Hi Moneypit, I agree with your statement but consider this. The reason the ABS light comes on is because the system sees an open circuit (high resistance) at one of the speed sensors. I should have added that when measuring the resistance of a speed sensor it's a good idea to do some twisting or pulling at the stress points of the sensor cables while watching for changes in resistance. The older 124's had lots of problems like this, especially with the front sensors. MB modified the sensors and mounting brackets to fix it. The improved sensors have cables which are reinforced at the mounting points, reducing the chance of failure.

On your car, the weak sensor probably drops the AC voltage to the point where ABS thinks one wheel isn't turning. That's why the pump comes on and the pedal vibrates. I've found that this usually happens at very low speeds, usually during the last few feet before the car comes to a stop. Nowadays the same problem exists (weak sensors) but causes different symptoms. On later model MB's with traction and stability systems the problem often becomes apparent when driving off from a stop, not when coming to a stop. The system sees three wheels turning faster than the fourth and intervenes. The pump can be heard running and the traction indicator either lights up or flashes. The faulty sensor usually doesn't cause any fault codes to be stored because it's still operating within the parameters of the system. The only way to pin-point the problem is to observe the actual values of ALL the sensors while driving, until the fault occurs.