[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
How transmisssion pressure affects throttle
This is a brief explanation of how pressure in an automatic transmission
*might* affect an engines' throttle.
Most automatic transmissions have a downshift of kickdown linkage between the
throttle and the transmission. Usually, this connects to a special lever on
the transmission. This is what gives you "passing gear" or "full throttle
downshift". On many cars, you can actually feel the extra pedal effort needed
to engage this lever at about 2/3 throttle position. This linkage actuates a
hydraulic valve inside the transmission. So, it works something like this...
You're driving at 55MPH, you want to pass, you depress the throttle to the
floor (feeling the detent), the linkage on the throttle moves a linkage to the
transmission, a lever on the transmission moves, a hydraulic valve inside the
transmission moves to sense the lever position, the transmission downshifts.
Cars with automatic transmissions have been made this way forever.
Now suppose that something is wrong inside the automatic transmission, like
maybe a high pressure hydraulic leak. Then, just maybe some of this high
pressure happened to get directed into the downshift hydraulic valve. Then
maybe the downshift valve could pull against the throttle linkage with enough
force to cause wide open throttle. Then maybe you could believe the
transmission caused the unintended acceleration. This might hold water unless
you've ever looked at the linkage. In all cars I've ever seen with automatics
(chevy's audi's fords), the downshift linkage is "arranged" to prevent this
through slots and other failsafe methods. If you look at the linkage, you would
not believe the downshift linkage could affect the throttle ...unless
something was modified. This is what the 60 minutes expert did. As a
hypothetical case, they jammed the downshift linkage to make a solid connection
to the throttle. Then they modified the transmission to allow external
application of high pressure hydraulic fluid directly into the downshift valve.
I saw them do this on TV, and yes, the engine accelerated on its own during
these conditions. They were grasping at straws, and the found one.
John Q. Public can easily understand having a truck rigged with firebombs as
being misleading, but most folks don't understand how the downshift mechanism
DEC MAYNARD, MA
>Well then could someone please explain to me how pressurizing the
>transmission would directly affect the throttle position in
>a 5000? We're still talking KE-Jet here right? This isn't
>completely obvious to me...anyone care to explain?