How does the A4WD work?

[Sqrly]

I just want to know because I don't.

I've run my vehicle through the different settings only to check that it all worked. I'll probably never use it.

What I think I know.

You turn the switch to AWD and the front axles lock in and a clutch somewhere comes into play that can send between 5% and 100% power to the front diff too. The rear always gets 100% power.

Questions:

Where does this front axle locking happen?

Where's the clutch and how does it work?

What rules does the system follow?

What data does it use and where does this data come from?

PS I tried searching "how does awd work" and +how+does+awd+work and I got "some of your words (how, does, awd, work,) are too common and were ignored.

 

[TequilaWarrior]

There's something to clear up first.

I think the question you're really trying to ask is how does A4WD work... NOT "How does AWD work".

The distinction is important. There are 2 basic transfer cases available in the GMT360 (3, actually, but the 3rd is a separate topic altogether). AWD (All Wheel Drive) transfer cases have no dials or controls whatsoever and are controlled by a module that uses ABS signals (independent fronts and a single rear for some years, 4 wheel signals for some years), to determine whether it should engage the front axle.

If you have a control dial you have 4WD, not AWD. The setting on the dial that is A4WD is "auto, 4wd" and is "sort of" like all wheel drive. I believe it pre-locks the front disconnect but leaves the transfer case clutch un-engaged. It too uses ABS sensors to determine when it should engage the TC clutch. 2WD obviously leaves the TC clutch disengaged and the front disconnect disengaged. 4HI engages the TC clutch and disconnect. 4LO shifts the TC to low range and engages the clutch and disconnect.

The third transfer case is technically a differential - and was only available in the Trailblazer SS and the Saab 9-7x Aero.

 

[TJBaker57]

There's something to clear up first.

I think the question you're really trying to ask is how does A4WD work... NOT "How does AWD work".

The distinction is important. There are 2 basic transfer cases available in the GMT360 (3, actually, but the 3rd is a separate topic altogether). AWD (All Wheel Drive) transfer cases have no dials or controls whatsoever and are controlled by a module that uses ABS signals (independent fronts and a single rear for some years, 4 wheel signals for some years), to determine whether it should engage the front axle.

If you have a control dial you have 4WD, not AWD. The setting on the dial that is A4WD is "auto, 4wd" and is "sort of" like all wheel drive. I believe it pre-locks the front disconnect but leaves the transfer case clutch un-engaged. It too uses ABS sensors to determine when it should engage the TC clutch. 2WD obviously leaves the TC clutch disengaged and the front disconnect disengaged. 4HI engages the TC clutch and disconnect. 4LO shifts the TC to low range and engages the clutch and disconnect.

The third transfer case is technically a differential - and was only available in the Trailblazer SS and the Saab 9-7x Aero.

Let's further clarify a couple of things.

The 4WD systems don't use the ABS wheel speed sensors at all. The transfer case has its' own front and rear propshaft speed sensors that are wired to the TCCM (Transfer Case Control Module). It is these two sensors that are monitored by the 4WD systems.

The A4WD selection of the selectable 4 wheel drive systems does indeed lock the front axle disconnect, coupling the right side CV axle to the intermediate shaft that runs through the engine oil pan from the front differential to the "intermediate shaft bearing assembly" (front disconnect).

Additionally, in the A4WD 4wd selection the transfer case shift motor DOES slightly engage the transfer case forward clutch sending a small amount of torque to the front propshaft.

And again, the 4WD controller monitors the front and rear propshafts speeds to detect rear wheel slippage, NOT the ABS speed sensors.

The ABS wheel speed sensors are wired solely to the EBCM (electronic brake control module) and are used solely for the antilock braking system.

 

[TJBaker57]

Questions:

Where does this front axle locking happen?

Where's the clutch and how does it work?

What rules does the system follow?

What data does it use and where does this data come from?

There is a device bolted to the right side of the engine oil pan. This device couples or decouples the right side CV axle from the 4wd system.

Here is a look inside that unit showing some of how it works....

Where's the clutch and how does it work is a little more complicated to follow. Here's a good starting point. the power flow diagrams in particular can be hard to follow. Some day I will grab a used unit at the upull yard and disassemble it to really get familiar.

Transfer Case Operation: NVG 226 (NP8) | offroadTB.com

offroadtb.com

The TCCM (transfer case control module) uses inputs from the mode switch on the dashboard, the two dedicated propshaft speed sensors, an "encoder ring" sensor inside the transfer case shift motor, and a status signal from the front axle disconnect actuator.


We can get really detailed into the control system and I believe you have the aptitude to understand it.

The electrical concepts used for the mode switch and the encoder ring are basic and widely employed. Getting familiar with them can be extremely useful in diagnosing many many vehicle sensors.

 

[TJBaker57]

Here is a pretty good look at the transfer case internals...

Exploded Views — 2002 Chevy Truck TrailBlazer 4WD L6-4.2L VIN S Service Manual | Operation CHARM

Detailed repair manual for the 2002 Chevy Truck TrailBlazer 4WD L6-4.2L VIN S.

charm.li

 

[Sqrly]

There's something to clear up first.

I think the question you're really trying to ask is how does A4WD work... NOT "How does AWD work".

Correct. I edited the title of the post in case anyone looks for this info in the future.

