Please refresh my memory on AWD/Full Time 4 wheel drive.

c good

Active Member
Original poster
Dec 8, 2011
473
There are some nice GMT360's out there like the Saab with the 5.3 but they have Full Time 4 wheel drive.

I understand it doesn't have a 4 Lo or 4 Hi option. Is it really that much inferior to the traditional 4x4 system as on my Envoy?

What are the advantages and disadvantages of them compared to each other? Off-road capabilities, mechanical problems, etc.
 
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TollKeeper

Guru
Dec 3, 2011
7,342
Brighton, CO
It does hurt the fuel mileage some, and shorten the life the tires. But not enough that I stayed away from them.
 

Mooseman

Moderator
Dec 4, 2011
23,693
Ottawa, ON
First of all, it's not a full-time AWD, it's a non-selectable part time system. It's exactly like putting your Envoy's 4x4 in auto. The front wheels are always locked and the transfer case engages them only when it detects slippage in the rear wheels. I call it the "slip/bang" system because it literally bangs in the front wheels when it detects slip. To me, it sucks.

The TBSS and Aero are the only ones that have full time AWD where all the wheels were engaged all the time.
 
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NJTB

Hobbyist
Aug 27, 2012
611
Flemington, NJ
I had questions about this on my 2015 Acadia. Apparently AWD is 80% front wheels, 20% rear wheels.
Never did get an answer if it goes into higher numbers in the snow, but was assured they're good in the snow.
 

Mooseman

Moderator
Dec 4, 2011
23,693
Ottawa, ON
I had questions about this on my 2015 Acadia. Apparently AWD is 80% front wheels, 20% rear wheels.
Never did get an answer if it goes into higher numbers in the snow, but was assured they're good in the snow.
That is a full time AWD. The GMT360 AWD (except SS and Aero) and 4x4 system in auto uses an encoder motor on the transfer case to momentarily engage the front wheels just like a solid 4x4, 100% to front and rear wheels. After the slippage is over, it shifts back to rear wheels only.

I think the SS/Aero AWD has much more bias, like 60% rear and 40% front because of the power and traction encountered while driving it in a "spirited" fashion.
 

TJBaker57

Guru
Aug 16, 2015
2,241
Colorado
So is it precisely the same Transfer Case in both RPO NP8 & NP4?? Only the shift motor and TCCM and such are different? I had thought the actual innards of the transfer case would be different yet this page from a parts supplier indicates even the single speed "AWD" version has the internals for the low range and all, just never gets used. Seems wasteful.

Screenshot_20230124-081214.png
 

TequilaWarrior

Hobbyist
Dec 5, 2011
511
Central Pennsylvania
Let's clear up a couple of things... some of this is repetitive of info given above, but for clarity sake....

The Trailblazer SS and Saab 9-7x Aero have a completely different AWD system. It is a purely mechanical, "Torsen", style AWD transfer case. It has a standard bias that applies a fraction of torque to the front at all times (@Mooseman mentioned 60/40 split). This system is VERY much a differential, when a slip condition is present, torque is directed towards the wheels with the most traction. There are no electronics involved in this process. Many actually call the transfer case a differential instead of a transfer case, because that's what it is. This system is also nearly bulletproof.

In all other cases, the transfer case is electronic. There is a motor that shifts the tcase in and out of 4x4. In non-AWD vehicles, the tcase is two speed. The electronics are interlocked so that it cannot go into LO range without being locked into 4WD. ANY time 4wd is commanded, the front differential disconnect is engaged so that the front output shaft on the transfer case drives the actual front differential instead of spinning in the disconnect.

In AWD vehicles, the front disconnect is never disconnected(I'm not sure if there even is a "disconnect"). The motor that shifts the tcase into and out of 4x4 is what gets actuated in a slip condition. It happens very fast and works very well in a healthy vehicle. There is also no LO range. The transfer case is single speed only. That the front disconnect is never disconnected creates drag loss negatively impacting fuel economy.

(Circling back) In ordinary 4WD vehicles, the setting marked "A4WD" is a a mish-mash of 4wd and AWD. It connects the front disconnect (like the AWD vehicles) as soon as the setting is selected and then monitors for a slip condition, engaging 4WD when a slip condition is sensed. For reasons unknown to me, this tends to cause the "slip-bang" that everyone dreads and can lead to "grenading" drivetrain components.

