mercedes sprinter 313cdi abs light on dash?

hello,i have a problem with my abs ...fitted a drive shaft second hand with the abs sensor intacted,the sensor worked on the van i got it off so that should not be the problem, i rejoined the wires from the sensor to the abs wires running to the hub..reset the ecu...lights went out ..drove it 50 meters lights back on again.hooked it up to the ecu reader...it telles me speed sensor /abs / problem cant figer it out ...the abs sensor should be fine,can any one help