Auto 4x4 mode

[XTREEME]

I remember reading something here a while ago about someone saying that the Auto mode is not that great at reacting to wheel slippage.

This morning was our first good snowfall since I got the TB, so I finally had a chance to try it out for myself. I have to agree with that claim of slow reaction time. I can't believe how different it is from the full size truck's version of the Auto mode. My Av's system had an excellent reaction time, and I don't think I ever had to use regular full time 4x4 mode due to that.

Another thing I noticed this morning is how much more unstable the short wheelbase of the TB is on snow covered on ramps compared to the long wheel base full size trucks. Yikes, I almost did a loop de loop this morning.

[04TBEXT]

Xtreeme, Your two vehicles have different transfer cases. Especially the electronics which command apply of the front axle(s). My '04 and your '05 T/Bs have the same transfer case set-ups. My '02 Suburban has yet different set-up comparative to the set-up on your '03 A/V. I agree about the full-size vehicles in that my Suburban engagement rate is quicker (maybe more noticable) than the T/B, but I never thought the engagement rate of the T/B to be slow. Just my opinion on the reaction time, but by looking @ the set-up differences (especially the electronic architecture), I can see how the T/B lags behind.

[02EnvoySLE Guy]

Interesting... I've never really thought it to be "bad" at reacting; of course I don't know if I'm feeling the limited slip (G80) in the rear kicking in before the 4x4 has to or not but it seems fine to me.

But then too, I'm used to the short wheelbase trucks (had a blazer before this) and I didn't have the auto mode at all before.

[XTREEME]

Too be more specific, I had never felt a lag at all in the 03 Av, but I did have a lag in my 99 Silverado, the first year of the Auto option.

Maybe they are still working out the bugs in the TB unit?

[Envoy Fan]

Another thing I noticed this morning is how much more unstable the short wheelbase of the TB is on snow covered on ramps compared to the long wheel base full size trucks. Yikes, I almost did a loop de loop this morning.

Does your TB have the G80 locking rear axle? Could the difference also be in passenger tires vs the truck tires you were used to?

[02EnvoySLE Guy]

Too be more specific, I had never felt a lag at all in the 03 Av, but I did have a lag in my 99 Silverado, the first year of the Auto option.

Maybe they are still working out the bugs in the TB unit?

if there is a different brand/model of Tcase in the TBs, I would think it's possible the programming or just the physical unit won't allow it to engage as quickly. Or maybe that it won't engage at all above a certain speed.

I agree with EF that the G80 could make a big difference (I know I notice it) but as far as tires, you'd have had to be running some damned agressive truck tires to make THAT much difference between them and the Contitrac TR's.... Maybe some BFG A/T T/A KDW's or something.

[04TBEXT]

Xtreeme... Leave it up to GM...the '99 had a different transfer case set-up; all with AUTO 4WD capabilities. The '99 RPO NVG 246NP8 - simplistic in design. In 4WD Auto, the system monitors front vs. rear propeller shaft speeds. When a speed variance is determined the transfer case encoder motor would activate and engage the front & rear propeller shaft together. When propeller shaft speeds returned to the same speeds, the encoder motor would disengage to 4 WD Auto "inactive mode"....somewhat of an "on" & "off operation. The '03 A/V RPO NVG 248NP8 was similar to the NVG 248 except it housed an additional clutch pack. The encoder motor, operating with the same propeller shaft inputs, would engage and/or disengage this clutch pack based on comparative propeller shaft speeds. This clutch set-up would apply like the clutch in a manual transmission car, with 100% slippage-to-0% slippage at full engagement (maybe the reason for a comparative lag time). Lastly, our T/Bs... RPO NVG 226NP8 is a fully active transfer case, front & rear propeller shaft are fully engaged. This transfer case uses a wet clutch set-up (like an automatic transmission). The transfer case control circuit has the ability to apply pre-determined amounts of "torque apply" expressed in a percentage. Slippage, by design, is limited up to 5% apply a speeds under 15 mph i.e to avoid front tire lock-up an full turns on dry pavement, etc. At higher speeds, the percentage of torque applied (clutch slippage reduction) is determined by the differences in front-to-rear propeller shaft speeds. This torque application is applied again, via the transfer case encoder motor. This may be the reason for a lag time again as engagement is applied as torque increases. Gradual torque..gradual engagement from 0% up to 100%

[XTREEME]

Does your TB have the G80 locking rear axle? Could the difference also be in passenger tires vs the truck tires you were used to?

