0411 Transmission Flaring - Tuning or Trans issue

[LRT]

....

[lionelhutz]

#1. The transmission line pressure doesn't go from 0 to 96 psi. It goes from about 60 to 180 psi.

#2. The force motor current tables typically do not go down to 0A.

#3. Picking 96 "psi" in the tune tells the PCM to use the lowest force motor current, which is not 0A. So, lowering the table closer to 0A can increase the maximum line pressure.

#4. Suggesting to use a shift kit as the way to modify the line pressure makes little sense, because modifying the line pressure response to the force motor current really doesn't do anything different than ramping the currents in the table down to a lower final value to get a higher line pressure, until you do reach the limit of 0A.

The "psi" in the tune is just a number. The PCM uses the tables to pick a value from 0 to 96, and then goes to the force motor table and gets the current to apply to the force motor. So, the force motor current sets the actual line pressure.

I was just in a Canyon tune with a 3L 4-banger and the 3-4 shift pressure was 90 "psi" across the board and the shift times were all 0. The lowest force motor current was 100mA. So, suggesting the force motor currents could be lowered a bit or to use 0 time for the 3-4 shift just isn't that shocking...

[LRT]

....

[lionelhutz]

I'm not arguing, I'm posting facts.

4L60e line pressure:

When you force command the "line pressure" between 0 and 96 via software or a scanner, the ACTUAL transmission line pressure on a stock transmission should vary between about 50 to 180 psi.

https://www.atreman.com/wp-content/u...e-Pressure.pdf


Also attached is a stock 2002 Camaro force motor table showing the usable part of the table reaching a minimum of 98, NOT 0. FYI, the tune NEVER uses the 100 column because those high current values would drop the line pressure to a very low value and cause bad slipping. I didn't do create this, so I have no idea why GM did it but the column is irrelevant anyways. The tables vary and usually go to 96 on older stuff. I have seen 0 in the highest or 96 column too, but the tune was limited to 90 "psi" so it NEVER used the 96 "psi" or 0A column.

[evilstuie]

To interupt the argument for a second.

I made the changes you suggested LRT, unfortunately I also got carried away and broke the cardinal rule of logic and changed some other stuff on the spark tables and speedo calibrations, which ran like crap and couldn't test the transmission state in anything other than idle basically.

When I got the car up to around 40km/h (maybe 15mph) and let off the throttle the transmissions felt like it was locking up and releasing over and over again for 0.5 seconds in succession until I put my foot back on the throttle.

Obviously the behaviour is bad, but is this a good sign that the transmission is ok and that I just need the tuner to calibrate / swap out the transmission segment, or is it a sign the transmission is faulty?

Also to throw it out there, is it possible the cause is that the transmission selector cable is not set properly and the trans could be between gears/engaging both?

[lionelhutz]

I'm just trying to post how the transmission works. The PSI stuff in the PCM is all labelled wrong since it's really more of a percentage where 0-96 is 0-100% of line pressure which is about 50psi to 180psi on a stock transmission. It's easier to setup the transmission when you actually understand what the PCM is doing. As for LRT, I can't help that he doesn't like being told he is wrong, but I'm not coddling him and letting him pass his wrong information as fact just to make sure his ego doesn't get bruised.

As for your transmission - either it was an internal issue or the PCM was rapidly shifting it between 1st and 2nd gear. At 0 throttle you have a 1mph spread in the shift speed. The speed has to go over 14mph to upshift and below 13mph to downshift. So, unless your speed signal was fluctuating by at least 1mph, the PCM should not have been rapidly shifting back and forth between gears.

You should be able to check the shifter cable by holding the shifter so the detent doesn't do anything and then moving the shifter so you feel where the transmission clicks in. For example, I have a floor shifter that can shift between N and OD without pressing the button. So, to check I hold the button and move the shifter to N and then release the button and try to move the shifter to R. It's adjusted right when the shifter just barely moves before hitting the stop in the shifter. I then checked OD and other gears a same way.

Overall, my feeling is that a big part of making an automatic vehicle fun for you to drive is tuning the transmission. On my stuff, I've probably done at least as many tunes for transmission shifting compared to engine tuning. On my pickup, I licensed it with HP tuners just to re-tune the transmission.

And you should pick-up a line pressure gauge if you want to be verifying the transmission is OK and also tuning it. It's a similar importance as using a wideband for the engine.