Please help me to understand AWD vs 4WD systems

[KingpenM3]

My ignorance will be obvious here, but my wife drives a '16 4wd suburban that had a 4wd malfunction this weekend. While driving down the Interstate in "Auto" 4wd mode (it has been in that mode since we bought it), it gave a "service 4wd" message, and then it became obvious once we exited the interstate that the vehicle had locked itself in 4wd (high), difficult to steer, tires squeaking on pavement and the mode selector button won't let me switch the drive mode at all. So here are my questions, why is the 4wd in the suburban so different than the Awd of my hummer, which can take the tightest of turns? Is it as simple as the suburban "4wd" is the same as the h3's "lock" mode? If so, why don't full size vehicles have a mode similar to the h3's awd? Why is there nothing between 2wd and 4wd lock? I'm assuming the entire 4wd systems must be quite different between the h3 and a full size truck?

After a weekend of fruitless googling a fix to get it back into 2wd, I now realize that we should keep the suburban in 2wd, and not auto, unless we are actually in a low traction situation. But who knows, maybe we just got real unlucky on this one. Just disappointing that there was no way to get it out of the 4wd without getting it to the dealership.

 

[skeptic]

4wd = transfer case is locked, front and rear drive shafts turn at the same speed. Not to be confused with locked diffs where right and left wheels on a given axle are locked and spin at the same speed.
awd = through the use of a differential or viscous coupling or whatever else might be out nowadays power is sent to all 4 wheels but is not locked front to rear.

While you probably shouldn't drive the 'burb in Auto 4wd mode all the time, I suspect your problem is unrelated. It's a fairly common issue with full size GMs, one we ran into with an Avalanche we had years ago. I forget exactly what it was, but it was an easily replaced electronic bit of some kind. Just noticed it's a '16, that doesn't sound right for the electronics that failed in my Avalanche. Dealer should fix you up and tell you what broke.

To your question of why AWD for Hummer and 4wd for other Chevy trucks, I don't know. I prefer something like what I had on an old Cherokee I had, 2wd/full time 4wd (essentially AWD)/4-hi/4-lo. Let's you drive in 2wd when it's dry, full time 4wd when the roads are slick or there are patches of snow or ice here or there, and 4wd when you really need it.

 

[KingpenM3]

Thanks skeptic, that helps me to understand what is going on for sure!

A lot of what I found online was about the engine software not being able to communicate with the TCCM. I check and I had a good TREC fuse, then pulled it for a few minutes. Didn't get rid of the message or problem. I just know it is going to be frustrating if they come back and say basically, "we cleared the code, everything is working and you should be good to go". Then I'll always be worried about it happening again.

eta: Also frustrating to know that it could be the software, a wired connection, the switch, or the actual actuator that could be the problem. I checked every connection I could find in and under the vehicle to try to catch something simple. Oh well, I need to take a deep breath and remember it should be all under warranty.

 

[650Hawk]

As skeptic said, our 4HI (unlocked) is actually AWD, and allows front and rear driveshafts to turn at different rates. 4HI Lock and 4LO Lock are the same as a normal 4 wheel drive vehicle.

Chevy's full size trucks & SUVs are typically an old school type 4wd. In 2WD, the transfer case outputs only to the rear. In 4WD (both HI & LO), the transfer case outputs front and rear at the same speed, and the front hubs also lock. In AUTO 4WD, the hubs lock, but the transfer case outputs only to the rear. But with the hubs locked, the front drivetrain is all turning, so when slippage is detected, the transfer case slips easily into 4WD mode and back out as required.

Better to run in 2WD mostly unless on slippery roads, although like any other, it's always good to occasionally cycle through all the modes.

BTW, a common issue on Chevys is the encoder motor. It is pretty easy to replace, but the part is costly. I recommend going the OEM route, NOT aftermarket; that's one of those parts that, for some reason, the aftermarket ones will not last and will end up failing quickly. Because of that, the added cost ($400 vs $250) is worth it. I chased a similar issue for nearly 2 years on my wife's Tahoe ('06). Replaced dash buttons, encoder motor 3 times (aftermarket), and finally an OEM encoder solved the problem.

Edit: Just saw that you're still under warranty, so the cost of the part is moot.

 

[KingpenM3]

Thanks for the info hawk!

I just got off the phone with the dealership and they are going to replace the 4wd control module. Now it's just a matter of my wife having to use the hummer as a 4 kid hauler all week! She is going to be ready to get the burb back for sure.