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Old 08-08-2007, 12:34 PM   #1
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How does Automatic Tranny choose?

I just reread the thread about EOC in autos and had this question... How does an a/t choose which gear to go into when shifting from N to D at speed?

I normally don't EOC over 45mph, but there are many times that I would coast in N at 55-60 for a downhill slope then shift back into D when the speed lowers. As long as I give a little rev to 2000rpm before shifting, there is no odd noises or feelings noticed. I assume that it's just throwing the tranny into 4th gear, but how does it know what gear to put it in? I have no knowledge of how trannies work, but it doesn't seem like it tries 1st gear and says "nope, let's try second" on soforth.
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Old 08-08-2007, 01:12 PM   #2
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It will depend on the transmission and what technology it uses. If the car has a TCU, I would venture to guess there is a VSS input that it uses. If the car doesn't have a TCU, then I think they use either a fly-ball counterweight or fluid logic. The kick-down cable - running from the transmission to the throttle body - also tells the transmission if the car is at WOT and typically selects the lower passing gear.

Take a look at the valve bodies on some old auto transmissions - crazy stuff!
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Old 08-08-2007, 03:18 PM   #3
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The simpler ones (pre-computer) decide it hydraulically, based on throttle, engine RPM, and speed.
http://en.wikipedia.org/wiki/Transmission_Control_Unit
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Old 08-08-2007, 03:33 PM   #4
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My Subaru is a 97, so that's probably the case
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Old 08-12-2007, 10:08 AM   #5
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I actually took my transmission apart last sunday. I'm now driving on it, so I guess I didn't let all the voodoo dust out.
In this case, the extent of the car's electronic control is the ability to disable the torque converter lockup mechanisim. I'm pretty sure this thing is the speed-measuring device, as it spins with the differential casing:

Not exactly a simple design, but actually similar to the manual trans more commonly found in CRXs.

A main shaft in the front, connected to the torque converter, a counter-shaft just behind it with various gear sets between the two (which make up the transmissions different "speeds") in constant mesh but engaged or disengaged via hydraulicly controlled clutches (the big cylindrical thing on the main shaft is a pair of clutch packs back-to-back), instead of shift forks and syncros like you would find in a manual trans. The countershaft is in constant mesh with the differential casing, the differential drives the half-shafts (aka axles), which are connected to each front wheel. The reverse gear actually still uses a shift fork - it's basicly an extra gear added between the main and countershafts, which reverses their direction of rotation relative to eachother.

So yeah, it's hydraulic logic in this case. A matter of valves, springs and balanced hydraulic pressures. As you pick up speed, the pressure from the speed measuring thingie increases the likelyhood of shifting, while more throttle input delays it. Engine RPM seems to just be along for the ride, as the trans shifts at a lower RPM when the torque converter is locked (and therefore not slipping)
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Old 08-13-2007, 05:05 AM   #6
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Neat Bobski - the auto trans is the only "black box" left for me on a typical Honda.
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