[V8] cruise control troubleshooting

[Ed Kellock]

It's coming up on my 3-year anniversary with my 93 V8.  The cruise control
has never worked on it, until yesterday!  Unfortunately, it was a fluke and
short-lived.  Nevertheless, I think this solidly establishes a foundation
from which to begin my diagnosis.
 
I believe my kick down switch, full throttle switch, whatever it is at the
firewall is also fubar.  The car has never reliably kicked down properly in
relation to full throttle, in Sport mode or otherwise, compared to my 94 and
even my 92 parts car.
 
The conditions that even prompted me to think to try the CC were as follows:
merging onto interstate, trans in Sport mode, other merging traffic ahead of
me not making any attempt to match highway speed, I merge into the right
lane at no more than 45, trans has shifted into probably 3rd by then, check
middle lane and it's free if I kick it, so I floor it and manually shift all
the way down.  The car did not respond to my "request" to shift to a lower
gear, which I've grown accustomed to, so I shifted back to D right away.  
 
As I returned the lever to D, the car finally decided to downshift to
second, and VROOOOM.  Very nice.  But why?  It will only downshift
appropriately about 5% of the time at best.
 
I've always thought the downshifting was a function of the
kick-down/full-throttle switch on the firewall.  But for some reason, this
time, I thought about the cruise control.  I'm not sure why.  I hit it and
it engaged.  I even recognized the soft click from under the passenger side
of the dash.  I played with it all the way to my destination, setting it,
disengaging it with the switch or with the pedal, hitting resume, hitting
accel, etc.  
 
I was picking someone up at the airport and was able to not shut off the
engine during my short wait for them to find me.  I was able to use CC again
for a time after that, but it disengaged once on its own and later, nothing,
back to "normal".
 
The troubleshooting for CC in the the V8 Service Manual is 8 pages and the
majority of it is focused on the switch.  The very first specific step
however is to test the full throttle switch...  car off the ground, engine
on, trans in D, hit full throttle briefly looking for battery voltage.  The
next tests are to check for battery voltage with the gear selector in 3 and
then 2.  It does not repeat the full throttle action for either of the other
gears.  
 
Other than broken wiring, the only other thing it mentions is the
transmission control unit.  I believe that is okay as I have not seen fail
safe mode ever and otherwise the car shifts fine, behaving differently in
Sport or Manual, and locking the torque converter at about 50mph.  So I
think the TCU is okay.  
 
I had finally decided to bite the bullet and pull the dash top off to go
through the troubleshooting steps, but I feel like it may not be necessary
now.  The rest of the checks are all related to the stalk switch, pedal
switch, and vacuum pump.  Since, during my brief fling with CC yesterday, I
was able to confirm vacuum and switch operations, I feel like my issue is
definitely with an input to the CC system related to transmission or the
full throttle switch. 
 
So, given the scenario I experienced wherein this seemed to coincide with
the correct operation of the kick down, I am of course looking for a golden
egg in all of this.  Or is it a wild goose chase?
 
I'm going to do some other reading of the service manual and see what I can
find about the full throttle switch, testing it and its function, as well as
other info about the transmission in regard to downshifting.
 
If anybody has any input on this, I would really appreciate your nuggets of
knowledge.  I feel like I'm very close to getting this one resolved.
 
Thanks!
 
Ed