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ABS when locked

Discussion in '80-Series Tech' started by hobbes, Jan 27, 2004.

  1. hobbes

    hobbes SILVER Star

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    I've read several times that it is obvious that you want to have ABS disabled when the CD is locked and that the nice part of the VC is that it allows ABS. I've searched. Why is ABS with a CD locked so bad?
     
  2. landtank

    landtank SILVER Star

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    With ABS active all 4 wheels need the ability to turn independantly. Because braking is determined by wheel slip if 2 or more are mechanically linked there would be no slip so the ABS would never turn on for those wheels. With the center diff locked both front and rear drive shafts are mechanically linked so not all 4 wheeels can turn independantly. You might have heard or read the term "driveline wideup", this is tension that accures in the drivetrain when the Center diff is locked and the vehicle is driven on a high traction surface. It's that mechanical link that causes the tension because there is no slip between the wheeels from turning at different rates.

    Hope this is what you were looking for.
     
  3. cruiserdan

    cruiserdan SupportingVendor Emeritus Moderator

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    ABS relies on individual wheels to react to changes in adhesion. If the center diff is locked and a front tire locks up, that lockup is transfered to the rear axle by the locked transfer case and the ABS computer can't tell which wheel is locked.
     
  4. Radd Cruisers

    Radd Cruisers

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    Oh Crap, I cannot make sense of this as I am a old time cruiser guy.

    Does centre dif mean transfercase?

    Rob
     
  5. Big_Moose

    Big_Moose

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    ABS monitors different wheel speeds to modulate the vehicle braking to allowing the vehicle to stop in a controlled manner. As others have mentioned, with the center differential locked (same as a part time 4wd cruiser in 4wd Hi-Range], the ABS system cannot apply / modulate the braking due to different signals generated from the wheel speed.

    Rob

    The 80 transfer case has a differential
    , which allows the fulltime 4wd system to function on high traction surfaces. The viscous (VC) coupling was used in later models to assist soccer moms in tackling the puddles at the malls ;) The VC locks up quicker (not a solid lock - fluid lock) allowing greater traction control to minimize wheel slippage. The "Center Differential Lock" is a electromechanical locking mechanism, which turns the cruiser into real 4wd = to the old cruisers 4wd hi range.

    Joe​
     
  6. Radd Cruisers

    Radd Cruisers

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    Thanks, that now makes sense to me now.

    Now you have me thinking how this is controled, throught the ECU or some seperate device?

    I hope its throught he ABS and does not rely on the ECU, I need to see a good skematic of the wiring and figure out how it all works.

    Rob
     
  7. hobbes

    hobbes SILVER Star

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    Cdan, thanks. That was the explanation I was looking for. I knew it was obvious, just not so to me ::)

    john
     
  8. Eduardo96FZJ80

    Eduardo96FZJ80

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  9. landtoy80

    landtoy80

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    This is the guts of the center diff.
    #3 is the vicsous coupling is locks up when one driveshaft/outputshaft spins more than the other
    #2 is the fork that is run by an actuator. It is locked by the CDL switch on the dash(if you have one) or by ECU when you put it in L

    When #2 is unlocked, #7 and #8 can turn freely (All wheel drive)
    when #2 is locked, it makes #7 and #8 turn as one (4h for you old farts)

    #1 is how it shifts from L and H