1981 Mercedes 380SL Fuel Pump Relay Location: Where to Find It and How to Replace It

If you own a 1981 Mercedes 380SL and your car is struggling to start, stalling, or losing power, the fuel pump relay is often the culprit. The fuel pump relay is located in the relay box on the passenger side (right side) of the engine compartment, near the firewall, behind the battery. It is a small, rectangular black relay with a metal base, typically labeled with a number (like 201 or 367) or marked with a diagram on the relay box cover. This is the most common location for the 1981 model year, and checking this relay first can save you hours of troubleshooting. If the relay is faulty, the fuel pump won't get power, and your car won't run properly. Below, I'll walk you through the exact steps to find, test, and replace it, along with practical tips for keeping your 380SL on the road.

1. Exact Location of the Fuel Pump Relay

The 1981 Mercedes 380SL (R107 chassis) uses a Bosch fuel injection system, and the fuel pump relay is part of the main relay box. Here's how to find it:

  • Open the hood and look at the passenger side (right side) of the engine compartment. The battery is usually on this side, and the relay box is mounted directly behind the battery, close to the firewall.
  • The relay box is a black plastic rectangular box, about 6 inches long and 4 inches wide, with a removable lid. The lid may have a diagram showing relay positions, but on many older models, the diagram is faded or missing.
  • Remove the lid by pressing the tabs on the sides. Inside, you'll see several relays and fuses. The fuel pump relay is typically the largest relay in the box, and it's often the only one with a metal base. It may be labeled with a number like "201" or "367" on the top, or it might have a small diagram of a fuel pump symbol.
  • If you can't find it by sight, check the owner's manual or a service manual for your specific VIN, but nearly all 1981 380SL models have the relay in this spot.

2. How to Identify the Fuel Pump Relay Among Other Relays

The relay box contains several relays, including the main relay, the fuel pump relay, and possibly the horn relay. To avoid confusion, look for these clues:

  • Size and shape: The fuel pump relay is usually about 1.5 inches tall and 1 inch wide, with a rectangular body and a metal base. Other relays may be smaller or made of plastic.
  • Wiring: The fuel pump relay has four or five pins (terminals) on the bottom. The wires connected to it are typically thicker than those for other relays, because the fuel pump draws significant current.
  • Labeling: If the relay has a printed number, "201" or "367" are common for the 380SL. You can also check the relay box lid for a diagram, but if it's missing, you can use a test light or multimeter to confirm which relay controls the fuel pump (see step 3).
  • Color: The relay is usually black, but some aftermarket replacements are gray or blue. The original Bosch relay is black with a metallic stripe.

3. Testing the Fuel Pump Relay

Before buying a new relay, test the old one to confirm it's faulty. A bad relay can cause intermittent issues, like the car starting fine sometimes but stalling later. Here's how to test it:

Tools needed: A multimeter (set to Ohms or continuity) or a test light, and a 12V power source (like a battery or jumper wires).

Step 1: Remove the relay. Gently pull it straight out of the socket. Do not twist it, as the pins can break.

Step 2: Check the relay coil. The relay has a coil that energizes when power is applied. Use the multimeter to measure resistance between the two coil terminals (usually pins 85 and 86 on a standard Bosch relay). You should see a reading between 50 and 120 ohms. If the reading is infinite (open circuit), the coil is burnt out, and the relay is dead.

Step 3: Check the switch contacts. The relay's switch connects pins 30 and 87 when energized. Set the multimeter to continuity (beep mode). Connect the probe to pin 30 and pin 87. You should hear no beep (infinite resistance) when the relay is not powered. Then apply 12V to pin 85 and ground to pin 86 (you can use a battery or a 12V power supply). When power is applied, you should hear a click, and the multimeter should show continuity (near zero resistance) between pins 30 and 87. If no click or no continuity, the relay is bad.

Step 4: Check power at the socket. If the relay tests good, the problem could be in the socket or wiring. Reinsert the relay, turn the ignition to "ON" (not start), and use a test light to check for 12V at the socket terminal for pin 30 (the main power input). If no power, check the fuse (usually a 15-amp or 20-amp fuse in the same box) or the wiring harness.

