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Pete
Pete, Car Enthusiast
Category: UK Car
Satisfied Customers: 24698
Experience:  Over 17 years experience diagnosing and repairing automobiles.
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My name is ***** ***** I have a 2008 ML350 the brake lights

Customer Question

Hi my name is ***** ***** I have a 2008 ML350 the brake lights wont turn off once pressure is applied to the brake and released There is no switch in the ML350 so I have been told it could have something to do with the real SAM unit
JA: Thanks. Can you give me any more details about your issue?
Customer: the brake lights wont turn off once pressure is applied to the brake and released
JA: OK got it. Last thing — Mercedes Mechanics generally expect a deposit of about $18 to help with your type of question (you only pay if satisfied). Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 10 months ago.
Category: UK Car
Expert:  Pete replied 10 months ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Has anything after market been installed on your vehicle recently that was wired into your vehicle?

Was anything done on the vehicle recently that may have caused this issue here at all?

Let me know so we can continue,thanks Pete.

Customer: replied 10 months ago.
Nothing after market has been installed. The car was recently serviced and a full diagnostics was done. I recently replaced an ABS sensor on the car but there are no warning lights on the dash and the vehicle is working as normal.
Customer: replied 10 months ago.
When the car is first started the brake lights are off, only once the brake pedal is pressed then released do the lights NOT go off again and remain on until the car is turned off.
Expert:  Pete replied 10 months ago.

O.k.,the brake pedal when pressed activates the brake booster and the brake booster then signals the esp control module which in turn signals the rear sam to turn on the brake lights.There should be a sensor in the brake booster that activates the signal to the esp and so on to the sam unit.This sensor can fail and cause this here.I believe this sensor is the brake booster pressure sensor or brake pressure sensor and is usually located on the front or bottom of the brake booster itself.Usually the brake booster has to be replaced on some models in order to replace this sensor.

Usually if this was the rear sam at fault the issue would occur at all times and in this case it only occurs after the brake is applied so to me from here this sounds like this brake booster sensor is at fault needing replaced.

I would have this checked/tested further to verify this is the cause here.This usually requires a scan tool that can communicate with the brake system or a dealers sds machine to see if this sensor is sending an incorrect signal needing replaced.It is possible the rear sam is at fault but not as likely as the booster/sensor.

https://www.dropbox.com/s/xu6mn1js9q469ag/boos-01-29.gif?dl=0

https://www.dropbox.com/s/2d0yu0p3wut19o1/booster-01-29.gif?dl=0

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Customer: replied 10 months ago.
Thanks Pete, I will try your suggestion and see if that is the issue. I will send feedback.
Expert:  Pete replied 10 months ago.

Your welcome.Let me know if you need anything else here and feel free to rate my help here as this is how I receive credit for my time.

Thanks Pete