All of your symptoms except one lead me to believe you have a speed sensor problem.

The one symptom that messes it all up and suggests an ECU problem is that the light is on and "both systems are working." That is about impossible. I'm guessing you are wrong when you say they are working. If not you have two problems: one for who is turning on the light and two for why the controller doesn't recognize the light is out of its control. The controller will not work with the light on or disconnected.

Have you tested the ABS on some wet grass? Does it work? What other evidence do you have of it working since it is a non intrusive system. Does the traction control triangle light when accelerating on wet grass?

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician