ABS Stands for Antilock Brake System. That means that the tires wont lock up under heavy braking, the ABS sensors are three, one on each front wheel and one on the rear diff pinion. If the system is working as it's supposed too the sensors monitor the wheel speed by a method of small electric current produced by a magnetic field from a small magnetized ring the sensor reads from. If the electric current from one wheel gets too low the computer controlling the system reads it as a wheel locking up and activates a high pressure electric motor inside the ABS valve box. The electric motor maintains maximum braking power while electric valves alternate the brake fluid between the locking wheel and a bypass, as soon as the bypass opens the brake fluid pressure to the wheel drops and allows the wheel to turn for a split second, as soon as the bypass closes the wheel brakes again. If your mechanic does not know this than maybe he should not be fixing something composed of more than one object.

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.