Mercedes-Benz W124 ABS light comes on

The most common causes are a broken wheel speed sensor wire, dirty or rusty sensors and toothed rings, and too low an electrical voltage. The wire usually breaks at the points where it flexes. Brakes are safety-critical, so have a workshop check the result.

There are three ABS sensors: one at each front wheel and one at the rear. Check the wiring, clean the sensor and toothed ring, and measure the resistance. The front sensors are about 1 kilo-ohm, the rear sensor slightly less; readings vary, so compare the right and left sides.

Yes. A weak battery, a poorly earthed hydraulic unit or the overvoltage protection relay can turn the light on, and the battery voltage must be over 13 V. Tyres of different sizes at the front and rear or a large tyre pressure difference change the sensor pulses. The brake light switch contacts can also cause the light.