If the control lamp burns constantly, ABS is not ready for operation. A Renault or Bosch workshop will help you find the cause. System errors are accumulated in the control unit and can be read from it using a special device. Thus, only a suitably equipped Renault workshop can determine the cause of the malfunction.
In any case, you can independently check the contact connections of the control device, relays, speed sensors and the hydraulic unit.
Note. Recommendation: even if the ABS warning light is on permanently and the ABS is not ready for operation or is defective, you can drive your Renault 19 without fear. The braking system then works in the same way as on a car without ABS.