I have an AWD and have never experience anything but traction. When I do have a slip I notice when the AWD kicks in, but it's smooth and quiet and I can just "go".

Things to keep in mind:
The rear differential makes a HUGE difference in traction. Open vs Limited Slip vs G80 (Locker) will materially affect how much traction you actually get.
Maintenance is KEY. If tcase fluid isn't changed at appropriate intervals, the clutches inside the tcase "stick"... in a 4wd tcase, this may not be noticeable most of the time, but in an AWD it cause "crow hop" and will very negatively impact drivability at low speed. It also causes excessive tire wear and noise and wear and tear on other drive line components.

As for the NP126 and NP226 both containing a low range... this would be the first I've heard of it. Were it true it would mean there is potential to mod AWD vehicles to be true 4x4 and I've never seen any such mods anywhere. I HAVE seen mods to selectively engage AWD instead of letting the TCCM do it, but that's all. I'll look around though.
 
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TequilaWarrior

Hobbyist
Dec 5, 2011
511
Central Pennsylvania
Let's clear up a couple of things... some of this is repetitive of info given above, but for clarity sake....

The Trailblazer SS and Saab 9-7x Aero have a completely different AWD system. It is a purely mechanical, "Torsen", style AWD transfer case. It has a standard bias that applies a fraction of torque to the front at all times (@Mooseman mentioned 60/40 split). This system is VERY much a differential, when a slip condition is present, torque is directed towards the wheels with the most traction. There are no electronics involved in this process. Many actually call the transfer case a differential instead of a transfer case, because that's what it is. This system is also nearly bulletproof.

In all other cases, the transfer case is electronic. There is a motor that shifts the tcase in and out of 4x4. In non-AWD vehicles, the tcase is two speed. The electronics are interlocked so that it cannot go into LO range without being locked into 4WD. ANY time 4wd is commanded, the front differential disconnect is engaged so that the front output shaft on the transfer case drives the actual front differential instead of spinning in the disconnect.

In AWD vehicles, the front disconnect is never disconnected(I'm not sure if there even is a "disconnect"). The motor that shifts the tcase into and out of 4x4 is what gets actuated in a slip condition. It happens very fast and works very well in a healthy vehicle. There is also no LO range. The transfer case is single speed only. That the front disconnect is never disconnected creates drag loss negatively impacting fuel economy.

(Circling back) In ordinary 4WD vehicles, the setting marked "A4WD" is a a mish-mash of 4wd and AWD. It connects the front disconnect (like the AWD vehicles) as soon as the setting is selected and then monitors for a slip condition, engaging 4WD when a slip condition is sensed. For reasons unknown to me, this tends to cause the "slip-bang" that everyone dreads and can lead to "grenading" drivetrain components.

I have an AWD and have never experience anything but traction. When I do have a slip I notice when the AWD kicks in, but it's smooth and quiet and I can just "go".

Things to keep in mind:
The rear differential makes a HUGE difference in traction. Open vs Limited Slip vs G80 (Locker) will materially affect how much traction you actually get.
Maintenance is KEY. If tcase fluid isn't changed at appropriate intervals, the clutches inside the tcase "stick"... in a 4wd tcase, this may not be noticeable most of the time, but in an AWD it cause "crow hop" and will very negatively impact drivability at low speed. It also causes excessive tire wear and noise and wear and tear on other drive line components.

As for the NP126 and NP226 both containing a low range... this would be the first I've heard of it. Were it true it would mean there is potential to mod AWD vehicles to be true 4x4 and I've never seen any such mods anywhere. I HAVE seen mods to selectively engage AWD instead of letting the TCCM do it, but that's all. I'll look around though.
As a slight "aside".... I was just digging around for info regarding internal similarity between NP126 and NP226 tcases and found a place selling master rebuild kits for both. I was surprised to find that both rebuild kits have the same part number.... and disappointed to find that they're charging $63 more for the NP226 kit. The kits are identical, down to the part number.....
2023-01-24 12_15_10-GM NP-226 TRANSFER CASE MASTER REBUILD KIT 2002-UP — Mozilla Firefox.jpg
2023-01-24 12_14_56-GM NP-126 Transfer Case Master Rebuild Kit, 2002-Up — Mozilla Firefox.jpg
 
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TJBaker57

Guru
Aug 16, 2015
2,241
Colorado
NP126 and NP226 are NOT internally identical. There is no planetary in the NP126

That is what I expected to see. I was surprised when I saw the image I posted above and also noted the same rebuild kit number for both 126 and 226 as you saw. Thought it was very strange but didn't look further.

