Many think that when if comes to the trans , We only need be concerned with MPH-RPM-TPS . And with many after market controllers like USSHIFT AND HOLEY THIS IS TRUE,
But NOT with the factory PCM
ALL OF THESE THINGS COME INTO PLAY

TPS- Tells PCM/TCM where the throttle is at this along with other sensors help the computer to CALCULATE - SHIFT POINTS-LINE PRESURE-KICKDDOWN-TCC ENGAGEMENT AND RATE.

RPM-This along with the TPS-VSS-MAF-MAP help the PCM/TCM also to determine SHIFT POINTS- LINE PRESURE-TCC-KICKDOWN.

VSS-This along with TPS-MAF-RPM-MAP again help determine all the above.

MAF- Determines LOAD with calculation using TPS-MAP-RPM-VSS Which is a primary determination of line pressure. (A dirty of malfunctioning / out of calibration MAF can lead to a quick demise of the trans. )

MAP- Helps determine load to set line pressure particularly with SPEED DESNSITY TUNES as engine vacuum along with TPS and RPM are used to calculate line pressure. (This is why some may have noted with boosted application and speed density they can get pressure spikes on downshifts AKA harsh downshifts particularly when some sort of overun mod is done like the sonnax HD 2-3 valve or like my own FTOVERUN MOD )

IAC-Can affect line by holding idle up during decel causing higher air flow through MAF making PCM/TCM command higher line pressure.

O2 SENSORS-Affect IDLE quality and mixtures at all throttle positions . This can cause issues with IAC- ENGINE MISSFIRE affecting line pressure and lockup (Example and what made me write this , My right bank O2 is bad right now , I have had 3 go bad in last 4 years . Notable changes in Trans operation are harsh engagement due to higher idle PCM compensating for lean mixture in my case. Later and much harsher shifts. I have noted this each time I have had one fail).

MISSFIRE DETECTION-This can cause random lock/unlock of converter or no lockup at all. Cars with large cams run into this when cruising in 4th gear and in particular cars with high ratios (Numerically low) as they can get into the lope of the cam which the PCM sees as missfire and can/will lock/unlock the converter or disable it all together potentially causing trans overheat or that odd wear you see at times with the FWD and 3-4 clutch set spine area if allowed to go on to long. While you can disable missfire detection to solve the lockup issue it will do nothing about spine wear. Best solution here is to raise the speed at which the car can engage 4th and lockup.

ALTERNATOR OUTPUT- Ran into this last year with my own car. My alt voltage due to a bad diode was down to 12.5 to 13.2 volts. I noted more aggressive part throttle shifts and occasional 2nd gear starts . Either because the solenoid was inadequately powered or AFL pressure off. Have not given it much thought. But replacing alternator fixed it. Note even with lower voltage and bad diode battery remained charged and car start drove this way about a month before fixing.

KNOCK SENSORS- Interesting here, This again my own experience. I have a TRIPLE DISC converter and those of you that have them know they make a rattling noise notable at idle but present even when light throttle cruising and coast. My PCM was hearing this and determined it was spark knock and retarded my timing resulting in missfire which then resulted in random lock unlock under low speed cruising.

INCORECTLY PROGRAMED TIRE SIZE, REAR RATIO.-Messes everything up , Shift times pressures, shift timing . DO NOT DRIVE THIS WAY PERIOD .

HIGH STALLS WITHOUT PROPER TUNING THAT CAN RESULT IN POOR OPERATION, PPREMATURE SHIFTING,LATE WOT SHIFTING, LIMP MODE.


All these things all work with each other to determine many aspects of trans operation . this is why when someone calls me about trans function issues particularly drivability and they have engine codes. I have them fix those first as many times it is one or more of those rather than a trans issue, Though ignoring them can indeed result in a real trans issue and or failure in short order.
No matter HOW WELL BUILT and what SUPER DUTY PARTS you have any and all of these items can KILL YOUR TRANS.

Oh and little note on OBD1 cars and perhaps OBD2 but have not had lots of experience with OBD2 yet in tuning.
Line pressure setting. At anything above 95 percent of max you risk a delayed of no shift issue at WOT. Over the years I have had many calls particularly over a no WOT 2-3 and found that they had been trying to fix by raising the line to no avail. This is because both ends of the 2-3 valve have use AFL pressure and the 2-3 valve has no spring (well except my own) . The shift solenoid B cannot vent fast enough in this condition and will not move the valve to make the shift or moves it very slowly. I have literally had people lower down to as low as 85 percent of max to solve the issue which to many tuners seems counter intuitive. But works. My addition of a spring to the 2-3 valve however eliminated those calls.

On OBD1 99 percent is max. If you try and go to 100 or higher the actual pressure will go very low. I noted this with my Impala in early days of tuning it, I could set the line at 99 percent and get a firmer shift but 100 up it actually slipped during the shift . I placed a gauge on it and noted this way at 110 percent of max my line at WOT dropped to 80lbs which was same as my idle line. I am not sure why. I can only guess the PCM cant see a 100 in the box but can see a 99.0 . Maybe it see 100 as 0.00. I am not a tuner by any means so maybe an actual tuner can explain this. Again note this was only an observation with my own OBD1 car I have no idea about OBD2 set ups.

 

And Now That I have OBD2 on the Impala and was working out new OS and fuel mixtures/trims/timing seems I have noted somehow these also affect the trans shift quality,
I started out with a really crappy base tune . It literally shifts so soft it scared me so i put in manual 1 and went right back home. I flashed the PCM for much higher line and went for another drive , Car still ran like crap, But shifts felt good . With some help worked through the running setup (stuff i didnt know like LS injector firing or is diff wth LS engines along with ignition, I had done ignition but not injectors.
Long story short once we got that and mixtures decent the car shifted like someone was hitting you in the rear even at light throttle . So had to flash the tables back down and now good,
But what I learned here while i dont know why or how, When I was tuning on the ob1 Tune what I did with the engine / fuel / spark even running bad it had little effect on the trans shift quality. However with the new O411 PCM and the 2002 OS they make a huge difference and I have no doubt feeing the shift that first drive that if i had continued I could and would have done serious damage to the trans.
So in summary Yes a crappy engine tune with no changed to the trans setting can indeed burn up a trans short order, This is for those who think they can run a base crappy tune after mods with car running poorly to the tuner to be tuned, By the time you get there , You might need a trans.

 

No one (other than original software engineers) know the exact algorithm the GM PCM uses to determine trans line pressure at any moment and the shift speed. We know that higher engine load translates to higher line pressure, but exactly how is engine load determined? Likely some combination of MAF, MAP and fuel flow. Transients (quick changes in load) are also an issue; slow response in a rise in load could lead to excessive slippage; slow response to a lowering in load could result in harsh shifts.

I think what Frank is saying is that after engine mods, be sure the tuner (hired or you) is experience enough to know how the engine and tuning changes might affect the transmission, it life and the shift feel. And that changes to the torque converter often require reducing misfire threshold and perhaps even knock sensor sensitivity. Just slamming a hot cam into an engine and only changing the fueling tables is not going to cut it.