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01-13-2011, 12:41 AM #1
What does GM's "Automatic Four-Wheel Drive" actually do?
I thought this may have been posted somewhere else, but I'd be darned if I can find it anywhere. Driving an '07 Silverado with the new body style (if it even matters), and it's got this nifty thing that GM calls "Automatic Transfer Case". What on God's good Earth does the auto function do? I fully understand the functions of 2 and 4 wheel drive systems, when to use them and when you can royally eff up the transfer case.
I've asked around some shops, and one guy said the auto function engaged the front axle and transfer case but didn't supply more than a few % of the available torque from the engine until the rear wheels began to slip. Another shop told me that it only engaged the front axle if it sensed wheel slippage in the rear and it was a strait 50/50. A third shop told me that it pulsed the gears in the transfer case to engage and disengage the front axle (with the front hubs locked in), and increased the frequency of this action if it sensed that the front and rear were not at similar rotation speeds. Who, if any, actually knows what really happens? I ask because I put about 450 miles on it a week with my new job, and you don't call in just because it snowed the night before. it'd be good to know what my truck is doing before I need a new pair of skivvies.
Additional information, the description of the auto setting from the owners manual reads as follows, and no it's not a typo: "AUTO (Automatic Four-Wheel Drive): This setting is ideal for use when road surface traction conditions are variable. When driving your vehicle in AUTO, the front axle is engaged, but the vehicles power is sent only to the front and rear wheels automatically based on driving conditions. Power is sent to the front and rear wheels automatically based on driving conditions. driving in this mode results in slightly lower fuel economy than Two-Wheel-Drive High." If I had to guess what happens, is it's some sort of variable torque transfer case (because it's not really a differential) and it applies more up front only as needed? If it didn't, I could see the same issues happening as if you drove on pavement without the center differential.
Am I seeing this right, or just wandering lost through the woods on this one?
Thanks guys!*Insert witty signature here*
01-13-2011, 08:36 AM #2
Look up corporate bulletin #01-04-18-001F titled driveline characteristics for all-wheel drive and four-wheel drive. This explains it all very well. I found it in the chiltons web site that is a sticky somewhere on this site. In a nutshell the AUTO 4X4 has a center differential with clutch packs (think POSI-TRACK rear end) that will allow for different speeds of front and rear axles and in 4X4 Hi and 4X4 Lo this differential is locked.
I suppose the amount of power transferred from front to rear depends on how tight the diff. is and the difference of traction from front to rear.
Last edited by dpeter; 01-13-2011 at 08:44 AM. Reason: additional thoughts2000 GMC YUKON SLT, 5.3L tow pkg, G80 rear/w 3.73 gear
01-13-2011, 09:54 AM #3
Thanks dpeter! I can usually find any answer I need, but I never thought of calling it Active-4WD or AWD-On-Demand in my searches. I found two bulletins with that number that didn't explain much but do mention the clutch packs. One is here from '03 http://www.denalitrucks.com/documents/TL_Jan03.pdf which called it Active-4WD. and the other is a doc download (dated Feb-08)here http://www.justanswer.com/uploads/BO...iscription.doc referring to the system as AWD-On-Demand and giving a little more information. This one also says you can use the auto system on drive pavement without the wheel-hop in corners because of clutch pack. That's good to know. Helps me select which mode to be in for the situation.
Posi! I wish someone would have said that before on any random site. I would have done the standard 'AH!' of understanding and went on my way. And for those of you following along (or don't fully understand positraction / limited slip differentials.) Visit http://auto.howstuffworks.com/differential.htm I love this site for explanation of complex concepts. The visual aids save the day!
01-13-2011, 02:49 PM #4
This feature is explained in your owners manual. In a nutshell it engages front axle however does not send power to it until computer detects slippage in rear tires. I dont use this feature much but will use it on the highway with slightly icy conditions or when it is raining heavily and hydro-planning is in issue. But for the most part in my situations, if I need 4wd I need 4wd .. not of the automatic crap. I think GM took this feature out in later silverados correct me if I am wrong....
01-13-2011, 04:09 PM #5
yeah, I felt it kind of explained what it's general purpose was, but I was curious as to the how more than anything.
Side note, you've the same cab style as me... how much room does the pair of subs take up?
01-13-2011, 05:35 PM #6
You're welcome bornagainbiker. The link to justasnwer is the same document I referenced. I have owned 4 wheel drive vehicles of many types and they have been called many things by their makers...Auto trac, Activetrac, suretrac, smart trac, or whatever will get you to buy theirs and not the others. How they engage and to what extent they engage has never been satisfactoraly explained to me, only that "It knows".
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