Help Me Understand My Avalanche

Artic-AK-AV

SM 2003
Full Member
Ok Ive only had my Av for about a month. And im learning new stuff about it everday :0:. Now on to one thing i dont understand. What is the diffrence between "auto 4x4" and 4 HI . Thanks in advance for your input ;D
 

Ger

Full Member
I think what happens is this , in 4x4 you are locked in with the front end and the rear end both turning, in auto what happens is your hubs are locked but it is not in 4x4 until the truck senses some slippage then it puts it in 4x4 instantly.
Auto just saves you putting it in and out of 4x4 if you are one questionable roads.
If I knw it is bad then I still use 4x4 but if it is patchy then auto is better on fuel and still works good
 

Chief

CAFCNA FOUNDER
Charter Member
SM 2008
SM 2007
SM 2006
SM 2005
SM 2004
SM 2003
Full Member
Ger is correct...to further answer the question (maybe I should just sticky this post that covers this)...

2-Hi - should be used 90% of the time. Dry pavement, improved gravel or dirt roads, packed dirt, light rain or snow when wheel slip is not likely.

Auto-4WD - should be used when wheel slip is POSSIBLE. Heavy rain, spotty road conditions like patches of ice, snow, deeper mud, loose gravel, unimproved dirt road where rocks might get displaced. Unless you live in a snowbelt area or drive down miles of unimproved dirt roads you should not be in full time Auto-4WD.

4-Hi - should be used when wheel slip WILL happen. Deep snow, loose sand, rocky terrain, steep terrain, deep mud, water crossings over 10", icy conditions, pulling or pushing on a slippery surface like a boat ramp, etc. You should never EVER drive in 4-Hi on pavement where wheel slip won't happen as you can damage the system. Generally speaking, you shouldn't exceed 55 MPH in 4-Hi - if you are you're either driving to fast or you don't need it for the conditions.

4-Lo - first, if you HAVE to use 4-Lo, you're probably not where you should be. Good for using in extreme conditions - very deep snow, very soft sand, deep soupy mud, pulling or pushing up a steep/slippery ramp (like a boat ramp). Extended driving in 4-Lo should be avoided to extend the life of your vehicle. Although you can achieve highway speeds in 4-Lo you should generally stay below 30 MPH. Again if you're going faster than that you're probably going to fast for conditions or you don't need 4-Lo - only excpetion to his would be very deep sand or mud where momentum is key.

A final note. You should engage your 4WD system (4-Hi) every 4 to 8 weeks. If you don't have an area you can do this then it is best to engage it on a road where the speed is under 55 MPH and where you can drive straight and steady for 1 to 3 miles. This is enough to lubricate all the parts. Never steer on dry pavement in 4-Hi as it binds the differentials.
 

marc_w

Full Member
I don't think our trucks have hubs... ??? or at least lockable ones.

Aren't the wheels a direct drive to the front CV'd axles?

My understanding was Auto 4x just puts the truck in 'ready' mode. As it is, the front driveshaft already spins full-time... even in two wheel drive mode. With auto engaged, it's just letting the computer know to go ahead and throw the engaging switch in the front diff, when needed.
 

ShapeShifter

PM 2013
PM 2012
SM 2011
SM 2010
SM 2009
SM 2008
SM 2007
SM 2006
SM 2005
SM 2004
SM 2003
Full Member
marc_w said:
I don't think our trucks have hubs... ???? or at least lockable ones. ?

Aren't the wheels a direct drive to the front CV'd axles?

My understanding was Auto 4x just puts the truck in 'ready' mode. ?As it is, the front driveshaft already spins full-time... even in two wheel drive mode. ?With auto engaged, it's just letting the computer know to go ahead and throw the engaging switch in the front diff, when needed.
This is what the shop manual has to say:

Page 4-333
The front axle control circuit consists of an electric motor which engages and disengages the front axle. The front axle actuator's motor consists of a Permanent Magnet (PM) motor, a worm gear controlled plunger, a front axle switch and an electronic control circuit all within the actuator assembly.

The front axle actuator has four circuits connected to a front axle feed circuit, front acle ground circuit, front acle control circuit, and a front axle switch circuit.

Whenever a shift to AUTO 4WD, 4HI, or 4LO is requested, the automatic transfer case control module engages the front axle by grounding the front axle control circuit though a current limiting driver.
This acutator motor is on the front differential.

