Let me share with you my experience with this. One day my ABS light came on and off she went to the mechanic. He tested the ABS hydraulic module - it was bad. I purchased another module (used) and brought it to mechanic. Module good.. ECM unit had a problem. The partition of the ECM that signals to the ABS system was out. I purchased another ECM (used). He put in the new ECM with the old ABS module -- still no ABS. My truck needed both parts to correct the problem entirely. I asked the same question, whether something happened between the two parts that caused them both to malcfunction. My ECM in every other way was fine. Just the ABS signal was affected.