Euro model 1992 500 SL

The ASR light is allways on once I start the engine and move forward 20 Feet. The ABS system is not working -= when I slam on the breaks to test it the wheels lock and skid.
The MB dealer swaped out the ABS control unit and changed the ABS sensors - still the ABS did not work and the ASR light still goes on...The MB dealer is sugesting to change the "potentameter"?? and the Activator Motor .... now ... at a high cost $2400 ... ?

Any suggestions ...?
Thanks
RA