Re: GM400

[Thomas Alberts <talberts@aero.odu.edu>]

TEA> 	Incidentally, the 1-2 shift under full-throttle and with the kickdown =
TEA>	engaged is supposed to occur around 55mph which should correspond to =
TEA>	something like 5000rpm.  Without kickdown and under heavy throttle it =
TEA>	should be near 45mph, and under light throttle 5-10 mph which would of =
TEA>	course be way below stall speed.  Your shift point of 22-2300 rpm sounds =
TEA>	like a transmission problem to me.
	
KM>	I have noticed on my XJ-S that the 1-2 shift under light throttle does =
KM>	occur ~10mph.  My kickdown isn't working at the moment so I can't verify =
KM>	that shift point.  But under heavy throttle the 1-2 shift is around =
KM>	20mph.  I would like it to shift at 45mph under heavy throttle.  Perhaps =
KM>	I have a transmission problem in addition to the kickdown.
	
If you've ever looked at the logic diagram for an automatic transmission, you will
appreciate that there is nothing simple about how the decision of whether to shift
is made.  Anyway, in simple terms, the governer which determines shiftpoints based 
on speed is only part of the logic.  The vacuum modulator is how the transmission
"knows" whether the throttle is light or heavy.  The modulator operates a valve
spool that has direct input to the shifting logic.  The modulator is easy to get at 
and it is easy to check whether it is really getting the manifold vacuum signal.
This might be a good place to start.  It is also simple to check whether the
kickdown solenoid is getting a signal at full throttle (terminal on left of trans.)
and if you listen carefully you can even hear whether the solenoid is engaging.
If it is not one of these two problems, then the required repair could be beyond
what you want to get in to.

Thomas E. Alberts

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