Re: How transmisssion pressure affects throttle

[moxley@eagle.columbiasc.ncr.com]

On Aug 26,  9:03am, Bill Samaras DTN 223-7322 26-Aug-1994 0839 wrote:
> Subject: 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.

Good explanation from Bill with one exception. The throttle cable on the
automatics was connected from the pedal to an arm attached to the transmission
"full throttle downshift" actuator shaft. Another rod was attached to this arm
that operated the throttle plates making a rigid connection between the
'downshift' shaft and the throttle plates. Therefore, the external modifications
made to the transmission would cause the 'downshift' rod to rotate
and operate the throttle linkage.

I recall that Audi changed the linkage design to a failsafe method as Bill
 described probably when the shiftlock was incorporated into production.

> Then they modified the transmission to allow external
> application of high pressure hydraulic fluid directly into the downshift
valve.

> ...bill
> DEC MAYNARD, MA


Just as a bit of trivia, I caught a television news report where London
reporters were attempting to interview Lady Di who managed a speedy escape at
the wheel of a black Audi convertible. Perhaps Audi is the choice among some of
the rich and famous.

Dave
moxley@eagle.columbiasc.ncr.com