Mercedes 420 SEL ABS brakes are always on, even at very slow stops...

The optical sensors need to be cleaned. They are on the inside of the wheel and are accessed by with an allen wrench. They get metal shavings on them and it makes them do that.

Yup pretty common problem. Sensor is magnetic and seems there is no shortage of metal filings around those brakes - guess what happens next? :^)

The (front) wheel sensors have the cable (look like TV cable) routed near the shock towers.  Find them and disconnect.  Then measure the resistance.  Both sensors should be less than 1400 ohms and the reading should be close to each other.  The rear sensor (only one) is on the differential which has different reading (and I forget). The system works this way: when wheels turn, the sensors develop small voltage.  The computer constantly compares the voltages and engages ABS when voltages differ over a threshold.  If the sensor is out of range, the voltage is out of range too.  Even when wheels turn in same speed, the voltages make computer 'think' they are not. The voltages generated by the sensors can be measured (on rollers) but it is not easily done on jacks.  If I remember right, the procedure can be found in the service manual. Bert Cross wrote: My 1986 420 SEL's brakes seem to have the ABS always on.  At the end of a stop (fast or slow), the pedal shudders at the end of the stop. Neither my mechanic or I can figure it out.  The sensors seem to be working.  What else could it be? Thanks, Bert

Try clean the teeth behind the disc