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  #37  
Old 02-15-2012, 07:33 PM
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Re: Jerking motion upon engine start? Easy fellas

Quote:
Originally Posted by xj31 View Post
Case Number: S1121000013
Release Date: 09/07/2011
Symptom/Vehicle Issue:
At start up, the customer may complain of a vehicle bump or the vehicle feels as though it lunges.
Discussion:
The MP3023 two-speed transfer case is an "on-demand" active four-wheel drive transfer case that provides optimum traction for a wide range of conditions, while maximizing driver convenience. When operating in 4WD Auto mode, this sophisticated system anticipates and prevents slip by redirecting torque as necessary through the use of an internal clutch.
If a turn is made, there will be a slight difference in rolling radius between the front and rear axles. The difference in rolling radius results in the axles rotating at different speeds. If the driving conditions or road surface requires the 4WD system to engage, the application of the transfer case clutch will attempt to prevent the axles from turning at different speeds. This will cause the driveline to "wind up." The extreme case of this is "crow hop" which is the binding experienced when making a tight turn while driving in low range when the transfer case is locked.
In high range, the clutch is automatically applied during various driving situations. If the clutch is applied while the vehicle is brought to a stop and there is any driveline windup present, the clutch will actually hold the vehicle in position after the transmission is shifted to park. In this specific situation, driveline windup is holding the vehicle stationary, not the park pawl.
By design, if the transfer case clutch is engaged when the ignition key is turned to OFF, the system leaves it engaged. This is a precautionary measure to ensure that if four wheel drive was required to get to a specified location, the transfer case remains in 4WD with the ignition key OFF so that the parking system is more effective due to the additional traction that the front wheels can provide.
On the next ignition cycle sequence, the system now has to reestablish the proper high range default position. The transfer case may move the clutch back to the default starting position, getting it ready for the next drive cycle. Because the clutch may have had to be released, the driveline would then "un-wind" allowing the parking pawl to hold the vehicle stationary. When this occurs, the customer may experience a slight bump or it may feel as the vehicle lunges at start up. This condition is normal and cannot be eliminated.



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  #38  
Old 02-18-2012, 11:34 PM
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Re: Jerking motion upon engine start? Easy fellas

Finally resolved, thanks xj31!
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  #39  
Old 03-23-2012, 11:21 PM
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Re: Jerking motion upon engine start? Easy fellas

Reading that case makes perfect sense, as I usually have the problem when parking in a lot, like at the mall. I drive straight in to my garage and it never lunges there.
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Old 03-24-2012, 10:51 AM
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Re: Jerking motion upon engine start? Easy fellas

Quote:
Originally Posted by jjmonta View Post
Reading that case makes perfect sense, as I usually have the problem when parking in a lot, like at the mall. I drive straight in to my garage and it never lunges there.

yep, same here. Thinking back it has only done it after turning into a parking spot....never at home in the garage or in the driveway. It's still strange, however.
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