This bulletin involves 2010-2016 GMC Acadia and 2017 GMC Acadia Limited vehicles. Some customers may comment that there is a clunk/thump noise on acceleration once after each key cycle that may be mistaken as coming from the suspension or engine compartment.
This noise may be caused by the second phase power-up self-test of the ABS. This self-test begins when the vehicle is driven at a speed greater than 9 mph and the EBCM has not detected any traction control/vehicle stability related concerns. This phase of the power-up self-test may be heard or felt by the driver.
The system solenoids and ABS pump motor are commanded ON and OFF to verify the proper operation and the EBCM verifies the ability to return the system to base braking in the event of a failure. The EBCM isolates all of the wheels by briefly closing the four isolation valves, and occasionally the driver may detect this by experiencing a momentary hard pedal.
In most situations, the ABS self-test noise should be considered normal. If a particular vehicle exhibits a noise that is louder than a comparable vehicle, the brake lines should be inspected for contact with the underbody, or the lines may be contacting one another. If any contact is present, the lines should be isolated to minimize the noise produced during the ABS self-test event.