How to put a Mercedes Benz car with a dead battery in neutral?

This blog will answer the following questions:  how to put a Mercedes Benz car with a dead battery in neutral? What are the causes of dead batteries in Mercedes Benz? When does a Mercedes Benz cannot go into neutral or drive?

How to put a Mercedes Benz car with a dead battery in neutral?

In order to put a Mercedes Benz car with a dead battery in neutral gear, the car will need a charged battery or auxiliary supply to operate the park pawl gear. Following is the process to put the car into neutral gear.

• If the vehicle is locked, use the mechanical key to gain access inside the vehicle.
• Open the engine hood and locate the jump post terminals.
• The jump post terminals contain a positive and negative port, remove the dust cap on the positive port.
• Connect a charged battery with jump cables or connect a battery charger to the jump terminal of the vehicle.
• After waiting for a few seconds turn the ignition key to position 2 or press the start-stop switch two times
• If there are no malfunction messages in the instrument cluster then depress the brake pedal and actuate the electronic selector lever.
• It can take multiple actuation of the electronic lever but once the transmission control unit receives the request to disengage the parking gear, the transmission can shift into neutral.
• Do not try to start the vehicle until the battery is fully charged and the battery charging system is working correctly.

Call the nearest workshop to arrange for roadside assistance or tow the vehicle to the nearest workshop for further diagnosis. Mercedes Benz uses a sophisticated battery monitoring program to detect the state of the battery charge. The vehicle will show warning messages like “Check 12 V battery” or “keep the vehicle running” if the main battery is in a low state of charge. 

The red battery symbol message is exclusive to the battery charging system and it is shown in the instrument cluster only when the battery is not getting charged. Do not drive the vehicle without consulting the workshop as it can lead to breakdowns.

What are the causes of dead batteries in Mercedes Benz?

The causes of dead batteries in Mercedes Benz are as follows.

• Parasitic drain

There are more than 100 control units in a Mercedes Benz and all of them communicate over different types of communication protocols like CAN, LIN, Most and flex ray. The control units are supplied by the 12-volt battery and there is an ignition ON signal which is received by all control units. The ignition ON signal tells the control unit that the vehicle is turned ON, similarly, the Ignition OFF signal is for when the vehicle is turned OFF. 

The status of ignition is sent by the electronic ignition switch control unit to all control units present onboard. A parasitic drain occurs when one of the control units stays active despite receiving the ignition OFF message. 

This malfunctioning control unit will not let the CAN data bus go into sleep mode and drain the battery even though the vehicle is turned OFF. Mercedes Benz authorised workshops are equipped with the necessary tools to identify the malfunctioning control unit and replace it. 

• Excessive internal resistance

Most Mercedes Benz cars are equipped with valve-regulated lead-acid batteries but unlike a flooded lead-acid battery, Mercedes Benz batteries have absorbent gel matt (AGM). 

Ideally, there should be zero internal resistance between the positive and the negative terminal of the battery but as the battery goes through multiple charging cycles the internal resistance of the battery can increase. Batteries with internal resistance greater than 1 ohm do not charge fully or retain their charge. 

The front signal actuation and acquisition control unit stores a fault code when the internal resistance of the battery increases. The remedy is to replace the battery as the state of charge is not enough to supply the power to onboard electrical systems.

• Unauthorised accessories.

Installing unauthorised accessories on the vehicle is not recommended by Mercedes Benz. The power supply to the unauthorised accessories like auxiliary lights or horns is usually taken directly from the main battery of the vehicle. This type of connection directly bypasses the vehicle battery monitoring systems.

Mercedes cars have alternator management and when the battery reaches an 80 per cent state of charge, the alternator goes into slow charging mode in order to save the battery life of the vehicle.

The unauthorised accessories cause additional battery drain and as it is not detected by the control unit it will not fully charge, thus leading to dead battery situations and breakdowns.

When does a Mercedes Benz cannot go into neutral or drive gear?

A Mercedes Benz cannot engage neutral or drive gear under the following circumstances.

• Faulty intelligent servo module

Mercedes Benz discontinued the selector lever which operated the detent lever of the transmission from the 7 G Tronic automatic transmission. Instead, there is an intelligent servo motor which actuates the shifter rod in the electrohydraulic valve body of the automatic transmission.

Over time the servo motor can break down causing the transmission to not shift into neutral or drive despite multiple actuation of the electronic selector lever. The only remedy is to renew the faulty intelligent servo motor ISM 

• Faulty electronic selector lever

The electronic selector lever is integrated into the steering column module control unit. It is very durable and long-lasting but the electronic selector module can malfunction causing it to not register the actuation of the lever. There will be a fault code stating ‘communication with the electronic selector module missing’ stored in the steering column module control unit. As the electronic selector lever is an integral part of the SCM it cannot be replaced separately and the entire steering column module must be replaced.

• Disturbed CAN network

The electronic selector module, transmission control unit, engine control unit and depending on the model the ISM, all communicate with each other on a drivetrain CAN data bus. The voltage levels on the CAN data bus must be within a specified range otherwise control units cannot communicate over the CAN network. An electrical short circuit in the electrical lines or anyone in the control unit can disturb the CAN network. The signal from the electronic selector module cannot reach the transmission control unit when the CAN data bus is disturbed.

The remedy is to identify the cause of the disturbed CAN network based on the diagnosis that it can either be an electrical wiring issue or a faulty control unit which must be renewed to rectify the concern.

Other articles about Mercedes Benz you may be interested in

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Conclusion

Mercedes Benz uses an auxiliary power supply in form of an additional battery or a capacitor to store enough charge required to put the vehicle in park when the main battery is completely discharged. It is for the security of the vehicle and if an auxiliary power source is not available to power the vehicle’s onboard electrical system a freewheel is installed on one of the rear wheels so that the vehicle can be towed away.