From: Wally Plumley [wplumley@bellsouth.net]
Sent: Thursday, January 03, 2002 10:11 AM
To: 928
Subject: [928] Re: Auto tranny question

At 07:06 AM 1/3/02, you wrote:
>I was curious if you could enlighten me in how the automatic transmissions
>are supposed to downshift in cars such as mine, 84 928S euro.  What I am
>really interested in is if the car is supposed to only be able to
>downshift via the kick down switch or moving the gear selector down when
>cruising in the 80-85mph range?  The reason I am curious about this, is
>because the car when cruising at that range is maybe turning over 2k rpms
>and it seems strange that when giving 1/3 or 1/2 throttle it does not just
>downshift like it does at say 75mph.  Yesterday morning was the first time
>that I can recall when my car would downshift with the 1/3 or 1/2 throttle
>from a cruising speed of 80-85.  Yesterday morning was an unusually cold
>morning for us, in the mid-high 30s.  Is it possible this is a vacuum
>related issue and the motor in the cold/dense air was just making
>more/less vacuum so the transmission was more happy to downshift?  I would
>say it was mor happy to downshift across the whole spectrum of driving it
>and not just at the 80-85 range.  To me, the car felt and responded how I
>think it should act, but my thinking could be wrong from how it is
>supposed to act.  I know it will be faster in such weather, but I am
>referring to its shifting characteristics.  Hope this makes sense, thanks
>for any help you can offer or insight, since I am not sure there is a
>problem, I am just wondering how it is supposed to work.


There are several things that control the shifting of the transmission.
Road Speed - There is a centrifugal governor that measures output shaft 
speed, and varies the hydraulic control pressure that controls shifting. 
Not adjustable.

Manifold Vacuum - The vacuum modulator measures engine power output by 
using manifold vacuum. Lower vacuum equals higher power output, so the 
transmission shifts later and harder. Higher vacuum equals lower output 
power, so the transmission shifts earlier and softer. Easily adjustable 
from under the car.

Throttle Position - The throttle position cable tells the transmission what 
the throttle position is. More throttle opening tends to cause downshifts, 
later upshifts, etc. Easily adjustable.

Kickdown Switch - The kickdown switch energizes a kickdown solenoid inside 
the transmission, and this suddenly and drastically changes the control 
pressure, which greatly encourages a downshift and retards the upshifts.

The final shift speeds depend upon a complex interplay of all of these 
factors. For example - You seem to feel that the transmission should 
downshift at 1/3 throttle. If it did, holding 80 mph up a slight grade 
would probably cause a downshift - NOT what you would want. The 
transmission uses throttle position, power output, and road speed to 
determine when to downshift - and usually does a good job of it.

You can make the transmission a little more eager to downshift by 
tightening the throttle position cable, and by adjusting the vacuum 
modulator - but it will always be more eager to shift, even when you want 
to simply flow with traffic.

It would be nice if we had a "Sport" button on the auto, as some Japanese 
cars had, to adjust the shift pattern for sporty driving or relaxed cruising.

Wally Plumley
928 Specialists






From: Thilo Corts [mailto:corts@webx.de]
Sent: Wednesday, November 13, 2002 5:16 AM
To: 928
Subject: [928] Re: Ignit timing and AT shifts


Rich, Tony,

This feature was introduced for the '87 MY.

Loosely translated from the Porsche dealer information book preceding
the S4 introduction:

"Since the engine has very high torque which must be handled by the
shift elements [.....] torque is reduced during shifting at higher
partial load or full load above 4000 rpm when gears 2 and 3 are
engaged.

To this end, a switch is disposed on the thrust bearing of brake band
B1, which sends a signal to the EZK control unit when opened and
closed. When the signal is sensed, timing is immediately retarted by
16° for 350 ms (when upshifting), which results in a torque reduction
of approximately 25%. Then, timing is ramped back up to normal within
150 ms. When downshifting under load, the same torque reduction is
applied, but only for 50 ms.

If the switch in the transmission is sensed to be faulty, rapid rpm
changes, as they occur during shifting, will also initiate the timing
retardation (fault mode)."

HTH,

Thilo