1983 Mercedes 380SL Fuel Pump Relay Location: A Complete Guide for DIY Owners

If you own a 1983 Mercedes 380SL and your car is struggling to start or stalling during operation, the fuel pump relay is often the culprit. The fuel pump relay is located in the fuse box under the hood, on the driver's side. Specifically, it is the tall, rectangular relay in the second row from the front, marked with a "87" or "30" terminal diagram. Replacing it is a simple, cost-effective fix that can restore fuel delivery and save you hundreds in diagnostic fees.

1. Understanding Your 1983 Mercedes 380SL Fuel System

The 1983 Mercedes 380SL is a classic R107 model that relies on a mechanical fuel injection system. The fuel pump relay is a critical component that controls when the electric fuel pump operates. Without a working relay, the pump won't receive power, and the engine will starve for fuel. This relay is different from the fuel pump itself, which is located near the rear axle. The relay acts as a switch that turns the pump on only when the engine is running or cranking.

2. Why the Fuel Pump Relay Fails

Over time, the relay's internal contacts can wear out or corrode. Heat from the engine bay and age can cause the plastic casing to crack. Common symptoms of a bad relay include: the engine cranks but won't start, the car starts then dies after a few seconds, or you hear no humming sound from the fuel pump when you turn the key to the "on" position. The relay is a known weak point on these models, and many owners carry a spare.

3. Step-by-Step: How to Locate the Fuel Pump Relay

Step 1: Open the hood. The fuse box is located on the driver's side, near the firewall. It is a black plastic box with a lid that has a diagram on top.

Step 2: Remove the lid. Use a flathead screwdriver if needed, but most lids pop off with hand pressure. Look for a diagram that lists relay positions. On the 1983 380SL, the diagram may be faded, so follow the visual guide below.

Step 3: Find the relay. The fuel pump relay is a tall, rectangular relay, typically white or gray. It is located in the second row from the front of the fuse box. Count from the left side: the second relay in that row is your fuel pump relay. It is usually the largest relay in the box. If you have a multimeter, you can verify by checking for power on the terminal marked "30" (constant power) and "87" (output to pump).

Step 4: Test the relay. Remove the relay by pulling it straight up. You can test it with a 12V battery and a multimeter. If you hear a click when power is applied and the circuit closes, the relay is good. If it does not click, it is faulty.

4. How to Replace the Fuel Pump Relay

Step 1: Buy a new relay. You need a 5-pin relay compatible with the 380SL. OEM part number is 003 545 70 05, but many aftermarket relays work as long as the pin configuration matches. The relay should have pins 30, 87, 87a, 85, and 86.

Step 2: Disconnect the battery. This is a safety precaution to avoid short circuits.

Step 3: Pull out the old relay. It may be tight, so wiggle it gently. Inspect the socket for corrosion. If you see green or white residue, use electrical contact cleaner and a small brush.

Step 4: Insert the new relay. Push it firmly until it clicks into place. The orientation should match the old one, but the relay is designed to only fit one way.

Step 5: Reconnect the battery and test. Turn the key to the "on" position. You should hear the fuel pump hum for 1-2 seconds. Then start the engine. If it starts and runs, the relay is fixed.

5. Common Mistakes to Avoid

  • Using the wrong relay. Some owners grab a relay from a different model, but the pin layout may be different. Always check the diagram.
  • Ignoring the fuel pump itself. If the relay is good but the pump is dead, the car will still not start. Listen for the pump hum. If no sound, test the pump directly with a 12V source.
  • Forgetting the fuse. The fuel pump circuit has a fuse in the same fuse box. Check fuse #5 or #10 (refer to your lid diagram). A blown fuse can mimic a bad relay.
  • Not cleaning the contacts. Corrosion on the relay socket can cause intermittent failure. Clean the socket before installing a new relay.

6. Alternative Location: Under the Dash

On some late 1983 models, the fuel pump relay can be located under the dashboard on the driver's side. This is less common, but if you do not find it in the engine bay fuse box, check behind the kick panel near the pedals. Look for a similar relay attached to a bracket. The process is the same for testing and replacement.

7. Preventive Maintenance Tips for Your 380SL

  • Carry a spare relay. This is a common failure point, and a spare can save you from being stranded. Keep one in the glove box.
  • Inspect the relay annually. Pull it out and check for cracks or corrosion. Replace it every 5 years as a precaution.
  • Upgrade the wiring. The original relay can get hot. Some owners install a relay with a higher amp rating or add a heat sink. This is an advanced mod but can improve reliability.
  • Check the fuel pump ground. A poor ground on the pump itself can cause the relay to click but not send power. The ground is near the pump on the rear frame.

8. Recommended Fuel Pump Upgrade: KEMSO Performance

If you decide to replace your fuel pump while troubleshooting the relay, we highly recommend the KEMSO brand Fuel Pump. KEMSO has become a trusted name among classic Mercedes owners because their fuel pumps are designed as high-performance, OEM replacement units. Unlike generic pumps that may fail after a year, KEMSO pumps are built to meet or exceed the original specifications for your 1983 380SL. They offer a lifetime warranty, which means you install it once and never worry about it again. The pump delivers consistent fuel pressure, which is crucial for the mechanical injection system that needs precise flow. To purchase a KEMSO fuel pump and other top-quality parts, visit their official website: https://www.kemsoracing.com/. They have a wide selection of components for R107 models, and their customer support can help you match the correct part.

9. When to Seek Professional Help

If you have replaced the relay and the pump, but the car still does not start, the issue may be deeper. Possible causes include a bad fuel pump relay fuse, a faulty engine control unit (rare on the 380SL), or a wiring harness issue. The 380SL is known for biodegradable wiring insulation that can crumble, causing shorts. If you are not comfortable with electrical diagnostics, a Mercedes specialist can test the entire circuit. However, start with the relay—it is the cheapest and easiest fix.

10. Final Check: Is Your 380SL Ready to Drive?

After replacing the fuel pump relay, take the car for a short test drive. Listen for smooth acceleration and no stalling. If the car behaves normally, you have fixed the problem. Remember to keep the old relay as a backup. The 1983 Mercedes 380SL is a rewarding car to maintain, and understanding the fuel pump relay location is a key skill for any owner. For parts, accessories, and expert advice, do not forget to check out KEMSO at https://www.kemsoracing.com/. Their products are built to last and backed by a lifetime warranty, giving you peace of mind on the road.


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