ABS Communication via CAN

SPECIAL NOTES: Some online sources, including one known official GM source list code U2108 with the following definition “U2108 – “CAN Bus Error ABS (4L30-E Transmission) Conditions”, but it is not known how, when, why, or where this definition originated.

Nonetheless, it is important to note that the 4L30-E transmission variant referenced in this definition is/was hugely problematic and unreliable, so it is probable that some failure modes on this transmission sometimes triggered or contributed to the setting of code U2108 on some GM products that date from the early 2000s.

Based on the above, we strongly recommend that you verify the transmission variant on affected vehicles from this era if code U2108 is present on older GM products before attempting a diagnosis of code U2108. Thus, if an affected vehicle is fitted with the 4L30-E transmission, the wisest course of action would be to seek professional assistance from a specialist transmission repairer to eliminate or confirm the transmission as the most likely cause of code U2108. END OF SPECIAL NOTES.

OBD II fault code U2108 is a manufacturer-specific trouble code that is (most commonly) defined by carmaker General Motors as U2108 – “Lost Communications with ABS/TCS Control System” and is set when the BCM (Body Control Module) detects that it has lost communication with the Electronic Brake Control Module (EBCM).

All modern vehicles, including General Motors products, use up to 50 (or sometimes more) control modules that monitor, control, and manage every possible aspect of the vehicle’s operation, and while all the control modules on any given vehicle are connected with each other via serial communications networks, some networks, such as CAN bus networks, are “stand-alone” networks that monitor or control critical functions.

We need not delve into the fine details of how the various communications networks on modern vehicles interact with each other here, but suffice it to say that the information some networks exchange is more important to the vehicle’s safe operation than other types of information.

For instance, the transmission of safety-critical information about the operation of the vehicle’s ABS system and other safety systems like Traction and Stability Control, which use the ABS system to work, will always take precedence over non-safety critical information. In terms of operating principles, all control modules that share a communications network have unique identifiers that all other modules on the network recognize.

Moreover, all the control modules on a shared network are also a) programmed to transmit a specified type of information periodically to all the other modules and b) to acknowledge receipt of such specified information transmitted by all the other modules on the network. While this arrangement might sound cumbersome, it is nevertheless an effective method of ensuring that all the control modules on a shared communications network remain in contact with all the other modules on the network.

For instance, if the traction control module stops working for any reason, all the other modules on the network will report the malfunctioning module to the BCM and each other when its periodic transmissions fail to arrive at modules that are directly associated with the traction control module.

This reporting function is programmed into each control module on a shared network as a safety precaution since the failure of one control module on a shared network may affect the operation of some or all control modules on that network.

For example, if the traction control module or the wiring between the traction control module and the BCM fails, the ABS module (or BCM) will typically disable closely related systems like stability control, adaptive cruise control, lane keep assist, and electronic brake distribution systems. However, as a general rule, whether these systems are disabled largely depends on the nature of the failure. In some cases, the BCM might even dis