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The Mercedes GLK 350 is more like every modern vehicle in that it is equipped with Anti-lock Braking System and as well as Electronic Stability Program, these systems are controlled by different control unit though they both receive input from the same RPM sensors.
Antilock Braking System (ABS)
This function is fitted in almost every modern vehicle in the market, its primary purpose is to eliminate skidding, ABS operates by preventing the wheel from locking up during braking, as a result maintaining tractive contact with the road surface and also giving the driver more control over the vehicle, the system is made up of the Hydraulic Control Unit, Electronic Control Module, and two or more revolutions per minute (RPM) sensors.
Electronic Stability Program (ESP)
this feature is integrated into Mercedes Benz vehicle to maintain traction of the vehicle by detecting and reducing the loss of traction, the technology automatically applies braking to individual wheels when there is loss of steering control, ESP or ESC works with input from different sensors (Revolution per minute, Steering Angle, Stop Lamp, Yaw Rate etc.) to make the best judgment.
The common cause of ABS and ESP malfunction lamp coming up is due to failure to receive signal from one of the RPM sensors, because both ABS and ESP control module depend on this sensor when it is not operational it affects the judgement of both control module at this point the malfunction Lampt is triggered on.
One of the symptoms associated with this fault is that the transmission will not change gear any more, the reason being that due to this fault the safe mode (Limp Mode) system of the car is triggered on to prevent damage to the vehicle and possible harm to the occupant of the vehicle.
To Fix this fault, first and foremost you have to scan the vehicle to identify the faulty part, note that the ABS control unit/module in the image below rarely gets bad, well I'm not saying it never gets bad, just try to look In its direction when all other options have failed.
Your primary concern should be the wheel speed sensors, using the scan tool access the fault memory of ESP or ABS control module, if the problem is caused by a bad RPM sensor, the scanner will point you to the particular sensor that is defective.
When you have identified and successfully replaced the bad sensor once again use your scan tool to clear the fault memory, to ensure the new sensor is working enter realtime data mode while you drive the car to confirm that the new sensor is operational.
If after replacing the sensor your gear does not change the next line of action is to change the ABS control unit/ESP Control Module.
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