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Before the advent of tranny computers, shift changes were controlled by pumps. The faster the car went, the more pressure was built until there was enough to pump the gears to the next level. (short version) Most auto trannies were 2 and 3 speed, which meant at least 2 pumps plus 1 for reverse.
With the advent of TCUs, the trannies could be shifted electronically depending on speed and engine rpm plus throttle position. This also allowed for a torque converter lockup capability in any gear, usually 3,4, and 5. It is my understanding that pumps are still used to push the correct gear into position and these pumps require very clean fluid.
Up shifting is very logical and quite easy to program. Down shifting is a different story and depending on the programming, quite problematic. Should a car downshift at partial throttle and low speed? Eventhough the engine rpm is above idle speed? If there is no decent sensor, how can the car downshift to maintain speed?
Occasionally, my 1998 V70 would down shift to 3rd or 2nd at 30mph on a downhill grade. My 1995 850 never did. My current behemoth has grade logic and will hold 30mph by down shifting.
The 5 speed V70 tranny will "hunt" for the correct gear when accellerating lightly. The TCU, in combination with the ECU, will attempt to find the "best" ratio for the load on the engine. So if you were in 3rd gear with lockup and going 20mph at part-throttle, turning to go slightly downhill, and gave it just slightly less gas, it will not downshift by itself. Cut the throttle completely and it might downshift, or just leave it in the gear it is in. If the downhill is more than 100 yards, it just might down shift unless your foot is moved to the brake. The brake is important here, that info is also conveyed to the tranny and the TCU might think you are going to stop, so why down shift?
I am glad I wasn't the person who wrote the program for the TCU! The number of variables and drivers would have been too much for me. Plus, my boss would hang me if I didn't improve MPG.
Just a thought,
Klaus
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My name is Klaus and I am a Volvoholic.
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