Noob 4x4 question!

[MTPockets]

Sorry for the noob Q, havent taken 'delivery' of our 05 TB yet :worried: , How exactly does the 4x4 systems work on these trucks? Does it have an electric shift on the fly (SOTF) actuator on the front axle and an electric clutch in the transfer case?? Just wondering how the "automatic" 4WD works. Never had that on my other shift on the fly part-time 4x4's

Thanks!
Joel

[the roadie]

Sorry for the noob Q, havent taken 'delivery' of our 05 TB yet :worried: , How exactly does the 4x4 systems work on these trucks? Does it have an electric shift on the fly (SOTF) actuator on the front axle and an electric clutch in the transfer case?? Just wondering how the "automatic" 4WD works. Never had that on my other shift on the fly part-time 4x4's

Noob questions aren't bad, but this issue *has* been extensively discussed in previous threads you can find with a search.

In short, the front axle is always active and has an open (non-limited slip, non-locking) differential. The transfer case has a set of clutches that can transfer torque coming from the input shaft to the front and rear output shafts. In 4WD HI and LOW mode, the transfer case clutch is commanded to go to maximum torque sharing by an encoder motor, which means minimum slippage between front and rear output shafts, but they are not in any way locked tightly together.

In A4WD, the encoder motor sends the transfer clutch pack to a "ready" position that is almost engaged, but not normally sending any torque to the front. If the computer senses an RPM difference (slippage) between the front and rear output shafts, the clutch engages with a bit of a time delay and sometimes a jerkiness, after the rear end starts spinning faster than the front. As the slippage decreases, and the input and output shafts reach the same speed again, the A4WD system lets the transfer clutch disengage a bit and you go back to the mostly RWD situation you started with.

Did you buy the G80 Eaton automatic locking rear differential option? If so, run a search for G80 and read my writeups on its operation in other threads. People have been stuck in 2 inches of snow on a hill without it. And congratulations. It's a great option.

The 4LOW state is about a 3-to-1 reduction gearing inside the transfer case, and can only be entered or exited with the shift in neutral, and the vehicle coasting between 0 and 5 MPH.

[SilverTB]

Do the TB's use the same Transfer case as the 1500 trucks? This is the first I've heard of a 4wd using clutches in the transfer case. I've always understood 4wd transfer case's are typically engaged or not vs. an awd that has clutches to make them more variable. You may very well be correct I just assumed the transfer case was the same or very similar to that transfer case I had in my 03 tahoe.

The tahoe had the auto setting but there was no clutches to it. The moment it slipped the TC locked. Nothing gradual or slipping like a clutch. All the auto setting did was engage the front diff and then wait for slipping to engage the TC.

I may be off base but I presumed the TB would be the same. A4wd would force the front diff to engage and automatically engage the Transfer case as needed. Where 4wd would force the front diff and TC to stay locked all the time.

[the roadie]

Do the TB's use the same Transfer case as the 1500 trucks?

I think new 1500s use the NVG 246, and I know we use a 226.

The New Venture Gear Numbering System

The first number is the number of speeds:

"1" = one speed (high range). Example – the NV 136.
"2" = two speeds (high range and low range). Example – the NV 241.
The second number is the strength:
The NV 241 is designated "4" in strength – tougher than the NV 231, but not
as heavy duty as their largest T-cases, which go up to a "7" (eg, NV 273,
for vehicles with a GVW of 17,500 lb).
The third number designates the type of T-case:
"1" = part-time 4WD
"2" = full-time 4WD with an open center differential plus lockable part-time
option – like the Selec-Trac NV 242 of the Jeep Cherokee and Dodge Durango,
or the Hummer’s NV 242HD AMG.
"3" = electrically shifted.
"4" = not currently used.
"5" = Torsen-type differential.
"6" = computer controlled multi-plate wet clutch, like GM’s AutoTrac NV 246.
"7" = GeroDisc – like the Grand Cherokee’s Quadra-Trac II NV 247.
"8" = not currently used.
"9" = viscous coupling.


