What is Mercedes Benz ECO mode?

This article will explain the Mercedes Benz ECO mode and answer the following questions: What is a Mercedes Benz ECO mode? What are the additional components required for the ECO mode? 

What is Mercedes Benz ECO mode?

The Mercedes Benz ECO mode is an automatic engine start and stop function aimed at reducing fuel and carbon consumption of passenger cars. It was first introduced in the 2011 C class facelift model and ECO stands for engine cut off. 

The general idea behind the eco mode is to turn off the vehicle when the car is waiting at stop signs or signals and thus saving fuel as the engine is not idling. The difference between turning off the engine and switching off the engine in ECO mode is that auxiliary components like radio, blower motor and other interior equipment still function. The driver does not need to turn the key to start the engine in ECO mode.

When the vehicle comes to a halt and the driver is pressing the brake pedal, ECO mode will activate and switch the engine off. The engine will start again as soon as the driver lets go of the brake and touches the throttle pedal. The front signal actuation and acquisition control unit is the master control for the ECO mode. 

It is constantly monitoring the voltage of the main as well as auxiliary batteries when the engine is not running in the eco mode. As soon as the voltage goes below 11 V and the vehicle is still at standstill, the SAM control unit will override the ECO function and start the engine. 

The ECO start-stop is available in ECO as well as comfort driver programs in the dynamic select. Customers can also create a custom drive mode in the intelligent dynamic select. Many customers think that the ECO function is always active but this is far from the truth. 

The ECO mode must satisfy certain pre-conditions before it is activated. The SAM control units also completed a systems check and indicated buying a green LED light on the ECO switch. In the latest models, it is represented on the instrument cluster by a green ECO symbol.

Following are some of the preconditions required for the ECO mode:

  • Vehicle speed

The vehicle speed must have exceeded the speed of 20 kmph once after starting the journey and the ECO or comfort drive program must be active.

The condition of the main and auxiliary battery must be good. There should not be any fault code in the fault memory of the front SAM control unit.

  • Air conditioning temperature

The Air conditioning temperature should not be set to the lowest settings, also the eco will only activate once the temperature set by the driver is reached and the air conditioning compressor is no longer required.  This condition is only applicable to models with air conditioning compressors driven by internal combustion engines. On models with high voltage electric AC compressors, this condition is satisfied more quickly.

The service brake pedal switch must be functioning correctly and there should not be any loose contact in the electrical coupler.

The ECO mode is activated once all the conditions are satisfied, the driver can turn off the ECO mode by simply pressing the ECO switch located either on the lower control or the upper control panel.

What are the additional components required for the ECO mode?

Mercedes Benz has been using ECO mode in almost all passenger cars for more than 10 years. Many components went through modifications but the core components responsible for the ECO mode remained the same. Following are the additional components required for the ECO mode:

  • Auxiliary battery

In the models before 2014, ECO mode mainly used an AUxiliary battery to power the vehicle’s onboard systems like blower motor of climate control, audio systems etc when the engine is starting in the ECO mode. 

An auxiliary battery is required because there is a 3-4 volts voltage drop across the main battery when the engine is cranked as the starter motor can draw up to 200 amps. Due to this voltage drop, The power supply can drop below 8 volts and there is no sufficient power to support other onboard systems like an ac blower motor, ambient light, radio etc.

The auxiliary battery is connected to a relay control by the front SAM control unit. It is connected to the main power supply for the onboard electric system when the relay is closed. The front sam control unit also actuates the relay one time when the vehicle speed of 20kmph is breached for the first time since the engine start.

The Auxiliary battery relay is located near the auxiliary battery and the positive supply terminal is connected directly to the relay. If there is a fault in the auxiliary battery relay, the ECO mode will not work and the green status LED will change to amber.

  • Voltage dip limiter

The voltage dip limiter was introduced with the launch of the fifth generation of the C class model series w205. The auxiliary battery was discontinued in the models above C class as the voltage dip limiter was able to limit the voltage drop across the main battery during the engine start in ECO mode.

A voltage dip limited is connected to the positive terminal of the battery and controlled by circuit 50 (starter circuit) relay. Depending on the construction, It has two separate electrical pathways, one path has a resistance builtin while the other is directly connected to the starter motor. When the vehicle is normally started current flows from a pathway that has no resistance as maximum current is required to start the vehicle for the first time. 

During the engine start in the ECO mode, complete rotation of the crank is not needed as the engine control unit saves the ignition angle needed for the next engine start. The start motor only cranks the engine for a few degrees.

 Hence the second resistive track is used to supply the starter and the engine can be started without turning off the ac blower motor and other interior equipment. If the engine is not started within 2ms then the voltage dip limiter is again switched to the first path and provides a full supply to the starter motor.

Thanks to the use of a voltage dip limiter the voltage level at the main battery can be maintained at 10 to 11 volts during starting the engine. The additional battery is no longer required to supply power to the vehicle. A small capacitor is used instead to put the vehicle in P when the main battery is disconnected or discharged.

  • Auxiliary oil pump

An auxiliary oil pump is required to maintain the oil pressure in the automatic transmission as the main oil pump is no longer driven by the engine. The auxiliary oil pump for the 7gtronic transmission is located just next to the torque converter bell housing and it is connected directly to the electro-hydraulic controller unit.

Like any other interior equipment, the auxiliary oil pump is also powered by the auxiliary battery when the engine is started in ECO mode. The auxiliary oil pump is integrated into the electric hydraulic valve body for the 9 g Tronic and dual-clutch transmissions.

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The ECO mode is a great way to reduce fuel consumption and it should be used in city stop and go traffic. There is no excess amount of fuel required to start the engine in eco mode and the driver can turn off ECO mode if the stop intervals are very small. It is possible to turn off the ECO mode entirely by making a custom dynamic select program and selecting it after starting the vehicle.