Where's the clutch and how does it work is a little more complicated to follow. Here's a good starting point. the power flow diagrams in particular can be hard to follow. Some day I will grab a used unit at the upull yard and disassemble it to really get familiar.

Transfer Case Operation: NVG 226 (NP8) | offroadTB.com

offroadtb.com

The TCCM (transfer case control module) uses inputs from the mode switch on the dashboard, the two dedicated propshaft speed sensors, an "encoder ring" sensor inside the transfer case shift motor, and a status signal from the front axle disconnect actuator.

The text is clear enough. I get how the T-Case works.

The front diff, the drivers side axle is perma locked in. In 2Hi are the spider gears spinning like crazy or is the front drive shaft spinning or some combination of both?

 

[TJBaker57]

Correct. I edited the title of the post in case anyone looks for this info in the future.



The text is clear enough. I get how the T-Case works.

The front diff, the drivers side axle is perma locked in. In 2Hi are the spider gears spinning like crazy or is the front drive shaft spinning or some combination of both?

My data records indicate a combination of both.

As you say the left side CV axle is constantly connected. So the drivers side "side gear" in the differential spins at the wheel speed rpm.

In 2Hi because of the rotational frictions inside the transfer case and differential the front propshaft does spin, but not as fast as the rear propshaft.

Knowing how a differential works one can see that if a side gear is spinning one way, and the propshaft is spinning below the speed required to rotate the carrier enough to keep up, then the opposing side gear will spin the opposite direction.

I have observed that the speed of the front propshaft and resulting reverse speed of the intermediate shaft/passenger side gear changes with the temperature of the transfer case/front differential/etc.

My front propshaft speed is faster as I begin to drive than the speeds seen after some driving at highway speeds once the fluids have come up to temperature.

 

[Sqrly]

My data records indicate a combination of both.

As you say the left side CV axle is constantly connected. So the drivers side "side gear" in the differential spins at the wheel speed rpm.

In 2Hi because of the rotational frictions inside the transfer case and differential the front propshaft does spin, but not as fast as the rear propshaft.

Knowing how a differential works one can see that if a side gear is spinning one way, and the propshaft is spinning below the speed required to rotate the carrier enough to keep up, then the opposing side gear will spin the opposite direction.

I have observed that the speed of the front propshaft and resulting reverse speed of the intermediate shaft/passenger side gear changes with the temperature of the transfer case/front differential/etc.

My front propshaft speed is faster as I begin to drive than the speeds seen after some driving at highway speeds once the fluids have come up to temperature.

I would assume then that when I shift to 4Hi while moving I am trusting a tiny computer to engage the T-Case clutch first and get the front propshaft up to speed before trying to lock in the front right axle so that the differental doesn't explode!

That's a scary thought LOL

I am reminded of a freind that literally blew the "auto-locking" front hubs off his ford ranger 4WD every time he went airborn in it while off roading. The dealer eventually replaced them with BW manual hubs and told him to go away

 

[TJBaker57]

I have observed that the speed of the front propshaft and resulting reverse speed of the intermediate shaft/passenger side gear changes with the temperature of the transfer case/front differential/etc

Here is a graphic from back when I was working with these data points. It shows what I mentioned about the front propshaft speed slowing as the system warms up.

 

[TJBaker57]

I would assume then that when I shift to 4Hi while moving I am trusting a tiny computer to engage the T-Case clutch first and get the front propshaft up to speed before trying to lock in the front right axle so that the differental doesn't explode!

I think they are fairly well simultaneously initiated.

It is exactly this that caused me to develop the equations to show the speed difference between the intermediate shaft and the right side CV axle.

I have seen values around 1100 difference at 65 mph.

Given this I tend to not engage any 4WD mode at high speeds.

 

[Sqrly]

Here is a graphic from back when I was working with these data points. It shows what I mentioned about the front propshaft speed slowing as the system warms up.

That's expected as the oil viscosity changes with temperature.

In my RC cars we actually use different diff oils to match road conditions. (They use very thick lube, like 10Kw silicone liquid)

 

[TJBaker57]

That's expected as the oil viscosity changes with temperature.

In my RC cars we actually use different diff oils to match road conditions. (They use very thick lube, like 10Kw silicone liquid)

I have wondered if the speed difference at various temperatures could be used as an indicator as to the condition of the AutoTrak II transfer case fluid and/or the front differential fluid.

I could do a controlled test recording data before and after a fluid change of both.

 

[Sqrly]

I have wondered if the speed difference at various temperatures could be used as an indicator as to the condition of the AutoTrak II transfer case fluid and/or the front differential fluid.

I could do a controlled test recording data before and after a fluid change of both.

You'd need data on the fluid itself. Does the viscosity change over its lifetime, does it's reaction to temperature change differ, if yes what are the changes.

Me, I stick my finger in it then smell it and possibly taste it and compare it to fresh fluid

 

[TequilaWarrior]

Let's further clarify a couple of things.

The 4WD systems don't use the ABS wheel speed sensors at all. The transfer case has its' own front and rear propshaft speed sensors that are wired to the TCCM (Transfer Case Control Module). It is these two sensors that are monitored by the 4WD systems.

Thanks for that. Somehow I missed that detail in previous system descriptions.