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Author Topic: Auto 4wd what controls to engage or not  (Read 1095 times)

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Seadoojetski

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Auto 4wd what controls to engage or not
« on: 12/18/17 11:46 AM »

Anybody know what controls the auto 4wd to engage or disengage? As soon as I put it into auto 4wd after about a minute of driving it will engage the 4wd and stay in until I put it back into 2hi. When I come to a stop it feels like itís tryiing to come out if 4wd but as soon as I start driving again it will engage. Sometimes pulling to one side  sometimes not. I am really looking to know what controls the auto 4wd. 4hi and 4low work ok. No service lights on either. Iím also trying this on dry pavement. Thank you in advance
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Vaeagleav

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Re: Auto 4wd what controls to engage or not
« Reply #1 on: 12/18/17 12:18 PM »

First mistake is not reading our owner's manual, it states NEVER run 4wd on dry pavement........whether 4wd hi, 4wd lo or auto 4wd.
The manual also states there may be a slight delay in engaging or disengaging and that is normal.
I'm sure one of the site's mechanics will answer your specific questions soon but as a general rule I believe it is wheel speed sensors that detect wheel slippage and help the computer determine which wheels to send power to to get traction back.
As a side note if you put your year and model AV in your signature then it will save you time having to answer that all the time since some problems or mods are year/model specific.
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Re: Auto 4wd what controls to engage or not
« Reply #2 on: 12/18/17 01:03 PM »

What model year?

Are you saying the 4WD switch selector is jumping into 4HI by itself and lights up as though you've chosen it, even though you've selected Auto4WD?
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Seadoojetski

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Re: Auto 4wd what controls to engage or not
« Reply #3 on: 12/18/17 01:16 PM »

03 1500 z71
I know about dry pavement and not to drive in hi or low 4wd. I am testing just driving straight and it almost immediately goes into 4wd in the auto setting. It does not jump from hi to auto and no lights blinking. Everything on the dash seems normal. Iím guessing itís not detecting wheel slippage but Iím not sure what to start with.
Thanks for your help
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #4 on: 12/18/17 02:59 PM »

I am curious how you know it is engaging 4WD in auto on dry pavement? Sound? Lights? Looking at the propeller shaft?

I've done a bit of research recently, learning about my own new toy. From what I understand, "Auto" mode engages the front axles to the front differential. Once engaged, the front propeller shaft will rotate when the truck moves, and I can definitely hear that on my truck. However, at that point the propeller shaft is free-wheeling in the transfer case, driven by the front wheels, but not receiving any torque from the engine.

Both front and rear propeller shafts have sensors so the Powertrain Control Module can monitor the respective RPM. If there is a difference in rpm between them, then the PCM progressively ramps up the power to a PWM driven electric clutch in the transfer case, sending power to the front. As the shaft speeds equalize, it eases off the power to the clutch. If and when the shaft speeds remain equal, power to the clutch will eventually return to 0 and the front shaft will again be free-wheeling, driven by the front wheels rather than driving them.


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Seadoojetski

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Re: Auto 4wd what controls to engage or not
« Reply #5 on: 12/18/17 05:19 PM »

I Can feel it through the steering and hear it engaging.
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Re: Auto 4wd what controls to engage or not
« Reply #6 on: 12/18/17 08:21 PM »

You should never be in 4wd on dry pavement. Even driving straight.

Normally I can't feel or hear mine going in/out of 4wd at all. Occasionally, I can hear a clunk, usually because I have my foot in it at the time.
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redheadedrod

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Re: Auto 4wd what controls to engage or not
« Reply #7 on: 12/18/17 11:21 PM »

Your truck is NOT in 4wd if you are putting it into Auto while on dry pavement. What you are hearing is that everything is spinning but it is NOT fully engaged. It will NOT fully engage until about a half second AFTER it senses slippage. It is normal. Do NOT drive in this mode unless you are on dirt or slippery road as you can damage the front differential the same as if you were in 4wd.

I am not familiar with what is engaging and what is not but I am sure someone who is will hopefully pipe up.

Rodney
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Seadoojetski

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Re: Auto 4wd what controls to engage or not
« Reply #8 on: 12/19/17 03:48 AM »

Ok thank you for all your help here.
Appreciate it
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #9 on: 12/19/17 07:46 AM »

Do NOT drive in this mode unless you are on dirt or slippery road as you can damage the front differential the same as if you were in 4wd.

How can it damage the front differential? The front diff is open, not a limited slip or locker. 4WD on dry pavement can potentially damage a transfer case, but even that shouldn't happen in Auto since the clutch is disengaged.

