2010 GMC Acadia Owner's Manual - Page 284

9-28 Driving and Operating Drive Systems All-Wheel Drive With this feature, engine power is always sent to all four wheels. It is fully automatic, and adjusts itself as needed for road conditions. When using a compact spare tire on an AWD vehicle, the system automatically detects the compact spare and disables AWD. To restore AWD operation and prevent excessive wear on system, replace the compact spare with a full-size tire as soon as possible. See Compact Spare Tire on page 10‑77 for more information. Brakes Antilock Brake System (ABS) This vehicle has the Antilock Brake System (ABS), an advanced electronic braking system that helps prevent a braking skid. When the engine is started and the vehicle begins to drive away, ABS checks itself. A momentary motor or clicking noise might be heard while this test is going on, and it might even be noticed that the brake pedal moves a little. This is normal. If driving safely on a wet road and it becomes necessary to slam on the brakes and continue braking to avoid a sudden obstacle, a computer senses that the wheels are slowing down. If one of the wheels is about to stop rolling, the computer will separately work the brakes at each wheel. ABS can change the brake pressure to each wheel, as required, faster than any driver could. This can help the driver steer around the obstacle while braking hard. As the brakes are applied, the computer keeps receiving updates on wheel speed and controls braking pressure accordingly. If there is a problem with ABS, this warning light stays on. See Antilock Brake System (ABS) Warning Light on page 5‑20.