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Noob questions: 4WD drive train

Requesting a detailed explanation of the design/operation of the 4WD drivetrain

Noob questions: 4WD drive train

Postby srr2 [OP] » Aug 06 2011, 3:30am

Well, this was an interesting buying experience. GM dealerships, at least those I visited, aren't the "used car" types of operations I anticipated. On the other hand, not one single salesperson I talked to (out of eight) was what I'd even loosely consider a "car guy". None of them knew anything about any technical details. ("How does the TPMS system work?" Got a wide variety of answers to that one, all of which were wrong, and a couple that must have been a product of a vivid imagination.) So in that respect, it was kind of unsatisfying. If these salespeople weren't selling cars, they'd be selling men's suits or insurance, most likely with the same lack of in-depth knowledge.

Anyway, one of my questions to everyone was "What, exactly, is in the 4WD system?". OMG. Everything but the kitchen sink. So I'll pose it here, since I still haven't been able to find a concise clear explanation of what's there and how it works together.

The truck is a 2011 Silverado 1500 with 5.3L AFM engine and electronic Auto transfer case. It has the AllStar package on it which includes some trailering options and the Eaton auto-locking rear differential. I did manage to find a post here that included a YT video from Eaton that did a great job of explaining how that works. Now I'd like to learn more about the other parts. So here are my questions:

1. Front axle
1a Differential: What is it? Ordinary open diff?
1b Hub locks: Are there locking hubs? How do they work? I've had "auto locking" hubs on a truck 25 years ago. Those required low-speed front-back movement to engage/disengage. There doesn't seem to be anything like that involved here. Are the hub locks, if any, inside the differential case or on the wheel? If there are no locking hubs, does this mean that the front drive components are always turning while the truck is moving? If not, why not?

2. Transfer case
2a What's in it? Differential? Locking differential? Viscous coupled diff? A straight shaft with clutches?
2b The various "modes".
2WD -- front drive shaft disengaged/freewheeling, right?
4WD auto -- supposedly it engages 4WD when it decides it's necessary. How? What exactly does it do? When does it engage/disengage? Is the transfer case operating as a differential in 4WD mode?
4WD "high" (AKA "Snowplow" setting) What's that, and what makes it different from 4 auto? While playing around with this on a test drive, I noticed wheel hop on turns, so it seems that the transfer case is locked front/back on this setting.
4WD "low" What makes it low? Is it a bona-fide low "stump-pulling" range? Engaging this mode sounds a little tricky if you take the owner's manual literally. How do YOU engage it?

Any other insight into the operation of the drive train would be greatly appreciated.
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Re: Noob questions: 4WD drive train

Postby lzn197 » Aug 06 2011, 5:00am

1a. Open diff.1b. No locking hubs. You have a front differential actuator that locks the right axle shaft to the differential at the same time your transfer case puts power to the front driveshaft. When you are in 2 wheel drive, both front axle shafts turn but are not mechanically connected to the front drive shaft.
2a. It has an electronically controlled clutch that can apply X percent power to the front driveshaft.
2b. 2 wd. Disengaged and freewheeling..............yes.
4wd auto. The front axle is engaged and The computer looks at % difference in speed between the front driveshaft and the rear via wheel speed sensors. The front axle maybe gets 5% torque via the transfer case clutch. When the speed differential between the fron and rear exceed X percent, the clutch adds more torque till the speeds then agree or go to 100% locked it 4 wheel drive.
4wd high is both front and rear axle getting 100% power.
4wd low. Inside the transfer case is a 2 to 1 reduction gear. You get full 4wd but now all power goes through the 2 to 1 gear reduction. That is why you have to almost totally stop the vehicle to get this gear engaged. Most people rarely use or need this gear. Basically it will double the amount of torque going to your axles. Treat it with respect. To engage, have your vehicle rolling real slow, put the transmission in neutral, select 4 low and wait for the little clunk then put the transmission in drive. I hope this helps. For more information:
1. Read your owners manual.
2. Purchase a service manual from gm. Don't buy the hanes manuals. They are junk.
3. Take some automotive classes.
4. Look through some youtube videos.
PS. I took the time to answer your questions. Will I be one of those persons you do not believe and you continue to ask others?
Gordon
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Re: Noob questions: 4WD drive train

Postby srr2 [OP] » Aug 06 2011, 5:30am

Thank you. That was exactly the kind of authoritative answer I was hoping for. But jeez, dude, why the chip on your shoulder? What did I do to deserve this bit of snark: "I took the time to answer your questions. Will I be one of those persons you do not believe and you continue to ask others?" Since I joined here less than 24 hours ago, I haven't had much of a chance to disbelieve too many people, ya know?

I did read the owner's manual on this subject. It has no information in it concerning the actual hardware. The only thing you find there is Push This and Turn That. There's nothing that gives you useful guidance into choosing one mode over another and what the exact differences are between them. I appreciate that you filled in those gaps quite well. Thanks again.
Original Poster
srr2
Firing on 8 cylinders [L1]
Firing on 8 cylinders [L1]
Posts: 81
Topics: 9
Blogs: 1
Joined: Aug 05 2011, 12:39pm
Years of membership: 2 years of membership2 years of membership
Location: Lehigh Valley, PA
Year: 2011
Make/Model: Chevrolet Silverado
Extra Info: 1500 Z71-4x4 5.3LAFM
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Re: Noob questions: 4WD drive train

Postby lzn197 » Aug 06 2011, 5:57am

Sorry for the chip. I have been repairing vehicles and answering questions for over 40 years and sometimes I can get a little grumpy. Sorry. Yesterday I had a friend call me and ask why his truck has some intermittent electrical issues. I answered his questions, site unseen and the first thing I asked was to check the battery cables. So tonight he calls me and says the truck still is doing it, intermittent electrical problems and now it wont start. I asked "did you check the battery connections". He says " I talked to so and so and he says that isn't the problem". So I grab my snap on 5/16ths strap wrench, go to his location, open the hood, twist the loose cable and ask him to start the vehicle. Starts right up. Loose cable, just like I said. The other guy with knowledge is still in high school. I have over 40 years. So did I have a little chip on my shoulder tonight? Yes I guess so but I had a good excuse. Have a great evening and I hope I answered your questions.
Gordon
2004 Chevrolet Silverado Extended Cab Z71
Semi-retired Master Mechanic
User avatar
lzn197
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Posts: 1774
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Blogs: 16
Joined: Apr 01 2010, 3:45pm
Years of membership: 4 years of membership4 years of membership4 years of membership4 years of membership
Location: Central Michigan
Year: 2004
Make/Model: Chevrolet Silverado
Extra Info: K1500 5.3L Z71 EX
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