From what I can see, the worst thing is some additional miles on the front driveshaft bearings spinning, and a reduction in fuel economy by spinning it. What am I missing here?
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redheadedrod

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Re: Auto 4wd what controls to engage or not
« Reply #10 on: 12/19/17 09:08 AM »

Someone will chime in if I am wrong but I am under the impression the front diff is locked when engaged.

Both wheels will spin at the same rate regardless which is why you can NOT run it on dry pavement. If you run it on dry pavement you will get serious wheel hop depending on how tight a turn you make. My understanding is that is a function of the differential and if you run it on dry pavement you will blow it up.

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Seadoojetski

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Re: Auto 4wd what controls to engage or not
« Reply #11 on: 12/19/17 09:58 AM »

I thought the whole point of driving in auto 4wd was it will engage when needed. It should never go into 4wd on dry pavement is my understanding.
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Re: Auto 4wd what controls to engage or not
« Reply #12 on: 12/19/17 12:26 PM »

I thought that too...it should only be turning the front diff but not engages unless it senses slippage then engaging the front wheels...you can feel the mass of the front diff turning...I did not know you couldnít drive in Auto...

Bill
« Last Edit: 12/19/17 12:28 PM by silversport »
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #13 on: 12/19/17 03:49 PM »

Someone will chime in if I am wrong but I am under the impression the front diff is locked when engaged.

Both wheels will spin at the same rate regardless which is why you can NOT run it on dry pavement. If you run it on dry pavement you will get serious wheel hop depending on how tight a turn you make. My understanding is that is a function of the differential and if you run it on dry pavement you will blow it up.

From GM's description of the 2004 Avalanche 4WD system:

"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."
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redheadedrod

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Re: Auto 4wd what controls to engage or not
« Reply #14 on: 12/19/17 11:22 PM »

From GM's description of the 2004 Avalanche 4WD system:

"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."

This sounds like the AWD system and not the 4WD system. Your going to make me pull out the manual... ;)

I know for a fact my truck makes all sorts of racket if I am on dry pavement when it is in 4wd and I turn. Same when in 4wd auto.
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #15 on: 12/20/17 03:14 AM »

This sounds like the AWD system and not the 4WD system. Your going to make me pull out the manual... ;)

Please do.

The AWD is also known as "Full-Time Four Wheel Drive"? I found this:

"The front axle on Full-Time Four Wheel Drive model vehicles does not have a central disconnect feature in order to engage and disengage the front axle. The left and right axle shafts are connected directly to the differential case assembly. This allows the axle shafts and the propeller shaft to spin continuously. The transfer case controls the amount of torque applied to the front axle. The remaining components are the same as the selectable four wheel drive axle."

From what I can see, the difference in front axles is that the part-time 4WD system has the clutch, fork and two piece output shaft allowing it to be disconnected whereas the full time system has a one-piece output shaft that is always connected. The respective front differentials appear to be the same. No?
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Re: Auto 4wd what controls to engage or not
« Reply #16 on: 12/20/17 03:44 AM »

front axle on 4wd AVY is open diffy which means power goes to the wheel which will spin the easiest.
SO it is really a 3WD system.

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Re: Auto 4wd what controls to engage or not
« Reply #17 on: 12/20/17 08:11 PM »

front axle on 4wd AVY is open diffy which means power goes to the wheel which will spin the easiest.
SO it is really a 3WD system.



         Your comment ďSO it is really a 3WD systemĒ
  Brought a memory. A guy I grew up with had I believe what Chevy called Full Time 4 Wheel Drive.
He bumped a guardrail popped off the left front hub cover things popped out.
That was basically all the damage.
 So there he sat unable to move.

 Iíve seen a demo where they jacked one wheel off the ground and the wheel in the air would be the only wheel turning. Then they said you thought you had a 4 wheel drive.

Was not long after Full Time came out. There was a kit to change the T-Case and hubs back to manual.

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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #18 on: 12/21/17 12:37 AM »

Iíve seen a demo where they jacked one wheel off the ground and the wheel in the air would be the only wheel turning. Then they said you thought you had a 4 wheel drive.

I saw that happen in person once. The fellow was trying to tow a small utility trailer up a steep and snowy hill. His front left wheel was spinning and the truck was just sitting there. He dug out the left front, got some kitty litter under there and the right rear started spinning instead...

