Big Dave, auto trac is not just push button 4x4 it is all time 4x4 that has a computer control telling it which wheel to send the power too when there is slippage including front wheels. I have a similar system on my ford and it is all time 4x4
Sorry to politely disagree. I worked for GM up through 1999. I spent two day of training on the autotrac system and it works just as I discribed. GM did offer an "all wheel drive" on the smaller trucks/suvs but not on the full size. Quoting from the training manual:
Auto 4WD: The transfer case control module engages the front axle. The transfer case utilizes a clutch pack similar to an automatic transmission. Under certain conditions the clutch pack is applied, locking the prop shafts together. If the transfer case control module detects a difference in front to rear wheel speed, it reacts within 200 milliseconds to apply the clutch pack.
2HI: The transfer case control module disengages the front axle and operates in 2WD HI only. In this mode the transfer case (NV246) operates exactly like previous 4WD systems in 2WD HI mode. Torque is transmitted to the rear axle only.
4HI: The transfer case control module moves the PWM motor to apply the transfer case clutch pack. It engages a clutch brake to hold the clutch pack in position, then engages the front axle. In this mode it operates exactly lke previous 4WD systems in 4WD HI.
4LO: the transfer case control module reverses the PWM motor. The motor moves the shift sector in th opposite direction into 4WD LO.
Neutral: Disengages both axles for towing.
A mode shift from 2HI to 4HI or Auto 4WD can be cmpleted in any transmission gear position and at any vehicle speed.
2HI, 4HI, or Auto 4WD to 4LO
4LO to 2HI, 4HI, or Auto 4WD can be accomplished only if:
The engine is running
The transmission is in neutral
The vehicle speed is below 3 mph
The transfer case motor/encoder signal is present.
Hope it clears it up.