This is the first I've heard of a 4wd using clutches in the transfer case. I've always understood 4wd transfer case's are typically engaged or not vs. an awd that has clutches to make them more variable. You may very well be correct I just assumed the transfer case was the same or very similar to that transfer case I had in my 03 tahoe.

Don't know what's in the Tahoe, but generally they have more serious off-road kit than we do. Wish it wasn't so.

The tahoe had the auto setting but there was no clutches to it. The moment it slipped the TC locked. Nothing gradual or slipping like a clutch. All the auto setting did was engage the front diff and then wait for slipping to engage the TC.

Cool. Wish we had a real locking transfer case as well, but alas.

If we did, it might overstress our wimpy CV joints and then we'd be making noise all the way back to civilization.:hissy:

I may be off base but I presumed the TB would be the same. A4wd would force the front diff to engage and automatically engage the Transfer case as needed. Where 4wd would force the front diff and TC to stay locked all the time.

Oh, I wish. But this isn't a serious off-road vehicle in GM's product roadmap. Only in our dreams. :yes:

[SilverTB]

Sorry I was off a little. Just went back and re-studied up and they sound very similar. When I hear clutches and TC I think AWD variable torque type setup and I don't believe that is what you were saying. Apparently the tahoe does have clutches but it is more of a shock preventer for A4wd rather than and variable torque split clutch. Which sounds the same style as the TB.

Sounds like the TC maybe different models but the systems pretty much work the same. I do have to say the Tahoes auto engaged smoother than the TB. Not sure if that was due to having a G80 or what but the tahoe didn't jerk noticiably. You could gas and go in the snow. For the TB I find myself peddling it a bit to prevent it slamming the front in.


Sorry to drag the thread down. Love the Tahoe and TB both. I hate to say it but I think I like the TB better than the Tahoe I just wish the TB had more mods availalbe.

[MTPockets]

Thanks roadie. So the front axle/diff/prop shaft is live all the time right up to the xfer case? I'm used to the rodeo where the front axle disconnected on the drivers side (via SOTF actuator), basically causing the open front diff & front drive shaft to go limp in 4x2 mode. Rodeos have the G80 posi rear. Do the TB's list "G80" on the firewall badge? Dunno if I have it or not. I know those buggers are sensative to having the lim slip add in there.

G/luck
Joel

[the roadie]

Thanks roadie. So the front axle/diff/prop shaft is live all the time right up to the xfer case? I'm used to the rodeo where the front axle disconnected on the drivers side (via SOTF actuator), basically causing the open front diff & front drive shaft to go limp in 4x2 mode.

Ahhhhh, I mistyped myself another time, reading about the wrong front diff in the shop manual. The full-time AWD in the Bravada has no front diff deactivation. The TB/Envoy indeed does have an actuator controlled by the transfer case controller, and locks a spline coupling when in any of the three 4WD modes. In 2WD mode the drive shaft from the transfer case to the front diff will not be turning. Sorry. :duh:


Rodeos have the G80 posi rear. Do the TB's list "G80" on the firewall badge? Dunno if I have it or not. I know those buggers are sensative to having the lim slip add in there.


The Eaton automatic locker is RPO code G80, and it's on the glovebox sticker.

[MTPockets]

Thanks man:thumbsup: I am loving this forum. I am spending too much time browsing already & I don't even have our TB yet! (waiting for my credit union to cut me the check)

Joel

[MTPockets]

In 2WD mode the drive shaft from the transfer case to the front diff will not be turning..

That's a great thing! IIRC, then just the ring gear side of the front diff spins in 2wd mode when the front axle disconnect is open, so the front diff input is dead. That all helps reduce wear and helps gas mileage. I know guys over on some of the isuzu rodeo forums (basically a GM drivetrain) yanked the front hub solid drive flanges and installed manual locking/unlocking hubs. Reason being, to keep the front half-shafts from spinning in 2wd to reduce wear and increase MPG's. The MPG thing never panned out! IMHO, it's not worth it for a DD.

G/luck
Joel