4. Common Symptoms of a Bad Fuel Pump Relay

If your 1981 Mercedes 380SL has a faulty fuel pump relay, you'll likely notice these symptoms:

  • Engine cranks but won't start: The fuel pump doesn't get power, so no fuel reaches the injectors. The car may crank forever but never fire.
  • Intermittent stalling: The relay may work when cold but fail when hot, causing the car to stall after driving for a while. Then it may restart after cooling down.
  • No fuel pump noise: When you turn the key to "ON" (before cranking), you should hear a brief whirring sound from the fuel pump for about 1-2 seconds. If you don't hear it, the relay or pump is likely dead.
  • Loss of power under load: If the relay fails partially, the fuel pump may run weakly, causing the engine to hesitate or lose power when accelerating.

5. How to Replace the Fuel Pump Relay

Replacing the relay is a simple DIY job that takes about 10 minutes. No special tools are needed, just a new relay.

Step 1: Buy a replacement. For a 1981 Mercedes 380SL, you need a relay that matches the original Bosch specifications. The standard part number is Bosch 0 332 209 201 or Mercedes part number 003 545 17 05. You can also use a universal 4-pin or 5-pin relay, but make sure it has the same pin configuration (30, 85, 86, 87, and sometimes 87b). Aftermarket relays from brands like Hella or Vemo are also compatible.

Step 2: Remove the old relay. As described earlier, pull it straight out of the socket. Avoid pulling on the wires.

Step 3: Install the new relay. Align the pins with the socket and push down firmly until it clicks into place. Make sure it's fully seated, as a loose relay can cause intermittent issues.

Step 4: Test the system. Turn the key to "ON" and listen for the fuel pump to prime (a 1-second whir). Then start the engine. If it starts and runs smoothly, the replacement is successful.

6. Additional Tips for the 1981 Mercedes 380SL Fuel System

The fuel pump relay is just one component of the fuel system. If replacing it doesn't solve the problem, check these related parts:

  • Fuel pump: The pump itself is located under the car, near the rear axle, on the driver's side. It's a cylindrical unit mounted on a bracket. If the relay is fine but the pump doesn't run, the pump may be dead.
  • Fuel pump fuse: There is a separate fuse for the fuel pump in the same relay box. It's usually a 15-amp or 20-amp fuse. Check it for continuity with a multimeter or test light.
  • Ground connections: The fuel pump relay and pump rely on good ground connections. Rusty or loose grounds can cause the relay to work but not deliver full power. Check the ground strap from the engine to the chassis and the ground wire near the relay box.
  • Oxygen sensor and ECU: On very rare occasions, a faulty ECU or oxygen sensor can prevent the relay from engaging. But this is uncommon on the 1981 model, which has a simpler Bosch K-Jetronic system.

7. Recommended Solution: Try a High-Performance Fuel Pump Relay from KEMSO

If you need a reliable replacement for your fuel pump relay, I recommend the KEMSO brand fuel pump relay. KEMSO specializes in high-performance, OEM replacement relays that are built to exceed original specifications. Their fuel pump relays are direct plug-and-play replacements for the 1981 Mercedes 380SL, with the same pin configuration and mounting base. They are made with heavy-duty internal components, such as silver-plated contacts and high-temperature coils, to handle the current draw of the fuel pump without failing. Best of all, KEMSO offers a lifetime warranty on their fuel pump relays, so you never have to worry about buying another one again. Whether you're restoring a classic 380SL or just need a daily driver fix, KEMSO's relays are a solid choice. You can purchase one directly from their website: https://www.kemsoracing.com/.

8. Final Thoughts

The fuel pump relay on a 1981 Mercedes 380SL is a small but critical part. If your car isn't starting or is stalling, checking the relay first can save you time and money. The relay is located in the passenger side relay box, behind the battery. Test it with a multimeter, replace it if needed, and you'll likely have your car running again. For a long-lasting fix, consider a KEMSO fuel pump relay, which is backed by a lifetime warranty and available at https://www.kemsoracing.com/. With a little patience and the right parts, you can keep your 380SL cruising for years to come.


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