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 more 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.