I was wondering what an automatic transmission monitors to shift. I have an a4 97 z28 and of course its different at wot and normal driving but i have come to the conclusion that trannys shift at certain mph all the time but can shift before at different rpms. What are the exact conditions that cuase a shift.

 

The pcm controls the shifts on 4L60Es. Basically it uses throttle position and speed to determine when it wants to shift. I think it also does some of it's calculations based on engine vacuum (MAP) as well, based on the fact that a 58 mm TB will make the trans shift differently. It's not an infinitely variable scale, it's more like a table I would say. For instance, any throttle input under a certain value (let's say 15%) will automatically shift at the minimum, when I had my auto w/ 3.23s the shift speeds were 10, 20, and 35 for 1-2, 2-3, 3-4. I don't think it actually will shift based on rpm though, based on the tendency of some auto cars to smack the redline before they shift. OBD2 cars might have a little more adaptable shift values, but I'm not sure. Some OBD2 cars do have a shift strategy that learns how to shift based on driver input- an agressive driver will make the trans shift more agressively, and the other way around too. So basically, if your friend borrows your late model car and when it comes back if feels "peppier", it's probably because he's been beating on it.

 

The 4L60E is electronically controlled. Shifts occur as a result of "A" and "B" shift solenoid scheduling. The 60E works as follows:

A Solenoid B Solenoid

1st Gear ON ON
2nd Gear OFF ON
3rd Gear OFF OFF
4th Gear ON OFF

The shift solenoids get their signal from the PCM (Powertrain Control Module), which is part of the computer.

The computer ges input from various sensors on the vehicle, such as the speed sensor, throttle position sensor, engine RPM's, etc. When the computer sees that all the parameters are met for a shift to occur, it sends the appropriate ground for the correct shift solenoid to energize.

It's a little more complicated than this, but this is just a guideline for basic operation.

Frank
CPT

 

Thanks guys, awesome info.

 

a specific ratio of the compound planetary gearset is needed at various throttle positions,engine loads and vehicle speeds to provide maximum performance and fuel economy.throttle position via the tps,vehicle speed via the output speed sensor,and engine load via the mass air flow or map sensor all feed an electrical signal to the computer or brain reporting these factors to it for analysis.shift scheduling .the computer then takes this information and compares it with the calibration pack stored in it and controls power and ground to solenoids which are turned on and off to control oil flow thru shift valves to the friction elements which control compound planetary gear set operation.there are other factors at work also .these include engine temp ,speed,brake on off etc.www.ckperformance.com

 

the brake on/off is for the TCC. and yes everyone else IS correct. now if we're talking about a non-electronic 4L then thats a different story...