453 Active Brake Assist Fault

Mod Details

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You get in your car, start it up and get stabbed in the eyes with a big error message about the active brake assist being inoperative. You click OK and the message goes away.

However, there’s still an error light showing on the speedo.

The issue is the front radar sensor. It’s used to warn you if you are too close to the car in front.

The radar sensor has 2 values, vertical and horizontal. These 2 values have to be within a set range or the car will spaz out.

I used MB Star Xentry Diagnosis. I’m convinced I saw the settings also displayed somewhere on DDT4All but I can’t find the page again. If you find it, let me know.

The radar sensor is module A90 in Xentry and the smart specific error code for an incorrect adjustment is C1B0878.

Once you go further in-depth on the unit, select actual values and the adjustment values, you’ll see what we are dealing with.

This car was lightly crashed at the front so the bracket on which the radar is mounted was slightly bent, hence why you can see the vertical axis is out of spec.
Vertical is +/-3° and horizontal is +/-2.5°

To adjust the vertical, you turn the top left adjuster.
To adjust the horizontal, you turn the bottom right adjuster.

How do you turn it? Annoyingly it’s not a standard size. You need a 3.5mm socket (closest in American is 9/64″). Let’s be honest, you don’t have one.

I bought this one from Amazon because it was cheap and arrived the next day.

So, I hear you ask, which direction do I turn it and by how much? Bad news is, you have to guess and eventually you’ll get it right.

Here is the problem. The values on the screen don’t change as you adjust the sensor. So you can’t dial it into position in your garage. The car measures the angle of the sensor over time. Xentry suggests that it takes about 30 minutes of driving to calculate the sensor’s position.

So, write down your measurements and make ONE adjustment (make a note of all of your adjustments). In my example, I adjusted the top left adjuster by 1 full turn clockwise. I then drove the car as normal for a couple of days. Then I checked again and made another adjustment. Obviously if your 1st adjustment made it worse, go in the other direction. I continued doing these small adjustments every weekend.

Suddenly 1 day I started the car and the error screen didn’t come up and the error light wasn’t on. I checked the values and I’d managed to get it close enough.

The error never came back.