ABS & traction control warning on Mercedes-Benz CLA & its solution

Sometimes the warning signs vanish on their own and then they light up again.

BHPian vattyboy recently shared this with other enthusiasts.

Hi, my fellow bhpians,

For the past couple of days, my Mercedes-Benz CLA was showing an error of - ABS and Traction Control inoperative on the instrument cluster. This warning vanishes sometimes on its own and sometimes it comes back again.

When this sign comes, the car goes into hazard mode in which the power steering deactivates, the Cruise control shuts off, and gear shifting is not smooth anymore. It becomes jerky. But there is no restraint on the speed limit.

Today I decided to get my car diagnosed at Mercedes ASS.

SA began with the OBD scan and found a fault in the front axle left side rpm sensor, he figured out that it needs to be replaced and I gave permission for the same.

The total cost of replacement of the 1 rpm sensor came out to be Rs 5200 (The cost of the rpm sensor is Rs 2700 and Rs 2700 for Labor).

Speaking about the overall experience of the ASS, I found their work to be very lazy. I reached ASS at sharp 10 am and they took nearly 6 hrs to complete this small job.

Here's what BHPian Dr.AD had to say about the matter:

Thanks for sharing your experience and I am glad you found a quick solution.

This is actually a very common problem with most Euro cars. That warning you saw on the instrument cluster almost certainly means a failed speed sensor. It has happened to many of us and has been documented many times on this forum.

For example, here is my experience. And here is the experience of @graaja who had the exact same warning and the exact same solution.

I know of at least five other cases in BMW and Mercedes where this happened. So yes, this is extremely common and nothing to worry about.

However, a key thing to note is that once one-speed sensor fails, it is very likely the other side sensor will fail soon too. This is why BMW changes these in pairs (they changed both of my rear speed sensors when the fault was in one of the rear speed sensors). In case they changed only one speed sensor in your case, be prepared to face a similar error again and do not get surprised that time.

Here's what BHPian Jeroen had to say about the matter:

Shocking to hear this, relatively simple job took so long. On some cars, it can be quite a problem getting these sensors removed as they get very stuck. E.g. VW Golf, especially some of the earlier GTIs had this problem.

ABS and RPM sensors are one and the same I believe. An ABS sensor measures the rotation speed (technically rotation angle) and thus the speed of the respective wheel.

I always find Mercedes has some very confusing terminology for some of its parts.

The part shown in your photograph is officially known as an electrical wiring harness. But it is an actual ABS sensor with the restive electrical harness and connector moulded onto it.

As you can see in this Mercedes part list, it is also known as the ABS sensor or front speed sensor.

Vehicle speed tends to be measured with a different sensor, usually mounted somewhere on the output shaft of the transmission. Although on some vehicles with four ABS/Speed sensors sometimes these are used for vehicle speed as well. Some systems use both and the speed is a derived measurement from 5 different sensors. Not quite sure why they would make it that complex, but there you are.

Check out BHPian comments for more insights and information.