4 wheel drive auto mode

[Spacido]

Hello everyone,

how does the "Auto 4wd" mode work on a Tahoe? On mine, I have the impression that his strength a lot on the front deck as if the four wheels were engaged.

I thought that the front axle was locked only in case of loss of rear wheel grip

 

[Fless]

Auto 4wd mode with the NVG246 transfer case should not engage the front diff until there is rear wheel slippage. Here are links to a couple of articles describing how the TC works and some diagnoses:

http://www.atraonline.com/gears/2006/2006-07/2006_07_10.pdf
http://www.atraonline.com/gears/2006/2006-08/2006_08_08.pdf

FYI, I believe that your year would have the 8v reference voltages as stated in the articles; later versions (like my '04) have a 5v reference. If you could verify your TCCM voltage to the encoder motor and/or dash switch I would be curious.

In Auto mode the TC varies the strength, if you will, of the power that is being sent to the front propeller shaft. I think it does this in order to make sure the prop shaft rpms are roughly the same as the rear driveshaft rpms for a smooth engagement.

 

[mikeyss]

2 HI - Front axle unlocked and all power goes to rear

Auto - Front axle locked in, all power still goes to the rear. If the rear slips, power is sent to the front axle and power flows according to what it needs to get the vehicle from slipping.

4 Hi and 4 LO - Front axle locked in, and the t case does a 50/50 split of power. 4 LO is a crawling gear which you will rarely, if ever, use.

 

[Fless]

2 HI - Front axle unlocked and all power goes to rear

Auto - Front axle locked in, all power still goes to the rear. If the rear slips, power is sent to the front axle and power flows according to what it needs to get the vehicle from slipping.

4 Hi and 4 LO - Front axle locked in, and the t case does a 50/50 split of power. 4 LO is a crawling gear which you will rarely, if ever, use.

Not sure about earlier versions, but everything I've seen is that in Auto there is always some power to the front prop shaft (through the clutch packs) but the diff is not engaged until rear wheel slippage. The prop shaft isn't "locked in."

 

[mikeyss]

The Center Axle Disconnect has to be locked when in AUTO, or else when power needs to go the the front axle, nothing would happen. The CAD can't lock and unlock that quickly in the event of sudden slippage.

 

[Spacido]

Yesterday, I suffered a very strong storm with a lot of rain. I engaged the automatic mode. I found the steering heavier and in a bend at low speed in front of my house, I felt that the front wheels were struggling to move like 2WD. a bit like I was in a 4x4

Did I do well to switch to "Auto" in these conditions? Or is it a way to book for snow or ice?

 

[Fless]

So, I'm not sure which TC the OP has. But for the NVG 246 (RPO NP8), in Auto it takes a good front diff actuator roughly 1 second to engage when commanded, which is when rear wheel slippage is sensed. You can remove the diff actuator, leaving it wired up, and use the Tech 2 (or 4hi mode) to command it on and off to check it. You can also bench-test it by applying voltage to the proper pins on the connector.

EDITED in blue

 

[mikeyss]

I have NP8 in my 05 Tahoe, and when I'm in AUTO, I had it on the lift and the front axle was locked in

 

[Fless]

By saying "axle" do you mean the differential, or the front propshaft on the TC? The propshaft will always be turning somewhat in Auto, although full power is not applied until/unless the rear wheels slip. And that's when the front diff actuator kicks in to engage the diff.

See if this video helps any:

 

[mikeyss]

I am meaning at the front diff

 

[mikeyss]

The motor engages the front axle. In mine, I noticed in auto, 4HI and 4LO, that motor has the front axle locked in. In 2HI, the front wheels were not locked in

 

[Fless]

The diff actuator should not be engaging when in Auto and standing still (no wheel slippage). Does your drivetrain bind on tight turns in Auto? It shouldn't.

 

[Spacido]

I did a test in auto mode with the wheels right in the wet grass and we can hear the front axle snap and make the vehicle go forward so everything is working properly.

But, turning right angle, for example, without skating the rear wheels, I feel a big resistance in the steering wheel and the front wheels that slide as if I was 4WD

 

[Fless]

You could disconnect the electrical connector from the front diff actuator and try it. If it still binds with the actuator disconnected, I'd suspect half shaft CV joints or a diff problem. Does the diff fluid look okay and filled?

Or maybe a hub. Does it do it on a left turn, or only a right turn? If only on a right turn, concentrate on the left side of the frontend. Not sure if the sway bar end links would cause this effect but worthwhile to make sure they're in good shape and tight.

 

[Spacido]

Yes, when I bought my truck in October 2018, I drained all the fluids, especially the automatic gearbox and the TC. Because in France, mechanics put everything and anything because they do not control all that.

And I was right, because it was not Autotrack II in the TC

 

[rjr]

While on the subject of the A4WD, on my 2015 Yukon, I find that when in auto, I hear a varying whine at low speeds. Sort of like a tire with an out-of-round thread [which it isn't]. Dealer says this is normal? Didn't do this when new; now 52,000 miles. Is this common?

 

[OneofFew]

I may not understand the auto 4x4 system well, but waiting for wheel slippage to lock in the front hubs- what could go wrong? LOL!
I would not expect auto 4x4 to be reliable and would leave it in 2hi mode until you actually need 4x4.

 

[Fless]

Auto is really for varying traction conditions, like a mix of dry and wet pavement. In full on snow or wet, 4x4 is best. Auto will engage parts of the drivetrain in and out as needed and theoretically save wear and tear. Don't use 4x4 on dry pavement, you'll get drivetrain binding and that can cause excessive wear and tear.

I wouldn't say that Auto is unreliable; one just needs to expect that it will take the front diff actuator a second or two to engage 4x4. Sometimes that's acceptable, and other times not.

 

[OneofFew]

Let's face it, it's a useless gimmick. If you go around a wet turn and the rear kicks out a but you counter-steer and are well in control...until the 4x4 kicks in and puts you in the ditch because you could not gradually recover from the slide, but catch traction unexpectedly.

 

[Spacido]

That said, I admit that you really have to do it on purpose to lose control with the Tahoe. It is quite stable, even in the rain.

In the GM documentation, there is really no explanation about the automatic mode and it's true that I like to understand how it all works.