what physically happens when selecting 4WD auto from 2WD

Discussion in 'General Chevy & GM Tech Questions' started by rmx250, Dec 16, 2013.

  1. rmx250

    rmx250 New Member

    Hi Everyone,

    I have a 2009 4wd Silverado. I would like to know what, if anything physically changes when changing from 2WD to 4WD AUTO. I need this information to go back to my dealer.

    I use 4WD quite regularly and it seem I used to be able to tell when I was in 4WD Auto and 2WD. It was either a slight sound or small vibration. I can no longer tell and my mileage seems to have dropped 1-2 MPG on average.

    No warning lights have come on. 4WD seems to be working normal.

    I took the truck in for a trans fluid/filter and transfer case fluid change and I asked if they could check the operation between 2wd and 4wd auto. When I picked up the truck, the dealer told me there were no codes and they test drove the truck and they just heard tire noise. It seems that if the computer doesn't tell them what's wrong, they can't diagnose it.

    Any ideas?


  2. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    When you engage Auto 4wd the front axle engages as well as a tcase switches to a 4wd mode that isnt fully locked and then the 4wd controller and traction controls monitor the amount of wheel slip at each tire. The clutch pack in the Tcase then engages when there is a high rotation difference between the front and rear end to give additional traction. the Tcase consistently varies how much the front end is engaged depending on what conditions are felt at the wheels.

    In normal 4x4 the whole system remains always engaged with no monitoring of wheel slip minus the ABS system.
  3. rmx250

    rmx250 New Member

    Thanks for the quick reply! That really helps.

    If the front axle did not disengage when switching back to 2wd would this cause a fault code?
  4. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    Its a possibility. But also its possible it wouldn't because on some trucks you can get a part that keeps the front axle always engaged. Best way to tell if the front end is stuck is to get the whole truck in the air and put it in neutral if spinning the front driveshaft causes the front tires to move in 2wd then there's an issue.
  5. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    @Conlan Rose, you are very close with your descriptions, good job. Just to add a little additional info, when 4 AWD is selected, the Transfer Case Control Module (TCCM) commands the encoder/shift motor into the 4 HIGH position. When locked into this position, the encoder commands the front passenger axle to engage. A return signal indicating lock will tell the TCCM that the t-case is ready.
    When front to rear slippage is detected, (speed sensors mounted on the t-case output housings) The TCCM commands the clutch to engage and send torque forward. When slippage is no longer detected, the TCCM commands the release of the clutch pack.

    The only comment regarding the front axle test you posted, is that the front diff is an open diff. By that, torque is only applied to one axle/wheel at a time. Unfortunately, it always flops to the wheel with the lest resistance. That's why a 2wd vehicle gets stuck when one drive axle wheel is on ice, even though, the other wheel may be on a hard surface.

    When you turn the front drive-shaft, as you suggest, one axle will always turn even though you may be in 2WD. The axle that should turn (because it is the one with the least resistance) is the passenger side. The inner half of the axle should be turning. If the disconnect coupling is engaged, the passenger side axle/wheel should no longer always be the wheel with the lest resistance. Resistance should be equal and either wheel might turn if the front drive-shaft is turned by hand.
    The problem though, is that to engage the disconnect, the t-case must be in one of the 4WD positions. this will possibly cause the rear drive-shaft to turn. If the truck is in Park, you will not be able to turn the front drive-shaft. If however, you have selected 4AWD, there will be some resistance, but the front drive-shaft should turn.
  6. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    I try my best to learn what I can! I was trying to get it all right off the top of my head.
  7. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Hey, you did a great job, I just tossed in a little more detail; which was from the top of my head as well, so there is probably more detail that I haven't covered. But if 3, or 4, of us toss out everything we know, we'll probably be close. (I can never find a thumbs up smiley).
  8. rmx250

    rmx250 New Member

    Great! Thanks everyone. This will help when I go back to the dealer.

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