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Ethics - Mercedes product

Wife experienced issue similar to what Toyota is going thru with Mercedes ML320. Tried to brake and vehicle accelerated to maintain speed. Had to stand on brakes and finally put in neutral. When stopped. brakes burnt. Mercedes can not find cause but says they replaced the brake switch and vehicle is not repeating problem. I believe they need to id problem before letting my wife back in vehicle, plus believe Mercedes should be responsible for replacing the breaks (discs, calipers, etc…). Wanted to get an unbiased oppinion?

What year and how many miles? Is the acclerator pedal hooked up to throttle body by a cable system or is it drive-by-wire? Has the cable, linkages, and throttle body been examined and lubricated?

Most of the Toyota incidents have shown that driver error is the most likely cause, hitting the gas instead of or at the same time as the brake.

It sounds as though the cruise control stuck in the On position and the dealer thinks that the cause was the switch on the brake pedal arm?

The trouble with that theory is that the brake switch is designed to fail in the ‘cruise Off’ position, so if that is the problem, it did not fail as it is designed to. This would not be unprecedented, however, as Jaguar had that problem once on a set of brake switches.

You did not mention the age of the car, but I would expect that Mercedes would cover the brake job. This is not the kind of complaint that they want aired (but then, I guess that cat is already out of the bag).

Well, yes and no. Most of us here already know about Mercedes and their inability to wire a car properly. It’s if the OP decides to go elsewhere, like, say, the news, that Mercedes would lose reputation.

This is where dilemma begins - Vehicle is 10 yrs old with 158K on it.
Vehicle has been meticulously maintained. If it needed it, it got it, even if it was “wont affect the car”.
I am not wanting “Something for Nothing”. But do not believe that FAILURE of a “Fail Safe” control would be something Mercedes Benz would want to a) Know about, b) Ensure it is not something that other vehicles and owners might “discover”. c)Repaired this problem d)Replace the brakes (which they have acknowledged / agreed are, as a result of the extreme temps, in need of replacement since they were glowing when my wife got out of the vehicle and their initial comment was “They are blue”).
At present, however, they have noted the following:

  1. They do not know what caused the problem
  2. They have repaired the car! - Replaced the brake Switch and it is not doing what we said it was doing.
  3. They will not provide guarantee or assurance that it will occur again
  4. They do believe it would be best to replace the brakes since they did experience such extreme temperatures, but they will not do that at their expense.
  5. It is not the dealerships responsibility. Need to deal with the Regional representative, but will not provide the persons number or address, only the 1800 dial a prayer to get a real reply # and to tell them this issue needs to be escalated to a supervisor. Here they tell you they will reply within 24 hrs. We are now entering week 5 of dealing with the dealership.
    Apologize for the rant, Just frustrated.
    Given todays economic situation and low vehicle sales, plus the business environment in response to Toyota’s issues, they would want to be a little more Customer Satisfaction oriented.
    I am grateful that there was very little traffic when this occurred on the freeway and that my wife did not think getting off quickly, because of the problem, would have been the best solution. If it had not been for the 2 miles and her ability to remain relatively calm, it could have been a different situation.
    Thanks for listening!

Everybody should be prepared for this situation, even practice for it. Practice slipping the transmission from drive to neutral. It’s such a simple thing to do but people aren’t prepared and they panic. This is not a new problem, I had the throttle stick open on me in my 1975 Fiat. People expect cars to be perfect and last 25 years with no problems. Stuff happens, be ready for it, and deal with it.

There’s probably a “black box” in your Mercedes that could shed more light on exactly what happened. I don’t know who can analyze the data from it or what good it will do.

I am of the opinion that a 10 year-old vehicle is no longer eligible for free repairs of any kind. I understand your annoyance, your consternation, and your concern for your wife’s safety, but everything has a limit and at 10 years and over 150k miles, I believe that you are past the point where you can reasonably expect Mercedes–or any manufacturer–to provide free diagnosis and repair.

And, despite your statement that you are not interested in “something for nothing”, it does sound like you want a free brake job. If you are hesitating to do this on your own dime, then your claim of being interested in safety does not sound serious. This car is not safe to drive with brakes in that condition. Whether you plan to sell the car, trade it in, or continue to drive it, those pads and rotors (and possibly the calipers) need to be replaced, and the brake hydraulic system needs to be flushed.

If your wife had the cruse control on, and the brake switch failed in the brake off position, then I suspect the cruise control could still be activated when the brake is applied, which I would guess the car would accelerate to maintain set speed even though the brakes were applied. So my question to the OP is.

Was the cruise control engaged at the time she applied the brakes. And if so did she hit the cruse control Cancel button? Which if the brake switch was defective should have shut off the cruse control.

Appreciate your reply. Do not want a free brake job as I have maintained the vehicle to a fault (mechanic can attest to that, not only that it is not a break job that it needs, it needs to be fully replaced due to overheating. What I do want is a reasonable explanation of why something happened and assurances that the failure of a Fail Safe issue will not happen again.
From your reply, I believe you to believe Mercedes should put a warning on vehicles that are reaching 150K Mi. It is not there. Do you believe it unreasonable to believe Mercedes should:

  1. Be able to explain why the vehicle reacted as it did and be able to resolve the issue with the same level of assurance as on any problem.
  2. At least cover the cost for the difference between a regular brake job and full replacement since it was the failure of a Safety System that cause the need for the brakes to overheat.

She does not remember if she engaged the cruise control or not, she rarely uses it when driving let alone short distances.
Mechanic has noted that if the brake switch failed it is supposed to automatically disable the cruise control. So which did come first? Chicken or the egg?

It is unreasonable to expect Mercedes to explain what happened. If they can’t duplicate the problem they can’t diagnose it or repair it. In this case they can only speculate.

What do you mean by “full replacement”? Do you expect them to replace every component of the brake system and then cover the difference in cost between a brake job and full replacement?

You are not entitled to any of this, period. It doesn’t matter if it is a safety system or not.

You are asking way too much of a car than is possible to deliver.

A brake switch is a simple contact button.
When it is pressed, it completes the electrical circuit, which then lights up the brake lights. There are no fail safe, double checking systems in the brake switch to deactivate ANYTHING in the event that it fails.

If its a combination brake switch, where one plunger triggers two sets of contacts, then one side activates the brake lights, while the other side would deactivate the cruise control.

Typically, there are two separate switches that are used. One is a brake switch, and the other is a cruise control deactivation switch.

Again, if the cruise control does not know your wife is pressing on the brake pedal, then it is 100% entirely up to your wife to manually turn off the cruise control system, via the control stalk. Typically, this is done by simply pulling it towards the driver’s body.

The real issue you are having is that this car has unnerved both you and your wife.
You should simply cut your losses, replace the worn brake parts, and sell the vehicle, this way YOU won’t have to worry about it ever again.

Until you do this, you won’t be happy.


There are no fail safe, double checking systems in the brake switch to deactivate ANYTHING in the event that it fails.

I don’t know about the Mercedes in question, but all of my cars have had double failsafes on the cruise control. The ones with fully electric cruise controls had an additional switch on the brake pedal that killed power to the electric motor of the cruise control. So, both switches would have to fail to have a problem. The vacuum activated cruise controls had a vacuum release valve on the brake pedal. My newest cars are throttle by wire. Their backup is a hydraulic brake pressure sensor. I find it hard to believe that Mercedes doesn’t have some redundancy in their cruise controls.
I wonder if there is any possible way that the OP’s wife was pressing the brake and the gas at the same time?

That’s what most of the Toyota black boxes show.

Unfortunately in those cases, you have to rely on the black box recording the right data - if the sensor detecting pedal position goes bad, and the car thinks you have your foot on the gas, the black box will record your foot being on the gas.

Sorry, but you’re entitled to nothing free on a 10 year old, 158k miles car.
At that point in time anything can happen.
This problem could have been caused by an intermittently sticking throttle cable (if not drive by wire), a sticking throttle plate due to not having the induction system serviced on occasion, etc. In a nutshell, normal wear and tear and maybe even neglect.

Assuming I’m reading this correctly, don’t be under the impression that Toyota was and is happily covering their problems in the name of good business. They were covering things up for years until backed into a corner and had to be dragged kicking and screaming into doing the right thing.
Do some net searching about this to determine if I’m correct; or I’ll post a few links if you want.