There is another motor in the transfer case that controls shifting the modes, and there is a "Transfer Case Lock Solenoid" which when released will lock the transfer case.

So, yes, there aren't "locking hubs" per se. However, when in 2WD, or Auto 4WD, the front half shafts may be turning, but the front diff and front propeller shaft are not. This is essentially the same as "unlocking" the "hubs" the only difference being that if we had true locking hubs, the front half shafts would not be turning in 2WD.

When switched into AUTO 4WD, 4HI, or 4LO, the front axle actuator motor is energized, "locking" the front half shafts to the front diff. Now, the front half shafts, front diff, and front propeller shaft always turn when the front wheels turn.

The difference between these modes is in the transfer case. When in 2WD, the transfer case is in a mode where power goes to the rear propeller shaft, and the front shaft is disconnected.

Put it in 4HI or 4LO, and the gears are set up so that power is driven to the front and rear propeller shafts. In these modes, the Transfer Case Lock Solenoid is de-engergized, and power is routed to the front drive shaft.

When In AUTO 4WD, the transfer case (and the front axle) are set up the same as in 4HI. The difference is that the Transfer Case Lock Solenoid is energized, preventing power from normally being transfered to the front propeller shaft. However, this shaft is still turning, being driven by the front wheel, even though it is transferring no power.

There are speed sensors on the front and rear propeller shafts. When the transfer case control module senses that these are turning at significantly different speeds, it de-energizes the Transfer Case Lock Solenoid, thereby allowing power to be transmitted to the front properller shaft. Once the slippage has stopped, the solenoid can once again be energized, reverting to 2WD.

So, yes, the front half shafts between the front diff and the hubs are always spinning, even in 2WD. However, the front diff and front propeller shaft between the diff and the transfer case are not spinning in 2WD, but they do spin in Auto 4WD, even if slippage is not currently being detected.


So, to summarize, this is what happens in the various modes:

2HI: Front axle disconnected, transfer case provides no power to front, lock solenoid de-energized.

AUTO 4WD: Front axle connected, transfer case routes power to front, but energized lock solenoid prevents power transmission. When slippage detected, solenoid de-energizes, providing power to front.

4HI: Front axle connected, transfer case routes power to front, solenoid denergized allowing power to front.

4LO: Same as 4HI, but an extra stage of gearing is enabled in transfer case, providing lower speed but more torque to front and rear.

NEUTRAL: Front axle disconnected, transfer case shifted to disconnect front and rear properller shafts from transmission.

-- SS
 

Artic-AK-AV

SM 2003
Full Member
Ger, Chief and SS. Thank you for your response. The roads in Alaska change from day to day. I was trying to figure out in which modes to run my AV in the changing conditions. I now have a better understanding, due to the superior knowledge of the members of this club. ;D
 

Artic-AK-AV

SM 2003
Full Member
Hey Chief, That may not be a bad idea making this sticky. Theres alot of good information in this post for new AV owners :cool:
 

midlifecrisis

Charter Member
SM 2003
Full Member
Holy Cow - that was a mouthful of information!!

If the 4WD system is that complex, how in the world do they get all wheel drive to work, especially on small cars like Subarus??
 

marc_w

Full Member
ShapeShifter said:
This is what the shop manual has to say:

Page 4-333
This acutator motor is on the front differential.
Yup, that's the one I was talking about. They used to be vacuum controlled on the older Chevy's. Finally they made it electric!

I thought it was located somewhere between the differentials (driveshaft) flange, and the pinion gear... Am I wrong there?

So, to summarize, this is what happens in the various modes:

2HI: Front axle disconnected, transfer case provides no power to front, lock solenoid de-energized.

AUTO 4WD: Front axle connected, transfer case routes power to front, but energized lock solenoid prevents power transmission. When slippage detected, solenoid de-energizes, providing power to front.

4HI: Front axle connected, transfer case routes power to front, solenoid denergized allowing power to front.

4LO: Same as 4HI, but an extra stage of gearing is enabled in transfer case, providing lower speed but more torque to front and rear.

NEUTRAL: Front axle disconnected, transfer case shifted to disconnect front and rear properller shafts from transmission.

-- SS
Hmm.....

I think this is pretty much what I was assuming...

The two front axles... from the wheels... spin full time not matter what. Hard connection.

The actual front ring, carrier, and pinion - I assume they spin with the wheels, because the axles are attached to the outer spider gears, which when turning at the same rate, spin the carrier...

