The GM trucks incorporate a hill-hold feature into the stability control system. Hill-hold is supposed to detect when the vehicle is on an incline with the brakes applied. When the driver releases the brake pedal, the valves in the stability control unit retain the pressure in the brake lines for up to 1.5 seconds. Once the timer expires or the driver applies the accelerator, the valves release the brake pressure.
During testing, Levine discovered that the system was not holding the pressure on the 7.2 percent hill for any of the GM trucks and notified the engineers on hand. Oddly, a later test found that the system was working properly on the steeper 16 percent grade. GM engineers investigated the issue with engineers from system supplier TRW and found an incorrect calibration value in the control software for the accelerometer used to detect inclines. This prevented the system from correctly detecting the smaller hill.
The code was corrected and updated software began rolling out to the production line this past Monday, July 19. The same software update is also going out to dealers so that trucks in stock on lots and in for service can also be updated. GM is still considering whether to issue a recall or a technical service bulletin for trucks that have already been delivered to customers.
[Source: PickupTrucks.com, TRW]