For 1999 all Hummer models get a new traction control system and anti-lock brakes to make driving easier both on-road and off. The traction control system, called TorqTrac4 (or TT4), automatically engages when it senses a wheel slipping. The system uses an electronic sensor on each of the four wheels to determine when a wheel is spinning. The sensors transmit a signal to a unit that determines how to regulate brake pressure. If the wheel speed is greater than the vehicle's speed, the unit applies brakes to that wheel; this engages the traction control system and transfers torque to the wheels that have traction. If the unit senses the brake is locked, it engages the anti-lock brakes.
Other refinements include finer wires in the heated windshield to reduce reflections at night, a revised fuel pump mounting to reduce engine noise, a modified instrument panel with several switches that are now illuminated, and a redesigned and simplified wiring system. Last, but not least, the windshield washer reservoir now holds more fluid so that drivers can play in the mud longer.