The front driveshaft though, Yes... it's true it's not getting actual power in 2wd, but with my experiences with my Blazer, it has HUGE amounts of friction to deal with. So much that it spins when the truck is in motion, even when not truly engaged. That's kind what I was getting at there... :)

It was so difficult to turn when parked in 2wd, that I had to get a skinny breaker bar in between the U-joints and crank it around. I had to drive with it disconnected a number of times, and the lack of drag was very noticeable in acceleration and gas mileage. (well, that was with the little 4.3 V6)

You 2-wheel drive guys are lucky in this respect!
 

new2Chevy

Charter Member
SM 2004
Full Member
At the risk of being nit picky...slippage is most likely to occur during the the first start of rain. Light or heavy...

But i have a question....couple of times i started driving and forgot it was in 4hi or 4 auto and i was on pavement and probably exceeded 60 miles before i noticed. Do you think i should be worried?

I do notice a very slight pull to the right sometimes, but it could just be my tires. I'm at 6k miles and haven't rotated yet. On occasion, i'll hear a slight moderat/high pitch hum coming from what seems to be the front end. But i tend to worry alot....i think i'll bring it in to get checked either way.

Sticky this topic is a good idea. This is my first 4x4 and the little info posted made me a bit smarter. It's simple questions that few will likely to ask.
 

golfdude18

SM 2003
Full Member
I live in Indianapolis and we just got our 3rd snow of the season.

I will slip and slide around in 2-HI like those mere mortal trucks but in 4-AUTO it plows on thru without slipping like a super truck should.

I've put it in 4-HI a couple of times but mainly to lub the transfer case like Chief had stated.
 

MagicMtnDan

SM 2003
Full Member
Thanks Shape Shifter for the detailed explanation.

I've had a number of people who, seeing my Av rolling, told me that my front shafts are turning while I was in 2-Hi. I was sure they were supposed to do that and you've confirmed that they are.

By the way, I have used Auto-Trac, 4-Hi and 4-Low (used them all while in Death Valley) and you can tell when the Av goes in and out of 4-wheel drive. Using 4-low is very necessary for that all-important torque - getting it to the wheels for low speed crawling (over obstacles) and to control descent (without using brakes!) on steep declines.

Happy four-wheeling! ;D
 

marc_w

Full Member
MudBug?? From ZR2.com??

If you have 4.10's, it's probably the typical whine most of us with them have. Mine is ALWAYS there between 45 and 55, whenI'm off the throttle.

If you were driving around doing tight turns in 4HI, you probably would have been "reminded" that you were that range by some nasty noises and binding. I did this in my first 4x truck. :7:

If you did mostly straight line driving, with at least a 'little' snow or ice thrown in, I wouldn't be all that worried.
 

midlifecrisis

Charter Member
SM 2003
Full Member
Last week I drove in 4WD Hi for about an hour in a snowstorm at about 30-40 mph. There was 1-3" of snow on the road at the time.

When I got out of the truck, I noticed a faint smell of a burning fluid. I've since had all the fluids checked and they are fine.

Should I be worried?
 

ShapeShifter

PM 2013
PM 2012
SM 2011
SM 2010
SM 2009
SM 2008
SM 2007
SM 2006
SM 2005
SM 2004
SM 2003
Full Member
marc_w said:
Yup, that's the one I was talking about. ?They used to be vacuum controlled on the older Chevy's. Finally they made it electric!

I thought it was located somewhere between the differentials (driveshaft) flange, and the pinion gear... Am I wrong there?

Hmm.....

I think this is pretty much what I was assuming...
Sounds like we are talking pretty much the same thing, the difference being just a matter of degree.

I have not crawled under my truck recently while I was switching it between different modes, so I can't tell you for sure what was spinning and what wasn't. :2:

But I can tell you that when I switch into AUTO 4WD, I do notice a difference. I don't so much hear it, as I feel the extra gear drag/vibration at times. My assumption is that the difference is that there are more parts spinning in AUTO as opposed to 2HI.