I had the G80 in the Av too.

I think the problem was that I had it in Auto 4wd and doing about 40 at the time. I guess I was expecting the front wheels to kick in.

[XTREEME]

Xtreeme... Leave it up to GM...the '99 had a different transfer case set-up; all with AUTO 4WD capabilities. The '99 RPO NVG 246NP8 - simplistic in design. In 4WD Auto, the system monitors front vs. rear propeller shaft speeds. When a speed variance is determined the transfer case encoder motor would activate and engage the front & rear propeller shaft together. When propeller shaft speeds returned to the same speeds, the encoder motor would disengage to 4 WD Auto "inactive mode"....somewhat of an "on" & "off operation. The '03 A/V RPO NVG 248NP8 was similar to the NVG 248 except it housed an additional clutch pack. The encoder motor, operating with the same propeller shaft inputs, would engage and/or disengage this clutch pack based on comparative propeller shaft speeds. This clutch set-up would apply like the clutch in a manual transmission car, with 100% slippage-to-0% slippage at full engagement (maybe the reason for a comparative lag time). Lastly, our T/Bs... RPO NVG 226NP8 is a fully active transfer case, front & rear propeller shaft are fully engaged. This transfer case uses a wet clutch set-up (like an automatic transmission). The transfer case control circuit has the ability to apply pre-determined amounts of "torque apply" expressed in a percentage. Slippage, by design, is limited up to 5% apply a speeds under 15 mph i.e to avoid front tire lock-up an full turns on dry pavement, etc. At higher speeds, the percentage of torque applied (clutch slippage reduction) is determined by the differences in front-to-rear propeller shaft speeds. This torque application is applied again, via the transfer case encoder motor. This may be the reason for a lag time again as engagement is applied as torque increases. Gradual torque..gradual engagement from 0% up to 100%

Interesting stuff. My obvious question would be - why couldn't they have just developed the Av's version for the rest of the vehicle line?

[02EnvoySLE Guy]

Interesting stuff. My obvious question would be - why couldn't they have just developed the Av's version for the rest of the vehicle line?

Based on his post, I'd have to guess the AV's version probably isnt/wasn't the most efficient of the three setups, and may not be as compatible with the rest of the driveline (given the torque curve of the I6 as compared to the V8 or other motors)

[thecarbonman]

Interesting... I've never really thought it to be "bad" at reacting; of course I don't know if I'm feeling the limited slip (G80) in the rear kicking in before the 4x4 has to or not but it seems fine to me.

But then too, I'm used to the short wheelbase trucks (had a blazer before this) and I didn't have the auto mode at all before.

I see it as you do, I feel the locking diff. hooking up not the 4x4. I do notice on off-road use in 4hi or 4lo the locking diff. doesn't seem to lock as quickly, because of less slippage. Auto4x4 doesn't react until slipping occurs, that I have noticed. :)

[02EnvoySLE Guy]

I see it as you do, I feel the locking diff. hooking up not the 4x4. I do notice on off-road use in 4hi or 4lo the locking diff. doesn't seem to lock as quickly, because of less slippage. Auto4x4 doesn't react until slipping occurs, that I have noticed. :)

Yeah it's almost like a "Dual Stage Traction Reaction" (whoa... sounds like a throwback term from a 70's disco song!) the G80 locks up to catch initial slippage, and then if it still needs some help, the 4x4 engages. Though I've RARELY had it go to the 4x4 on me unless I physically switch it into 4HI.

[ParadoxJr]

I just checked my option codes, and I have GU8 and G80.. Any Idea's???

[02EnvoySLE Guy]

I just checked my option codes, and I have GU8 and G80.. Any Idea's???

that would be a 3.73 locking rear differential. :)

[chivistb]

How do I know what kind of a locking rear differential I have on my TB?

[02EnvoySLE Guy]

How do I know what kind of a locking rear differential I have on my TB?

What do you mean 'what kind'?? If you have a locking differential (G80) it's an Eaton-manufactured unit.