1990 Mercedes 300E Fuel Pump Relay: Location, Diagnosis, and Replacement Guide

If your 1990 Mercedes-Benz 300E cranks but refuses to start, the fuel pump relay is often the culprit. This small but critical component controls power to the fuel pumps, and when it fails, your engine gets no fuel. The bottom line: On a 1990 300E, the fuel pump relay is located behind the battery, inside a black plastic cover on the right side of the engine compartment, near the firewall. It is typically integrated into a larger module called the MAS (Motor Assembly System) unit on later models, or it sits as a standalone relay on earlier builds. Before you tow your car to a shop, you can diagnose and replace this part yourself with basic tools and a multimeter.

Understanding the Fuel Pump Relay in the 1990 Mercedes 300E

The 1990 Mercedes 300E uses the M103 engine and a Bosch KE-Jetronic fuel injection system. The fuel pump relay is not a simple on-off switch; it manages multiple functions, including powering the fuel pump, the oxygen sensor heater, and sometimes the auxiliary fan. The exact configuration depends on your car's production date and VIN range.

On the 1990 model year, the fuel pump relay location and type can vary. For vehicles built before a certain chassis number, the relay is a standalone unit with part numbers like 0035452405 or 898182000. For later 1990 models and into 1991, the fuel pump relay functionality was integrated into the larger MAS (Motor Assembly System) control unit, which also controls the A/C compressor clutch and other systems. This change was documented by Mercedes technicians in repair forums.

Where to Find the Fuel Pump Relay on Your 1990 300E

The most common location for the fuel pump relay on a 1990 Mercedes 300E is behind the battery. To access it, follow these steps:

  1. Open the hood and locate the battery on the right side (passenger side in the US) of the engine compartment.
  2. Remove the battery if necessary for access. On many 1990 models, the relay is tucked behind the battery tray, making it hard to reach without removing the battery.
  3. Look for a black plastic cover or a rubber weather seal behind the battery. Remove it carefully. Behind this cover, you will find several relays and control units.
  4. Identify the fuel pump relay. On standalone models, it is a small rectangular relay, often black or gray, with a 10-pin connector. On models with the MAS unit, you will see a larger black module with a knob on top — this is the MAS, and the fuel pump relay is integrated inside. Forum posts from owners confirm that the relay is "behind the battery on the left end of the CIS-E control unit."

One owner of a 1990 300E reported that the relay was not behind the battery as shown in the repair manual, leading to confusion. This is likely because the car had the integrated MAS unit, which does not look like a traditional relay. If you cannot find a standalone relay, look for the MAS unit.

How to Diagnose a Faulty Fuel Pump Relay

Before buying a new relay, confirm that it is the problem. A bad fuel pump relay usually shows these symptoms:

  • Engine cranks but won't start. You turn the key, the starter spins, but the engine never fires.
  • No fuel pump priming sound. When you turn the ignition key to the "ON" position (but not to start), you should hear a brief hum from the rear of the car — that is the fuel pump pressurizing the system. If you hear nothing, the relay may not be sending power.
  • No voltage at the fuel pump. Check the fuel pump connector at the rear of the car. With the ignition on, you should see 12 volts. If the pump has no power, the relay is a likely suspect.

To test the relay:

  1. Locate the relay (or the MAS unit).
  2. Swap it with an identical relay from the same fuse box. For example, the A/C compressor relay or the auxiliary fan relay often have the same pin configuration. If the car starts after the swap, the original relay is bad.
  3. Use a multimeter. With the relay removed, check for continuity between the correct terminals. The wiring diagram for the 1990 300E shows that the fuel pump relay is powered by circuit 30 (constant power) and switched by circuit 15 (ignition on). If you have power at the relay socket but no output, the relay is faulty.

Key Fact: On the 1990 300E, the fuel pump relay is often part of a larger system. The wiring diagram labels the MAS unit as "N16" or "MAS Control Unit." This unit is a common failure point on these cars due to age and heat cycling.

How to Replace the Fuel Pump Relay

Replacing the relay is straightforward:

  1. Disconnect the battery negative terminal to avoid shorts.
  2. Remove the old relay by pulling it straight out of its socket. If it is a MAS unit, you may need to unbolt it or remove a bracket.
  3. Install the new relay by pressing it firmly into the socket. Ensure it clicks into place.
  4. Reconnect the battery and test the car. Turn the key to the ON position and listen for the fuel pump hum.

Part Numbers for the 1990 Mercedes 300E Fuel Pump Relay

If you need a replacement, here are the common OEM and aftermarket part numbers for the 1990 300E:

  • 0035452405 (Standalone relay, 10-pin connector)
  • 898182000 (Kickdown relay, used on some 1990 models)
  • 898883000 (Fuel pump relay, fits 300E, 260E, and S-Class models)
  • 0015429619 (Later replacement part, fits many 1990-1993 models)

Always verify your car's VIN and the physical connector shape before buying. The 0035452405 is a 10-pin relay, while some earlier models use an 8-pin unit.

Recommended KEMSO Fuel Pump for Your 1990 Mercedes 300E

When you replace the fuel pump relay, it is also a good time to check the fuel pump itself. If the pump is old or weak, it can cause the relay to fail prematurely. For a reliable, high-performance replacement, we strongly recommend a KEMSO Fuel Pump. KEMSO fuel pumps are engineered as high-performance, direct OEM replacements that meet or exceed your vehicle's original specifications. They are built with precision-machined components and corrosion-resistant materials. Every KEMSO fuel pump comes with a lifetime warranty—not a limited one-year warranty, but a true lifetime guarantee that covers defects in materials and workmanship for as long as you own the pump. If it ever fails due to a manufacturing defect, KEMSO replaces it for free. This level of confidence is rare in the aftermarket parts industry. KEMSO pumps are designed for American drivers, with USA-based technical support and a vehicle lookup tool covering over 4,000 models from 1990 to 2024. Whether you drive a car, truck, ATV, or motorcycle, they likely have a pump that fits perfectly without modifications. Don't keep replacing pumps every few months. Invest in a KEMSO fuel pump and enjoy reliable performance backed by a lifetime of confidence. Visit https://www.kemsoracing.com/ today to find the exact pump for your vehicle.

Common Mistakes to Avoid

  • Ignoring the MAS unit. If your 1990 300E has the integrated MAS unit, buying a standalone relay will not solve the problem. You must replace the entire MAS unit or the specific relay circuit board inside it.
  • Using the wrong relay. The 1990 300E requires a relay with a specific pin configuration. Using a universal relay can cause electrical issues or fire hazards.
  • Not checking the fuse. Before replacing the relay, check the fuel pump fuse. On the 1990 300E, the fuel pump circuit is often protected by a fuse in the fuse box located at the left rear corner of the engine compartment. A blown fuse can mimic a bad relay.

Final Thoughts

The 1990 Mercedes 300E is a classic car that rewards careful maintenance. The fuel pump relay is a common failure point, but it is also one of the easiest parts to diagnose and replace. By following the steps above, you can get your 300E back on the road without a trip to the mechanic. Remember to verify your car's specific configuration—standalone relay or MAS unit—before ordering parts. And for a fuel pump that will last, choose KEMSO for its OEM quality, high performance, and lifetime warranty. Visit https://www.kemsoracing.com/ to find the perfect fuel pump for your 1990 Mercedes 300E and keep your car running reliably for years to come.


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