The front axle theory of operation according to the shop manual is:

Page 4-124
The Selectable Fopur Wheel Drive (S4WD) Front axle consists of the following components:
  • Differential Carrier Housing
  • Differential Assembly
  • Output Shafts (Left and Right Side)
  • Inner Axle Shaft Housing
  • Inner Axle Shaft (Right Side)
  • Clutch Fork
  • Clutch Fork Sleeve
  • Electric Motor Actuator
The front axle on Selectable Four Wheel Drive model vehicles uses a central disconnect feature in order to engage and disengage the front axle. When the driver engages the 4WD system, the Transfer Case Control Module sends a signal to the electric motor actuator to energize and extend the plunger inside. The extended plunger moves the clutch fork and clutch fork sleeve across the inner axle shaft and the clutch fork shaft locks the two shafts together. The locking of the two shafts allows the axle to operate in the same manner as a semi-floating rear axle. A propeller shaft connects the transfer case to the front axle. The differential carrier assembly uses a conventional ring and pinion gear set to transmit the driving force of the engine to the wheels. The open differential allows the wheels to turn at different rates of speed while the axle continues to transmit the driving force. This prevents tire scuffing when going around corners and premature wear on internal axle parts. The ring and pinion set and the differential are contained within the carrier.
There is also an exploded diagram of the axle. From the looks of it, the clutch shaft is in line with the axle half shafts. It would appear to me that this clutch will disconnect the half shafts from the differential, preventing the spinning of the ring gears. However, it is not clear to me. It does not appear that the clutch is on the transfer case side of the diff.

Also, not visible on the exploded diagram is the amount of friction or drag on the shafts and gears. Experience is the best teacher there.

Attached is the exploded view of the front axle. What's not shown there is the electric motor that activates the clutch. It is mounted outside of the axle housing, is paralell to the front half shafts, and pushes on item 10 ff the diagram. Take a look at it marc_w, and let us know if it looks the same or different from your past experiences. Also, see if you can figure out what parts are connected/disconnected by that clutch. I'll leave it to people with more experience than me to determine what's really going on.


I'm curious to know how it really works, but my guess is that we're really getting down to the nit-pick details here.

For general information, it's probably enough to say what was stated in the earlier posts, where AUTO 4WD is "half on" waiting for the transfer case to give it power. To call it "locking the hubs" puts it in terms that many people can understand, even if it's not entirely technically correct. ;)

-- SS
 

Attachments

marc_w

Full Member
ShapeShifter said:
Sounds like we are talking pretty much the same thing, the difference being just a matter of degree.
Yeah, Definitely! ;D

I have not crawled under my truck recently while I was switching it between different modes, so I can't tell you for sure what was spinning and what wasn't. :2:
What movie was that there... Speed, with the guy under the bus? :)

I just have more close, personal time with the driveshaft of that Blazer than I could have ever wished for. My original one grenaded one rainy day on the way to work, in 2wd. That's the lovely way how I orginally found out it spins full-time. :rolleyes:

From yanking 'it', and a custom one I had built, in and out a number of times for all sorts of reasons... I found that the t-case provides a LOT of drag to the front driveshafts flange, and it (the flange, and if attached, the driveshaft too) rotates when the vehicle is moved.

The front diff's flange does not move with the vehicle when in 2wd. That flange is very easy to turn by hand, so I assumed (you know what they say about that) that the disconnecting mechanism was somwhere between that flange and the ring-gear. Basically, it sure didn't feel like I was spinning the carrier by hand!

But I can tell you that when I switch into AUTO 4WD, I do notice a difference. I don't so much hear it, as I feel the extra gear drag/vibration at times. My assumption is that the difference is that there are more parts spinning in AUTO as opposed to 2HI.
Yeah, that's definitely reasonable. I honestly haven't spent a whole lot of time in Auto. :cautious:

I use 4HI on ocassion to get going up tricky snow covered hills and what not, and boy is THAT noticeable! It grumbles and whines more than.......... me... ;)

Page 4-124
There is also an exploded diagram of the axle. From the looks of it, the clutch shaft is in line with the axle half shafts. It would appear to me that this clutch will disconnect the half shafts from the differential, preventing the spinning of the ring gears. However, it is not clear to me. It does not appear that the clutch is on the transfer case side of the diff.
WOW... This one surprised me. I didn't think the front diff operated like this. It's almost like a ARB air locker, with the locker on the outside of the diff. :) Yeah though, you are definitely correct here.

Take a look at it marc_w, and let us know if it looks the same or different from your past experiences. Also, see if you can figure out what parts are connected/disconnected by that clutch. I'll leave it to people with more experience than me to determine what's really going on.
Uhg... I haven't cracked the cases on a front diff (yet) (hope I never need to!), so this is beyond me... I tried piecing things together, but my brain works better holding pieces in my hand. :cautious:

I'm curious to know how it really works, but my guess is that we're really getting down to the nit-pick details here.

