The sensor (drivers side front) for the speedometer provides a signal to the ABS processor. The output is a square wave. You have to remember it goes thru a lot of wiring and connectors. I guess a test would be if the automatic conrol on the radio works, that would indicate there is a signal from it. MB gives values that can be measured on a volt meter or scope as the wheel is rotated. It also cold be a bad speedo since it is nothing but a meter getting voltage. The abs module is in the box at the base of the windshield in the engine compartment.