I have a 2003 SLK, which without warning when applying the brakes in a normal slowdown (35-50 MPH) will suddenly and violently lock up the brakes, overriding the ABS system and squealing the tires. The violence of this action is so severe that the car decelerates from the approx. 45 mph to less than 10 mph in a split-second. As soon as this occurs the vehicle senses that it has done something wrong and releases the braking action. It is extremely dangerous--especially if someone is behind you when it occurs. There are no trouble lights that appear and no codes are present when the dealer runs diagnostics on the car. Any suggestions? This has happened approximately 8 times in the past two months.

 

Very interesting!

A few more details like mileage, age, service history etc may help.
Anything else that you can add? For instance does this problem only occur when the brake pedal is / is not pressed. Hot/cold wet/dry weather. Going straight or around corners. etc?
Is your car manual/automatic.

Does just 1, 2, 3 or all brakes suddenly lock up?
is your foot on or off the brake pedal when this occurs? (Not sure how you will be able to answer this other than marks on the road)

Don't know why but something is saying master cylider return valve fault to me but would like to see if you can add anything further.

Stuart

 

Additional information requested is as follows: Vehicle has approx 51,000 miles, no exceptional service history except all required maintenance performed on schedule. One issue that I understand is common to this model was problems with the "check engine" light which was resolved by a one way valve replacement in the 02 sensor. The problem always occurs with the brake pedal depressed, although it has never occurred in hard braking action, always in slow down from approximately 45-35 mph. The incidents have always been in dry conditions in temperatures from 66 to 92 degrees farenheit. The incidents have occurred in going straight or in turns basically on level or slight downhill grades. My car is automatic with select transmission, although I always use the automatic only mode. Feels/sounds like it is both rear brakes that lock up and the braking action when the violent execution is initiated is equal to both sides of the automobile (doesn't pull or grab to one side or the other)

 

This is indeed a fascinating issue. Thank you for the extra information.

What makes you think it is the BRAKES that are being applied. Could it be a sudden and uncalled for transmission downshift or similar?

I have experienced similar transmission issues although finding the cause has eluded me.
I cannot think what in the ABS braking system could cause your symptoms particularly at this mileage.
I asked whether the problem was with the pedal pressed (lightly assumed) since this eliminates the possibility of a pressure build up in the system due to a non operational return valve in the master cylinder.
I wonder if the servo actuation valve could be bad? Mmm..

The whole concept of the ABS is that the wheels cannot lock (the modulator lets the pressure out of the affected brake line).
If required pressure is re-applied by the auxillary electric pump but once again as soon as lockup is approached the pressure is relieved.
If you had a problem with lock up under heavy braking that would be entirely different scenario to what you have described.

AND you state that you don't have an ABS or other sort of light on. (I assume the ABS light works at startup.

As I have stated the other possibility is some sort of random transmission problem.

Finally I don't understand the following quote "One issue that I understand is common to this model was problems with the "check engine" light (OK) which was resolved by a one way valve replacement in the 02 sensor."

 

I have not noticed an other than smooth transmission gear change at any time in the car. This violent braking action seems to be that and that alone, the rear brakes do lock up for a split second and cause the tires to squeal and almost as quickly and violently as it began it ends, but the car has decelerated from 35-45 mph to less than 10 in that split second. The ABS light does not come on that I have noticed but both the EBA and ABS lights function properly at start up as do all warning lights on the dash. Also, when this happens there is none of the pulsing that you normally feel on your braking foot with ABS systems when they activate. The brakes lock violently, squeal the tires and violently decelerate the car in a second or less and then everything returns to normal. The occurence is approximately once a week on the average. Happens so fast that you don't have the opportunity to be scared about it until several moments later, when you start the what ifs-- like lucky no one was close behind me, etc. Problem with the check engine light was with a one way valve which allowed oil from the engine to flow back into the wiring harness that supported the check engine light which shorted out the curcuit and turned the light on. Finally, as an afterthought the braking incidents always occur in normal slow-downs not in heavy braking actions.

 

Hi,
OK so you are adamant that it is the BRAKES are being applied.
The only possible source in the BRAKE system that could do this is the BAS (Brake Assist System) also known as EBA (Emergency Brake Assist).
Below is an extract as to how the system works...............

The Brake Assist System (BAS), an electronic system by Mercedes-Benz, interprets the braking behavior of the driver and initiates the full braking effect when it identifies an emergency situation. This reduces the braking distance substantially.
Here is the engineering foundation for BAS:
Conventional braking systems usually use engine vacuum to increase braking capability. Instead of relying solely on vacuum power to provide effective brakes, an electric pump pressurizes brake fluid to provide power assist for BAS (and the necessary brake force for 4ETS.... 4 wheel drive Traction Control if fitted).
The pressurized brake fluid is stored at about. 180 bar (2600 psi!) in a reservoir known as the accumulator. From the accumulator the high pressure brake fluid is supplied to the hydraulic operating cylinder in the master cylinder assembly. Commands to utilize the pressurized fluid comes from either 4ETS or BAS. When the brakes are applied and BAS deems it necessary, a lever connected to the brake pedal releases varying amounts of the high pressure fluid to assist in stopping the car.

Since this failure has significant SAFETY ISSUES I urge you to get the car transported to a main dealer immediately.

What I don't understand is how the BAS could be triggered on light brake pedal application. Looks like the ABS, or possibly the BAS, is however recognising the problem and letting the brakes OFF again as quickly as it is able.
THE QUESTION REMAINS WHY IS THERE NO ABS / BAS MALFUNCTION WARNING LIGHT. MAYBE THE SYSTEM HAS STORED A DTC (Diagnostic trouble Code) in memory which can be read by the main dealer.

Sorry I can't be of more help. Let us know what the dealer says.
Do not leave this problem unattended to.
Good luck,
Stuart

 

First, thanks for all the feedback. Yes, I have already taken the vehicle to the dealer three times for this deficiency. They have been unable to replicate the incident, have taken a look at possible diagnostic trouble codes and nothing appears in their diagnostics which indicates any trouble with any system on the vehicle except the problem that I had with the check engine light that I referred to in previous emails. I was in the hopes that this forum might shed some light on a very disturbing event which has not been able to be rectified by the dealer. Thanks again, best regards.

 

Will continue to think about this and do more research about exactly how the BAS works and the cause of the fault. Your symptoms are so specific and significant that I cannot credit that your dealer is unable to help even if only by contacting the factory. I suggest you put your fault in writing to MB thru your dealer just incase the "worse" happens should you choose to continue to drive which I have already stated I cannot support.
Where are you located?
Stuart.

 

Stuart--I had already reached much the same conclusion. Coincidentally the incident occurred again this morning, basically same parameters as previously described. I am in the Cincinnati, Ohio area. Thanks

 

somewhat similiar to mine. my 99 e320 was fine no issues. i got a flat and put on the temp tire for about 3 days. then, i got a tire put on the original rim (205/55/16). everything is fine as i head back towork. the following day, im headed to work in the morning and i felt a kind of slide as i turned a corner. (as if i was on gravel). the bas/asr light came on. i turned the car off and on again and still the same light. the light continually comes on now. i checked the scanner and found no codes with an obd II scanner. what now. please give me some advice about anything i can do other than take it to the stealer. lord knows .........