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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22737
Experience:  BEng hons Mech engineering, in auto industry 22 years
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ABS light on ALK200 4 yr ol 40k miles comes and stays on Why

Customer Question

ABS light on ALK200 4 yr ol 40k miles comes and stays on Why and is it safe?
Submitted: 1 year ago.
Category: Mercedes
Expert:  Matt replied 1 year ago.


If the ABS light is staying on then this would indicate a fault with the system which shouldn't effect the regular braking performance so the car is still safe to drive.

The quickest route to repair is to have the car plugged into a diagnostic machine which will read off the fault code stored in the ECU (providing the light is still on at this point) and indicate which sensor has failed or whether the fault is more serious.

However with ABS systems 99% of the time its a problem with one of the wheel speed sensors and I'd first check the connections at all four corners first for any signs of corrosion or chafed wiring.

The sensor is located in the wheel hub so you'll have to remove each wheel in turn to inspect them all.

Also check that the toothed wheel that the sensor points to is clean and free of any debris.

you can check each sensor's function by plugging the two output wires into an AC voltmeter and spinning the wheel quite quickly, you should get a voltage signal that increases with rev's.

Its possible that the fault is only a bad connection to one of the wheel sensors.

This may be remedied by removing each wheel in turn and unplugging each sensor and cleaning the contacts with a squirt of electrical contact cleaner. Then refitting.

and with mercedes If the battery has been disconnected then it is necessary to perform a zero point calibration of the steering angle sensor. To do this you need to start the car and turn the steering wheel all the way to the left and then all the way to the right 5 or 6 times

Customer: replied 1 year ago.
Thanks I am not a very technical person but will try above we were due to go away tomorrow for a few days in it So is the braking actually defective I guess is the question or should I postpone and take to Merc dealer
Expert:  Matt replied 1 year ago.


I'd double check that the brake fluid is up to the mark and be mindful when driving that the ABS will not be there to help stop in wet or slippery conditions

but otherwise yes you should be fine

and don't forget to try the zero point procedure I mentioned