Greetings Scooterwrench,
I like your no-nonsense process of elimination & concise problem description!
From over here it looks like there are 3 general areas to focus on, with the
goal of eliminating 2...and then diving down the rabbit hole for the remaining
area, learn as much as we can, so that we either fix it ourselves and/or be able
to understand the guru who is fluent in all things 700R4. (hint: I agree
w/Schurkey, I think he's on the right track. Disclaimer: I am
not a 700R4
guru by any stretch, just a troubleshooter throwing my 2 cents into the hat.)
****
OK, reading around the Internet it seems that you are up against a relatively rare
(or rarely reported?) problem. I did see some discussion about this in a couple of
'vette forums, and the discussion usually centered around a bad TV cable, with
a couple of anecdotal worn throttle body bushings thrown in for good measure.
...But before we start to load up the parts cannon, in the interest of science
(and your piggy bank) let's see if we can A) look at the big picture, B) figure
out some low-cost ad-hoc testing to tighten up the problem description, and
then C) figure out who we gotta find in order to confirm our troubleshooting
results, so that D) you can make an informed opinion on whether to tackle it
yourself, or track down the 700R4 savant that can fix this on the first try.
Big Picture:
Proper line pressure is critical to the longevity of the 700R4. (all autos, actually)
Constant too low = slipping/burned clutches. Constant too high = ancillary parts
failure due to shock loading of components due to harsh gear changes.
So, in order to hit the reliability/driveability sweet spot, the throttle valve cable
was added by the designers to establish a mechanical communication path between
the powerplant & 700R4 so that the tranny can know how much torque is
headed it's way & act accordingly. (Actually torque demand by the human
as expressed by the degrees of throttle opening.)
In English, as the human constantly varies the throttle opening according to
changing conditions, the TV cable translates this movement directly into
sliding the 700R4's internal throttle valve back & forth inside a bore.
This valve's movement in turn modulates the line pressure between
(using round numbers) ~
100 psi at idle to ~
200 psi at full throttle.
Makes perfect sense...but while you are reading the rest of this
keep in the back of your mind how many gazillion times that this magic
throttle valve inside your 700R4 has traveled up & down the associated
bore in the past 32 years?
What material is this bore made out of vis-a-vis the throttle valve itself?
(Hint: If we built a transmission today, would we pick metallurgy that would still
have factory clearances in the year 2055? Or maybe the bean counters would
insist on something more affordable in order to remain competitive in today's
price-conscious marketplace? :0)
Tightening up the problem description / Proposed process of elimination
Apologies for taking so long to set the table, but if we all agree on the same big
picture then a common consensus should be easier to achieve.
OK, so we have 3 possible areas to evaluate: wear in the throttle body, bad
TV cable, or a problem with the 700R4's internal throttle valve causing sticking
at the beginning of the travel / cocking in the bore / or wear somehow allowing
the line pressure being modulated to somehow be coupled back towards the
throttle/human?
What to do first?
Theoretical troubleshooters will assign a *probability* percentage
to each component, and insist on going from most probable to least probable,
in that order. (Think engineers directing the repair strategy that never leave their
cubicle, and don't comprehend that the 1st part is 6 hours of soul-sucking agony
to get to, while the 2nd part is an easy-peasy 5 minute R&R...especially when
the probability of these 2 parts is a 50/50 tossup! Ask me how I know this? :0)
Some troubleshooters swear by using
only price instead of probability to prioritize
their list. Most affordable part first, followed by the next most affordable, etc.
(Additionally, a lot of times price-sensitive troubleshooters are wont to make the
repair effort even more challenging by adding the 'cheap parts' variable to this approach.
NOTE: Despite all their focus on cost, this 'non-data-driven/wishful thinking' troubleshooting
strategy can easily blow the repair budget.)
Mechanics under the gun in a production environment (who aren't buying the parts)
will often prioritize the parts replacement by how long each part takes to swap out.
The 10 minute R&R will always be chosen over the 1 hour part...even if the 1 hour
part has a 90% probability of fixing the issue and the 10 minute part is only 10%. (!)
If asked about this, they simply don't care that the customer had to buy 2 parts in
order to fix 1 problem. (And in certain situations, they are incentivized by the number
of parts that are sold, making this even more problematic for the customer's piggy bank.)
I prefer a common-sense amalgam of all of the above. I take probability + price + how
hard is it to get to in order to come up with the most advantageous sequencing.
In addition, I like to find out how hard it is to prove/disprove that a particular part is
functioning correctly in order to come up with the biggest troubleshooting ROI.
****
Given the above, I want to start with the throttle body. Is there ANY appreciable play
in the throttle shaft, especially on the side where the TV cable is connected? In a
perfect world, the shaft has only 1 degree of freedom -- that is, it only spins smoothly
in the bushing, and doesn't move up & down/side-to-side/etc due to wear/excess clearances.
Even though throttle body bushing wear is a lower probability overall than a worn TV cable,
this failure is easily proved/disproved in <5 minutes by a visual inspection while
manipulating/futzing with the linkage.
Moving on to the TV cable, this is a perennial favorite part to blame, but at the same time I
would want to prove it's bad to myself before I swapped it. Some would argue that if
you disconnect it from the circuit and it moves freely by hand that you should be good to go.
Others insist that the constant back and forth motion of the inner steel cable may have worn
into the inner walls of the cable (think using piano wire to cut the goo surrounding a windshield)
so you need a tension load on the cable in order to stimulate the fault.
In order to do this (and also quantify the internal cable drag at the same time) I'd put a fish scale
on one end of the cable and pull on the other. In a matter of moments you should either gain
confidence that this part is not the perpetrator
or you have located the culprit & now can order
a replacement with confidence.
You must be registered for see images attach
Analog or digital is your choice...and the price of admission is way better than just
throwing a new TV cable at it & crossing your fingers. :0)
****
Assuming that you've proven to yourself that both the throttle body & TV cable are still in
serviceable condition, all that's left is the 700R4's internal throttle valve.
After several searches came up with the typical confusing mish-mosh, I finally found a
700R4 Whisperer by the name of "clinebarger", who neatly laid out the internal throttle
valve & how to correct the "TV valve sticking problems that plague 700R4's in a single
screen:
You must be registered for see images attach
Follow this
link, & then scroll down to entry #115 & start reading. (Note: This screen snap is #117.)
Of course, let's say that by the process of elimination we have convinced ourselves that the fault
lies with the malfunction of the 700R4's internal throttle valve. But we aren't done yet. Why?
Thinking back to the throttle valve vs worn bore discussion at the beginning of all this, we have yet
to determine if the throttle valve as
bad part the
Perpetrator of the symptom...or is it a
good part that is the
Victim of a worn bore after 3+ decades of service?
Check this out:
You must be registered for see images attach
Link:
****
OK, at this point I am on some pretty thin ice from a knowledge perspective, so
as a troubleshooter I must stop here & let more informed folks take over.
I know that we have covered a lot of ground...and if the 3 possible bad parts
were each <$10 apiece, and all were in easy reach in the engine bay, then I
wouldn't have gone through all of the above.
Instead, you have 2 relatively easy parts to prove/disprove...and once they
are eliminated, you are now looking at a somewhat esoteric/spendy fix
inside your transmission...requiring a genuine SME (Subject Matter Expert)
to put things right for the minimal number of parts & moolah.
****
Again, I really liked your troubleshooting approach, and I eagerly await
further info from your ongoing troubleshooting session.
Hope this helps -- best of luck in the hunt!
