What does it mean?
OBD II fault code is defined as “ Data bus: anti-lock brake system (ABS) control module – no communication”, and is set when communication between the ABS brake system control module and the PCM (Powertrain Control Module) is lost for a specified time. On most applications, the code will be set, and the CHECK ENGINE light illuminated after an interruption in communication of 250 milliseconds (0.25 sec).
During normal vehicle operation, all control modules are connected to a system known as the General Motor Local Area Network (GMLAN). In essence, this system monitors output data from all control modules involved in the operation of the vehicle, and it can be likened to a computer network that monitors operational systems in a complex, automated manufacturing environment where several controllers work together to deliver a final product.
In this case, the final product is the safe and efficient operation of the vehicle. In practical terms, the GMLAN system is a sort of clearing house for all information exchanged between the various modules that control systems like the ABS brakes (and associated safety systems like Traction and Stability Control), the transmission, fuel management, and others.
Moreover, the GMLAN system supervises communication between the various control modules, each of which contains prerecorded information about the type(s) of signals that need to be exchanged between them to ensure efficient and trouble free operation of the vehicle. In addition, the PCM expects to receive regular availability, or status, reports from each control module in the system.
In the case of the ABS control module, an interruption of more than 250 milliseconds will set the code, since an interruption in communication can affect associated systems like Traction and Stability Control, since these safety systems share some of the circuitry (and functionalities) of the ABS system.
To facilitate fault diagnosis, each availability report contains a unique code that identifies the control module that sends the report, thus enabling the PCM to identify defective modules correctly. The image below shows a typical ABS actuator with the control module contained in the black plastic box.
Note the pins in the connector; on this particular unit the ABS brakes, Electronic Traction Control, and Electronic Stability Control share the control module, hence the large number of pins in the connector. Note that this example is only representative of ABS actuators, and that designs, locations, and functionalities differ widely between applications.