I pulled him up with my 92 Roadmaster Estate Wagon. It was 2 wheel drive, but had the tow pack with limited slip rear diff.  :4:

I tried out Auto last night. Our neighbour was away and didn't feel it necessary to get her drive plowed. Then it thawed, rained and re-froze, making the plow row at the end of the drive mighty crusty. I drove in through in OK, but then backing out I got stuck for a moment, the rear wheel(s) spinning briefly until the front axle and/or rear locker kicked in. The truck started moving again and it powered on through. Pretty cool. Even in the old RMW I would have had to start rocking or digging to get through that pile.

(Once it gets light out, I'll go back with my other 4WD vehicle and dig her out with the front end loader.)
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2004Slickside

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Re: Auto 4wd what controls to engage or not
« Reply #19 on: 12/21/17 07:49 PM »

In auto mode
1. first the front axle locks (what you are feeling)
2. The clutches in the transfer case apply power to the front axle per the Transfer Case Control Module and speed sensors when wheels slip is detected

In4WD mode
1. first the front axle locks (what you are feeling)
2. The transfer case locks in and applies power to the front axle all the time
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #20 on: 12/21/17 11:52 PM »

I think saying "the front axle locks" is some confusion about what actually happens to the front axle. My understanding is that the two halves of the right hand axle engage together. An excellent (if somewhat dry) illustration of this principle can be found here, starting at 11:15:

https://www.youtube.com/watch?v=47m7QAPrpsI

The particular system shown there is from a Toyota, but comparing parts diagrams and service information I believe the GM system operates on the same principle.

For some, the phrase "front axle locks" seems to imply that the front differential itself locks. I think they are envisioning that it acts like the Eaton locker on the rear differential. If that was the case, then yes, the front wheels would be fighting each other in turns, and yes, damage could result from operating a locked front differential on dry pavement.

I found this on GMC.com:

"AUTOMATIC 4 HI

If road conditions frequently alternate between high- and low-traction areas, consider using the ďAUTOĒ setting found on select GMC electronic transfer cases. This setting allows your GMC to automatically distribute torque to the front axle by anticipating the need for additional traction. Shifting into ďautoĒ engages the front axle, but the transfer case sends power primarily to the rear wheels in normal conditions and the clutches modulate torque forward to provide stability and enhance traction to the vehicle. Although not always optimal for efficiency and wear of your vehicle 4wd driveline, AUTOMATIC 4 HI can be used on any road condition without risk of damaging your vehicle."


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Re: Auto 4wd what controls to engage or not
« Reply #21 on: 12/22/17 05:43 AM »

Engages is a better word for it.
« Last Edit: 12/22/17 05:56 AM by MS03 2500 »
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #22 on: 12/29/17 08:23 AM »

I have a Tech2, and took it along for a drive to see what happens in the system as it shifts through the various modes, particularly "auto". It was enlightening. Driving along, the front propeller shaft is actually turning, I assume due to the viscosity of the gear lubes in the front diff and transfer case. Not fast, mind you: a few hundred RPM compared to around 2,600rpm for the rear shaft.

Selecting auto causes the PWM transfer case clutch to ramp up, which brings the front prop shaft speed up to match the rear shaft speed. It happens pretty quick, in less than a second, so it's hard to read the exact percentage being applied to the clutch, but it does hit double-digits for a split second before it starts to drop again. Likely peaks between 10 and 20%. It starts to drop pretty much as soon as the front axle speed matches the rear axle speed. At that moment, power is applied to the front axle engagement system.

This sequence seems to usually happen 2 or 3 times in a row. I think that maybe the system is checking to see if the front axle splines actually engaged properly by looking for a drop in front propeller shaft speed compared to the rear when the clutch is released -- if it does, then the system tries again.

There is a similar ramping up of power (but for longer and to higher percentages) to the transfer case clutch when the rear wheels begin to spin. I really should have recorded the results to get exact numbers because it's hard to drive squirrely and watch the Tech2 at the same time.  :4:
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ca2kjet

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Re: Auto 4wd what controls to engage or not
« Reply #23 on: 12/29/17 07:48 PM »

Awesome information buickwagon!  :thumbsup:

Likely peaks between 10 and 20%.

Hm, imagine that, GM wasn't lying in their marketing. That seems to be right in line with their statement that Auto provides 15% power to the front when called for. I'd be curious to know if it's 50% when in 4WD.
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buickwagon

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Re: Auto 4wd what controls to engage or not
« Reply #24 on: 01/01/18 07:53 PM »

I'd be curious to know if it's 50% when in 4WD.

Well, it can hit 47% in Auto. 4WD seems to do something else.

