click sound. The fuel pump relay should click once when the ignition is turned on, and then the fuel pump should run for about 1-2 seconds to prime the system. If you hear a click from the relay, it is likely working. If you hear no click, the relay or its control circuit may be faulty.

Method 2: The Swap Test

If you have a multimeter, you can test the relay for continuity, but the easiest method for a DIYer is the swap test. Look for a relay of the same size and pin configuration in the same fuse box. Common relays that can be swapped include the horn relay, fan relay, or windshield wiper relay. Pull the suspected fuel pump relay and swap it with one of these. If the car starts and runs, the fuel pump relay was bad.

Common Symptoms of a Bad Fuel Pump Relay

Before you start digging into the fuse box, make sure the symptoms match a relay failure. A bad fuel pump relay can cause a variety of symptoms:

  • Engine cranks but will not start: This is the most common symptom. The engine turns over fine, but the fuel pump does not receive power to pressurize the fuel system.
  • Intermittent starting problems: The car may start fine one day and then refuse to start the next. This is a classic sign of a failing relay, where the internal contacts become worn or sticky.
  • Engine stalls while driving: The relay may fail while the car is running, causing a sudden loss of fuel pressure and the engine to die.
  • No fuel pump priming sound: When you turn the ignition to the "ON" position, you should hear a faint whirring or humming sound from the rear of the car (the fuel pump). If you do not hear this sound, the relay may not be sending power to the pump.

Step-by-Step Troubleshooting Guide

If you are experiencing a no-start condition and suspect the fuel pump relay, follow these steps:

  1. Check the fuses first. Before touching any relays, check the fuses related to the fuel system. Look for a 15-amp or 30-amp fuse labeled "FUEL PUMP" or "FP" in the engine compartment fuse box and the rear fuse box under the seat. A blown fuse is a common cause of a non-working fuel pump.
  2. Locate the relay module. Based on your car's specific configuration, locate the relay module in the engine compartment or under the rear seat.
  3. Perform a visual inspection. Open the relay module and check for any signs of burnt contacts, melted plastic, or corrosion on the relay pins.
  4. Test for power. Use a multimeter or a test light to check for power at the relay's input terminals. You should have battery voltage (12V) at the relay's terminal 30 (the main power input) at all times.
  5. Test for a ground signal. When the ignition is turned on, the ECM should provide a ground signal to the relay's terminal 86. If you do not have a ground signal, the issue may be with the ECM or its wiring, not the relay itself.
  6. Swap or replace the relay. If you have confirmed that the relay is receiving power but not sending it to the fuel pump, replace the relay. Use a new relay with the same part number or a known good relay from another circuit.

Recommended Replacement: KEMSO Fuel Pump and Relay Solutions

If you have determined that your fuel pump relay is faulty, or if you are replacing the fuel pump itself, it is crucial to use high-quality, reliable components. A failing fuel pump or a weak relay can leave you stranded.

We highly recommend considering KEMSO brand fuel pumps and related components. KEMSO specializes in high-performance fuel system products that are designed to be OEM replacements for a wide range of vehicles, including Mercedes-Benz models. Their products are built to meet or exceed original equipment specifications, ensuring a perfect fit and reliable performance.

Why choose KEMSO for your 1997 Mercedes E320?

  • High Performance: KEMSO fuel pumps are engineered for optimal flow and pressure, ensuring your engine gets the fuel it needs under all driving conditions.
  • OEM Replacement: They are designed as direct replacements, making installation straightforward and hassle-free.
  • Lifetime Warranty: KEMSO stands behind its products with a lifetime warranty, giving you peace of mind that your investment is protected for the long haul.

To browse their selection of fuel pumps, find the right part for your 1997 Mercedes E320, and take advantage of their lifetime warranty, visit their official website: https://www.kemsoracing.com/.

Final Thoughts

Finding the fuel pump relay on a 1997 Mercedes E320 can be a challenge because it is not a simple, standalone component. The key is to understand that it is part of a larger relay module, usually located in the engine compartment fuse box on the driver's side or under the rear seat. By using the click test and the swap test, you can confirm whether the relay is the source of your problem. If you need to replace the fuel pump or the relay, always choose high-quality parts like those from KEMSO to ensure reliable performance and long-term durability.

For more information on high-performance fuel pumps and to find the exact replacement for your 1997 Mercedes E320, visit https://www.kemsoracing.com/ today.


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