For general information, it's probably enough to say what was stated in the earlier posts, where AUTO 4WD is "half on" waiting for the transfer case to give it power. To call it "locking the hubs" puts it in terms that many people can understand, even if it's not entirely technically correct. ;)

-- SS
I agree completely... for both statements... :)
 

ShapeShifter

PM 2013
PM 2012
SM 2011
SM 2010
SM 2009
SM 2008
SM 2007
SM 2006
SM 2005
SM 2004
SM 2003
Full Member
marc_w said:
What movie was that there... Speed, with the guy under the bus? :)
LOL!

I use 4HI on ocassion to get going up tricky snow covered hills and what not, and boy is THAT noticeable! ?It grumbles and whines more than.......... me... ;)
ROFL!

Uhg... I haven't cracked the cases on a front diff (yet) (hope I never need to!), so this is beyond me... ?I tried piecing things together, but my brain works better holding pieces in my hand. ?:cautious:
I had trouble interpreting just how it works myself, which is why I posted it. Now, if I could just hold it in my hand and spin the gears while sliding that clutch fork back and forth, I'd be able to see what's happening. But just looking at the diagram? :rolleyes:

I traded in a 4WD Blazer for the Av, and I can say that the are sure are two completely different classes of vehicles. Even though the control panel is similar, I'm not at all surprised that the mechanics behind it are very different. The Av sure seems like a big improvement to me! ;D

-- SS
 

dmacker

SM 2003
Full Member
SS,

I was trying to figure out this very thing on my way back from Erie, while running in 4-Auto in deep snow. Thanks for the explanation.
 

Crexis

Full Member
Chief said:
4-Lo - first, if you HAVE to use 4-Lo, you're probably not where you should be. ?Good for using in extreme conditions - very deep snow, very soft sand, deep soupy mud, pulling or pushing up a steep/slippery ramp (like a boat ramp). ?Extended driving in 4-Lo should be avoided to extend the life of your vehicle. ?Although you can achieve highway speeds in 4-Lo you should generally stay below 30 MPH. ?Again if you're going faster than that you're probably going to fast for conditions or you don't need 4-Lo - only excpetion to his would be very deep sand or mud where momentum is key.
D'oh! >:D I guess perhaps I'm a bit hard on my Av then. ;). The difference in "slipperyness" of the conditions doesn't seem as critical to me as the torque multiplication. I've yet to be in mud or sand where I wasn't better off in 4hi... but trying to get a 3 ton Av to crawl over boulders... 4lo does wonders! You don't have to give it as much gas, thereby avoiding potential damage as you come down off of rocks. The extra torque on hand erases the need for a "running start" at many boulders and hills. 4lo is safer and actually easier on your Av (although Chief's point remains about wear and tear on the drive train) if you do any hardcore 'wheeling.

My last 4x4 had an old 2:1 transfer case and the Av's 2.73:1 is a pleasant surprise. It's no Rubicon (4:1) but it only takes a bit of throttle to tackle most situations.
 

Ger

Full Member
the only thing I didn't like about using the auto is ( and I don't do it any more)
when I had it in auto when I was hunting this year driving down the ack roads that were snowcovered . I parked in a field, when I went to leave the wheels spun and it locked in with a clunk.
I would say not to use it when you park , if you might get stuck use reg 4x4
JMO
Ger
 

marc_w

Full Member
I noticed that too. Every once in a while I'll catch it off guard, or not give it enough time to fully go into Auto (my fault), and I'll really give the driveline a jolt.

It mostly happens when the back wheels are on snow, or ice, and the fronts are on dry pavement. (like when pulling out of a parking lot, maybe)
 

ms38w

Full Member
Ok guys (& gals)...I am resealing a leaky front differential.  '02 Z71.  I have disconnected the passenger side cv joint and all but the final bolt where the axle connects to the differential.  I removed the little actuator motor to gain better access to the bolt,but no luck removing the bolt.  with the stabilizer bar and various tie rods,etc., there simply is no room to crank on the ratchet, and the body of the tool hits the axle housing,causing it to come off the bolt. I tried an air ratchet, but there is even less room for it in there. As the leak is coming from where the axle housing mates to the differential cover, and not the diff itself, I refuse to break the seal on the diff to remove the axle housing.  Has anyone had a similar problem & how did you get that upper bolt out?

Thanks.
 
Top