And not related to this thread but.....

The electronics are interlocked so that it cannot go into LO range without being locked into 4WD


From factory I would say this is true, however I have had my 2002 in 2 Low and NOT by disabling the front axle disconnect. I did this after seeing a "2 Low" mode in the Tech 2.

A shift into 4Lo first moves the transfer case to the neutral position, then engages the reduction gearing and lastly applies the clutch pack to the front driveline. The Tech 2 recognizes the point at which the reduction gearing is engaged but the clutch pack is not yet engaged as "2Lo.

PXL_20210919_193432046~2.jpg

I spoofed the encoder return signal to the TCCM such that the TCCM believed it was in 4 Low when the shift motor was actually not applying the clutch pack yet and thus was in 2 Low.

Granted, it would be far simpler and safer to just spoof the front disconnect. I just wanted to see if I could do it the hard way. :wink:
 
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TequilaWarrior

Hobbyist
Dec 5, 2011
511
Central Pennsylvania
That is what I expected to see. I was surprised when I saw the image I posted above and also noted the same rebuild kit number for both 126 and 226 as you saw. Thought it was very strange but didn't look further.

And not related to this thread but.....




From factory I would say this is true, however I have had my 2002 in 2 Low and NOT by disabling the front axle disconnect. I did this after seeing a "2 Low" mode in the Tech 2.

A shift into 4Lo first moves the transfer case to the neutral position, then engages the reduction gearing and lastly applies the clutch pack to the front driveline. The Tech 2 recognizes the point at which the reduction gearing is engaged but the clutch pack is not yet engaged as "2Lo.

View attachment 106463

I spoofed the encoder return signal to the TCCM such that the TCCM believed it was in 4 Low when the shift motor was actually not applying the clutch pack yet and thus was in 2 Low.

Granted, it would be far simpler and safer to just spoof the front disconnect. I just wanted to see if I could do it the hard way. :wink:
Hence my phrasing of it being the electronics that are interlocked.... it's the electronics you're spoofing (the TCCM, really). There's no internal linkage between the reduction and the clutchpack. The TCCM is responsible for the "timing", if you will.

I want to install a selector switch to engage my AWD to forcibly be engaged, I just haven't invested the time and effort into doing so.
 

Mooseman

Moderator
Dec 4, 2011
23,693
Ottawa, ON
I want to install a selector switch to engage my AWD to forcibly be engaged, I just haven't invested the time and effort into doing so.
I don't think it would work as there is no brake in the encoder motor to hold it. When I drove my Saab with a blown out rear differential, the front wheels would constantly engage and disengage, even when I would pull the fuse when engaged. It would slowly stop driving the front wheels. If you constantly keep power to the encoder motor to hold it there, it will burn out.
 

TJBaker57

Guru
Aug 16, 2015
2,241
Colorado
There's no internal linkage between the reduction and the clutchpack. The TCCM is responsible for the "timing", if you will.


I would say it is a difference of phrasing. Yes the TCCM controls the turning of the shift motor which turns the one shaft which does everything, engages the reduction and when turned further applies pressure to the clutchpack. Neither can be acted upon separately as the cams only move in unison. The sole criteria for what mode it is in is where the shift motor stops.
 

Mooseman

Moderator
Dec 4, 2011
23,693
Ottawa, ON
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MaroonMonsterLS1

Registered Member
Sep 17, 2018
18
Iowa
While we're on the subject here.
Does anybody know if a 2500 pickup transfer case has the same encoder output signal as the envoy encoder?
Say someone was doing a 4l80e swap and wanted to keep the 4x4 functionality like stock
Will the 3/4 ton truck transfer case "play nice" with the TCCM assuming you get a transfer case with the same encoder plug
 

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