Where is the fuel pump relay on the 1983 Mercedes Benz Sl 380?

In this article, we find out the location of the fuel pump relay in the 1980 Mercedes Benz SL 380 and answer the following questions: what is the function of the fuel pump relay? What are the symptoms of a bad fuel pump relay? How to diagnose a faulty fuel pump relay?

Where is the fuel pump relay on the 1983 Mercedes Benz Sl 380?

The fuel pump relay in the 1983 Mercedes Benz 380 SL is located under the dashboard. There is a fusebox in the front passenger or driver leg compartment, all the relays including the fuel pump relay are placed just above the fuse box.

The Mercedes Benz 380 SL was available in domestic as well S international markets mainly in America and Canada. Depending on the market, the steering wheel position was either on the left or the right side of the vehicle. 

Irrespective of the driver’s position in the car the location of the onboard electrical fuses and relays remained the same in the 380 SL model series R107. To remove the fuel pump relay, it may be required to remove the centre console panel and the fuse box cover. If the vehicle is right-hand drive, the access is slightly better than the left-hand drive vehicle but it is in no way inaccessible.

What is the function of the fuel pump relay in the 1983 Mercedes Benz SL 380?

The 1983 Mercedes Benz 380 Sl was a two-door convertible sports car, it was powered by a 3.8-litre V8 engine.  The design was an instant success in the domestic as well as the international market and it also remained unchanged throughout its production life cycle.

The Mercedes Benz SL model series R107 was sold in many different engine choices and they were targeted toward the American market. Mercedes Benz sold more than 200,000 units in the United States alone and the SL was the only two-door roadster model in the Mercedes Benz lineup.

The fuel system largely remained the same in the different models of the Mercedes Benz Sl. The SL 380 in 1983 used a  Bosch K-tronic with lambda control. The fuel in the fuel tank is pumped and supplied to the fuel distributor by a fuel pump. The fuel distributor then distributes the fuel flow from the fuel tank to 8 injectors. The fuel quantity is also adjusted according to the air mass going into the cylinder.

The main function of the fuel pump relay is to control the fuel pump that pressurises the fuel to 5 bars and delivers the fuel to the fuel distributor. There is no fuse for the fuel pump, the Mercedes Benz SL 380 fuel pump is only actuated by the relay. A relay is used to switch heavier load circuits with the actuation of a small load circuit. 

When the ignition key is turned to position two, the coil inside the relay is charged and it closes the switch. When the switch is closed the fuel pump gets the power supply and it starts working.

The current consumption of the fuel pump can be up to 8 A and it is not a suction jet pump. The fuel pump is located outside the fuel tank. The fuel pump relay has multiple terminals but the four basic terminals are as follows

  • Terminal 30

Terminal 30 is the power supply of the load circuit, battery voltage is always present on the terminal 30 sides of the relay. When the relay is switched the current flows through the load circuit and reaches the load. Depending on the onboard electrical system the terminal 30 power supply can be fused or it can be a direct electrical line from the pre fuse box.

  • Terminal 87

Terminal 87 is the output side of the load circuit, if there are multiple loads connected then usually there are fuses in the electrical lines leading to the loads. Without 87 signals the fuel pump does not turn on.

  • Terminal 85 & 86

Terminals 85 and 86 are also known as the actuation circuit, a power supply is always present in one of the terminals and when the coil needs to be magnetised, the other terminal is grounded and the circuit is completed. The actuation circuit can be power supply actuated or ground signal actuated, ground signal actuations are more common in Mercedes Benz relays.

Additional terminals are for circuit 15 signals and a fuel pump shut off function if circuit 15 is not present then the actuation circuit is not energised and the fuel pump does not start.

What are the symptoms of a bad fuel pump relay on a Mercedes Benz 380 SL?

Since there is no fuse for the power supply of the fuel pump, the relay is very crucial for the operation of the fuel pump. The following problems can occur due to a bad fuel pump relay:-

  • Long cranking

The fuel pump in Mercedes Benz 380 SL consumes a 6-8 A current, if the relay is not good it can cause under-voltage or low power supply at the fuel pump. The fuel pressure needs to be in a specified range and bad fuel pump relay are known to cause very slow pressure build-up in the fuel line leading to the fuel distributor. This results in insufficient fuel pressure and the engine turns for a longer duration until sufficient pressure is built up.

Once the pressure reaches 5 bars and desired fuel is supplied to all 8 injectors, the engine can start and maintain the idling engine speed.

  • Vehicle not starting.

A faulty fuel pump relay can result in no power supply to the fuel pump. In this case, the engine only turns but it never starts as no fuel is present in the fuel lines. This fault is more critical than the long cracking as the vehicle remains at a standstill. 

Replacing the relay is the only option and the fuel pump will work normally once the supply from the fuel pump relay is restored. 

How to diagnose a faulty fuel pump relay in Mercedes Benz 380 SL?

Diagnosis of a faulty electric relay is fairly easy compared to the 1983 380 SL compared to the models launched after 1985. One of the easiest methods is to replace the relay temporarily from a non-affected car and check if the fault occurs. 

If the fault does not occur after replacing the relay then it means the relay is faulty and a diagnosis can be further carried out to identify the cause behind the failure of the relay. If a spare relay is not available, then the voltage can be measured at the relay terminals. After turning the ignition key to position 2, there should be a 12v supply in terminal 87 of the relay.

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Mercedes Benz used a very durable relay in the fuel pump supply circuit of 1983 380 SL and usually, there is no fault unless there is a short circuit to the ground in the electrical line or an internal short circuit in the fuel pump. In that case, the short needs to be rectified before installing the new relay otherwise the new relay can also get damaged.