cracked open the  service manual  buried in this  laptop for the 3680.  

seems   either I misunderstood  my  guru,  or both of us were  'close, but no cigar'.

we both thought  that the solenoid  redirected oil to the other side of the shifter piston to move the  fork.   it doesn't.

one side of the piston  is  about half the diameter of the other.     the  small end  has constant  240 psi (17bar)   at all times.    
The  large end   gets  17bar  oil  when  commanded  to  shift to  low speed.   

the larger side  will overtake the  smaller  because of the  exposed area differential..   

simple and  elegant  once it's  explained!!   



I've  since  discovered/re learned  a couple  of  things-   
1. solenoid when  UNenergized   allows  oil to flow, and pushes  piston to  LOW.  I was  BassAckwards originally.  
oil on the far side is  simply  pushed back into the  17 bar  circuit.

2. the  Energized  solenoid   blocks  flow,   and  the  piston is moved  with the  ever present  pressure on that  side to  high range.  
Oil on the  far side of the piston is then  dumped back through the  spool..back to sump. 

3.   the switch,  dead center in the  round shift cover,   is not used to soley interrupt 4x4  like the  3545 does.
      it is activated  when  pulling the shift lever to the left,  and it tells  the autotronic  to  shift. (energize, or de energize the solenoid)  which  includes waiting for motion to stop,  plus intrupt the 4x4.  
So,   I've  established my  spool is  clean and free.  works  as it should.   

my next  step is to  pull the  shift cover,  and  check the piston seal and o-rings.. plus check for  freeness of  shifter fork/rail.


 

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