<VV> First extended drive -PG shift harshness issues

[Sethracer at aol.com]

In a message dated 7/6/2007 6:41:55 A.M. Pacific Daylight Time,  
contactsmu at sbcglobal.net writes:

Oh, I  forgot one thing.  As I come to a stop, the PG downshifts like   
a horse is kicking the back of the car, but not every time (70% of   
the time?).  Ideas?






Stephen - I don't know how much you know about the PG. The 2-speed has to  
upshift and downshift under differing loads. A full-out sprint onto the highway  
means a hard upshift under full throttle at about 45mph. A smooth drive up to 
35  will require a soft shift at 18-20 mph. The way the trans knows how hard 
to  shift is the vacuum modulator. It adjusts the shift hardness depending on 
how  much vacuum is present in the engine intake. A high vacuum, like a soft 
throttle  during a slow acceleration, causes a soft, slow shift. Stomp the 
throttle and  vacuum drops like a rock. As the engine accelerates, vacuum remains 
low and a  hard shift is specified. So a leak somewhere in the line from the 
metal  cross-over tube (the vacuum source for the modulator) to the 
transmission  modulator will cause the modulator to think a low vacuum condition exists 
in the  intake and a hard shift is needed. This occurs in either upshifts or 
downshifts.  In your case, the PG thinks you have "kicked-down" the trans into 
Low gear to  accelerate - hence the hard downshift. From the metal cross-over 
tube (which can  also leak at the carb bases where there are two short hoses 
providing the seal)  to the bent metal tube that runs into the transmission area 
and the hose onto  the modulator itself,  you have to check hoses and 
connections. Since they  are vacuum connections, Chevy thought they would never be 
subjected to anything  BUT vacuum, so no clamps were installed. Of course, it is 
possible that the  modulator itself has failed, but they are pretty reliable. 
The more common  failure mode is for the modulator to fail internally and let 
ATF be sucked  into the engine. That results in pinging, because of the low 
octane of the ATF  and smoke out the exhaust.  Another piece of good news is 
that they are  quite easy to replace. So check all your connections first, then 
make sure that  the throttle linkage is in proper alignment as it connects 
through the  throttle bellcrank on the trans. Then, if it persists, try swapping 
the  modulator out with a known-good unit. If the problem goes away, you may  
want to put a brand-new unit in. -  Seth Emerson 



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