I played a bit with the Tech2 in Snapshot mode, uploaded the data to TIS, graphed it and grabbed some screen shots.

This first sample shows what typically happens while changing modes. The light red line is the 2WD light, green is Auto, dark blue is 4WD high, light blue is the front axle switch, dark red is the encoder gear position and yellow is the PWM duty cycle % applied to the transfer case slip clutch.



So, starting in 4WD and selecting 2WD, the 2WD light starts to flash, the slip clutch goes to 47%, the front axle disengages, slip clutch goes to 0, 4WD light goes out, encoder gear to 2WD, 2WD light on steady.

Selecting Auto is similar, Auto light flashes, 47% on the slip clutch, encoder gear changes to Auto position, front axle engages, lights change.

The last one, selecting 4WD, is most interesting in that it's the only time I see the slip clutch ever go to 100% -- but then it drops back to 0!

I think the 4WD position of the encoder gear must engage a mechanical lock of some sort, and the clutch is just used temporarily to ensure speeds are matched before it is engaged. Similarly, when coming out of 4WD, I think the clutch takes the torque while the mechanical lockup is disengaged -- but that's a guess, I don't know for certain.

The other graph I'll post shows what happens when the rear wheels start spinning in Auto mode. In this case, red is the rear prop shaft speed, green is the front prop shaft speed, dark blue is the slip speed (ie the difference between front and rear) and light blue is the PWM duty cycle % applied to the slip clutch.



Driving at low speed in Auto on snow, I goosed the throttle just enough to break the rear loose, then held it at a gentle acceleration.

There's a sharp spike in the slip speed as the rear prop shaft speed increases quickly. Slip speed peaks and starts falling as the slip clutch kicks in. PWM to the clutch hits 47% about the same time as the two shaft speeds equalize. (Front and rear tires are all actually spinning at this point). The system slowly eases off the slip clutch as it drops to 11%. Prop shaft speeds decrease until the tires stop spinning and then slowly start increasing again as the vehicle speed climbs. Interestingly, even though the prop shaft speeds have now equalized the system holds the clutch at 11% until I ease off the throttle.

So, I would think 100% duty cycle on the slip clutch = 50/50 torque distribution and therefore 47% = 25/75 (approximately)and that 11% after the event represents about 5/95 -- enough to discourage a second occurrence while still under acceleration without undue stress on the components.

« Last Edit: 01/01/18 07:56 PM by buickwagon »
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ca2kjet

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Re: Auto 4wd what controls to engage or not
« Reply #25 on: 01/02/18 02:33 PM »

buickwagon that's some awesome information! Thanks for taking the time to do all this.  :thumbsup:
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Re: Auto 4wd what controls to engage or not
« Reply #26 on: 01/02/18 02:45 PM »

I know I have had the hum/vibration/etc if in the Auto AWD mode when I leave it in there by accident.  For the most part I never have it in Auto, but I do haul water all year round, and need traction sometimes in the city where I haul, and backing into my yard as well.

I sometimes forget to take it out when on the highway for a minute or so some days (like yesterday_ but the tell tale sound reminded me to put it to 2WD.  Im not perfect.

I do leave it in Auto sometimes in the city when driving when the roads are bad, just to get better traction from stop lights, etc.  But as soon as Im out of the city I turn it off

It would be nice if they put something on the dash as a reminder of what you are in.  The little light on the knob doesnt work well as a reminder, since its basically hidden when driving by the steering wheel
« Last Edit: 01/02/18 02:47 PM by spidey »
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dna9656

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Re: Auto 4wd what controls to engage or not
« Reply #27 on: 06/21/18 05:56 PM »

How can it damage the front differential? The front diff is open, not a limited slip or locker. 4WD on dry pavement can potentially damage a transfer case, but even that shouldn't happen in Auto since the clutch is disengaged.

From what I can see, the worst thing is some additional miles on the front driveshaft bearings spinning, and a reduction in fuel economy by spinning it. What am I missing here?
My 06 owner's manual says your drive train will wear prematurely if you drive on dry pavement in 4 wd.
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Re: Auto 4wd what controls to engage or not
« Reply #28 on: 06/21/18 06:05 PM »

Yes, in 4wd there is no slip between the front and rear drive shafts, so any speed differential due to turns, rolling circumference, etc. will cause strain and tire wear. But the front shaft is not normally engaged in Auto. 
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dna9656

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Re: Auto 4wd what controls to engage or not
« Reply #29 on: 06/21/18 06:09 PM »

 :thumbsup:
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