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Allison tranny stuck in neutral?

Chevrolet trucks GMC trucks Silverado trucks GMC how-to Chevrolet truck how-to truck troubleshooting Allison

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#1 RareCJ8

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Posted 18 October 2013 - 11:35 AM

2001 2500HD 8.1L with Allison 1000.  Warn M12000 winch with dual batteries.  Assisted in the recovery of a jeep that rolled into a deep gully.  Hard pulling and amp draw at about 10 degrees side hill-- near level.   Truck in park and ebrake fully engaged.   Hard pull really put load on system-- ran up RPMs to help recover some juice.  At one point winch stopped working due to insufficient power.  (Should have used the snatch block.)  Winching force actually pulled truck sideways about 4'.  While winching also stood on brake pedal and at times it was nearly all the way to floor.  

 

Relevant fact:  high RPM for about 10 minutes while tranny in Park and T case in 4 low.  Ran up RPMs while winching so alternator could generate amps.  Heavy amp draw ands stood on brake pedal (to hold position and not get pulled into the deep gully). 

 

Eventually got him out and was time to go.    Put truck into drive and....nothing.  Reverse...nothing.   Uh oh.  Toggle between 4 low, 4 high and 2 high.  Nothing.  No sounds, no indication of anything moving.  Now the rescue rig needs help.  Another vehicle pulled me onto level ground and turned off engine.  Let it sit for about 5 minutes pondering what to do so tried again.  Upon start up, all is normal and drove home w/o issues.  

 

Any explanation why/how a period of high RPM in park will neutralize the tranny?  Or did the T-case go into a fit?  since only a very slight side hill do not believe it was fluid starved.  

 

Thanks,

 

Mark


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#2 RareCJ8

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Posted 18 October 2013 - 11:42 AM

Immediately thought limp mode, but that should offer R and 3rd gear.  I had nothing.  Hope to have someone check for T codes and see if that offers any clues...


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#3 RareCJ8

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Posted 18 October 2013 - 11:48 AM

Am leaning toward this as problem:

 

NSBU switch

 

http://www.merchant-...h2001-2004.aspx


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#4 Moses Ludel

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Posted 18 October 2013 - 04:40 PM

I read the link, and I believe you're on it, Mark!  The chassis was in a strain and linkage could have been binding with the side pull.  The transmission stayed in Park, because the pawl was engaged; however, the shift valve may have been moving slightly toward Reverse position with the chassis twist. Out of sync (Park versus Reverse), according to the explanation, the NSBU switch would throw the transmission into Neutral override as a protective measure.

 

Question: Do you have cable or mechanical shift linkage to the transmission? Mechanical would be more susceptible to the side twisting of the chassis while the powertrain torqued in the opposite direction...

 

As a footnote, this degree of force on the Parking pawl can snap the pawl cog. Fortunately, in your situation, it sounds like you were side pulling, not straight ahead, which would be much worse. Better to plant your foot on the brakes in Neutral, the effect is the same, actually. 

 

The pawl engagement locks the output shaft of the transmission to the transmission case. Take a look at the pawl as an individual part: It's amazing that you can park a hefty vehicle on a slope with this arrangement. Always wise to set the parking brake in conjunction with using the Park mode. 

 

Sounds like the truck's powertrain system corrected itself on level ground once recycled through the Key-Off/Key-On modes.  At $125 for a new NSBU switch plus labor, try operating the truck for a while before doing any parts replacing.

 

By the way, did you get any fault codes? The article talks about that likelihood if the NSBU switch comes into play...

 

Moses





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