AWD, what's going on?

tealboy · April 1, 2006

I guess i don't understand how AWD works. My vehicle dipped into an unexpected hole and the rear hitch got hung up to the point I couldn't get traction at the rear tires. I thought the front wheels would have kicked in to pull me out, but they didn't seem to do that. The rears would spin fast, but the fronts were not spinning, even when i stomped the gas pedal. However, once i got out of the hole and tried to spin the tires on the soft surface/gravel, the fronts were grabbing and maintaining traction.

i guess i don't really understand how awd works, b/c the ditch puzzled me.

cadillac escalade awd

mmmikkke · April 1, 2006

TB,

I think maybe one of your front tires was also in the air. They both need to be on the ground or your front axle won't pull you. (The front axle's a standard axle, not a locking axle, so if one of its tires doesn't have traction, you get no help from the front axle.)

If both front tires were on solid ground, then something's not working right. They should have pulled you out.

Mike

firstruck · April 1, 2006

I know it sounds counter-intuitive but, you should have left foot braked while squeezing the go pedal. Appling the brake just enough stops the "free spinning" wheel allowing the power to go to the "stuck" wheel. The free spinning wheel takes very little brake force to hold it, there by equalizing the traction between the wheels. Or I'm talking out my azz again? Does this sound correct to anyone else?

the blur · April 2, 2006

Basically the transfer case sucks. I got stuck on ice with my AWD Denali. It's an electronic transfer case... That should have kicked in, when it senses loss of traction.

The older viscous coupled transfer cases were better.

and besides that, the Denali is not available with posi rear.

bowtygie · April 3, 2006

the denali truck website has a very detailed description on how this transfer case works the website has lots of info on these types of systems i would type the link but i don't know how

Butch 02 Denali · April 3, 2006

the denali truck website has a very detailed description on how this transfer case works the website has lots of info on these types of systems i would type the link but i don't know how

<{POST_SNAPBACK}>

That link is in my sig!

A brief explanation of the two major types of AWD used in the full size trucks.

The NVG149 vicous coupling transfer case. This is about as good as it gets for performance and traction. It has the capibility of transferring 0-100%, or anything in between, to the front or the rear as traction calls for it. Very strong system and rated for 2300 ft lbs of torque. It came/comes in the 2001 GMC C3, the 2002-2006 Sierra Denali (with a full posi rear on the 02-04), the 2001-2002 Yukon Denali and the 2001-2002 Escalade.

In 2003 the Yukon Denali and the Escalade changed to an open center diff(transfer case) and open diffs front and rear. The "AWD" is acomplished by the ABS braking system. As the ABS senses a wheel spinning faster than another, it applys the brakes to that wheel untill it matches the speed of the others. It works ok but not great as you've found out! Applying the brakes is not the greatest thing for performance either!

Hope that helps!

chuck16 · April 3, 2006

Not that this applies and it's a completely different beast but, when it comes to AWD nothing beats a Subaru.

I had a 1990 Blazer 4x4 and it was ok in the snow in stuff but, it was easy to get it stuck. My Wife has 2000 Subaru Impreza and we've driven through snow that almost high centered the car and it pawed it's way out on 15" rubber no less.

Now before everybody slams me about a Japanese vehicle I was merely stating that the American vehicles still have a little ways to go when it comes to getting true "AWD". Now having said that there is nothing that compares to the sheer pulling power that our 4x4 trucks have when all 4 wheels are biting in and doing there job.

Maybe someday there will be some common ground where the AWD and 4x4 become one and you get true 4 way posi with the brute force of our trucks.

Just my 2 bits worth.

firstruck · April 3, 2006

I stand corrected.

tealboy · April 3, 2006

ButchDenali, given my circumstances, do you know what happened to my system and why the fronts weren't grabbing while the rears were spinning? It is a 04 excalade.

mmmikkke · April 3, 2006

If your transfer case is operating as "open diff" and both front and rear axles are "open diff" then you'd need the brakes to stop all spinning wheels before you could get any traction on the ones that had grip. Are your rear brakes worn out or could they have been inoperative?

Butch 02 Denali · April 3, 2006

Firsttruck and Mike, your idea was sound and could work if the emergency brake was applied. That would stop the rear wheels from spinning and probably get the power transferred to the front like you needed!

Chuck16, yes the Subaru is a good AWD system, but it is exactly the same system used in the Sierra Denali's and Chevy SS with the NVG149 transfer case. These systems are as good if not better than Subaru's! No comparison to the 4WD in your Blazer.

Tealboy, you were "highcentered" meaning the frame was hung up on the ground and the wheels were in the air. The emergency brake could help like I mentioned above. Other than that, if you're alone with no tow, jack the rear up and fill in under the rear tires with anything you can find, then lower it down. Hopefully you could then drive out.