1999 Yukon - multiple PCM fuel programs?

[letsbangout]

Hi all. Related to my ongoing transmission problems as documented in other threads (harsh shifting from 1-to-2 and 2-to-1 when stopping), I recently replaced my post-cat O2 sensors. This was in response to both a P0141 code on one of the sensors, as well as a recollection that I once read about someone having the same 1-to-2 shifting harshness (which didn't follow the classic valve-body issues) and was rectified by replacing the entire exhaust system and the O2 sensors, under the theory that the exhaust was clogged and the ECU was telling the tranny to shift hard.

In my case, I can say that anecdotally, I've observed over multiple drive-cycles since changing the O2 sensors that it now shifts light as a feather on 1-to-2 and the 2-to-1 issue is almost un-noticeable while it's cold. When it comes up to temperature, the problem starts to come back to it's "normal" intermittent behavior. This brings me to my question:

While preparing to change the O2 sensors, I read in the Haines repair book that the PCM runs on a "default" fuel program while the truck is coming up to temperature, during which time it ignores the readings from the O2 sensors (among other things perhaps?) because it's waiting on them to reach 600 degrees before their readings are considered accurate. As I'm pursuing the hypothesis that these transmission issues could be caused by the PCM and not by something wrong inside the tranny, I'm curious if anyone has any more information from a shop manual or other authorative information source about the use of this "default fuel program" and at what point it converts to the real-time fuel map, and how the PCM affects transmission operation in relation to my problem or in relation to the multiple fuel programs, if at all?

Thanks in advance!

 

[gpracer1]

When you start your truck, it will be in "open loop" and not use the O2 sensors to determine the fuel mixture. It uses all the other sensors and uses a speed density tune that is pre set up in the PCM.

After a certain amount of time, ECT temp and O2 sensor activity, it has enough valid data to switch to closed loop and use the O2s to determine the mixture.

During any of these times the trans pressure etc will be changed by the pcm.

I would recommend searching for the 4l60E trans pressure control solenoid and some reading here.....
http://www.355nation.net/forum/i4-i5-engine-drivetrain/16646-technical-info-4l60e-transmission-adaptive-functions.html

 

[letsbangout]

Thanks for the reply, it has a wealth of knowledge. Particularly the link about adaptive shifting. There are a couple of things I'm still unclear on after having read everything:

1) Does anyone know how long it would take to re-learn or replace the adaptive shifting data? If I've already resolved my problem with the replacement O2 sensors, or if something else fixes it, how much lag time should there be before the tranny starts behaving right? Also I presume that where the guy refers to resetting the adaptive shift table with a scantool, he's referring to something like a Snap-on or GM proprietary scantool, yes? i.e. something beyond a standard OBD-II.

2) You mentioned the pressure control solenoid - what specifically are you asserting that might be wrong with it? Most of my re-googling of my problem points to possibilities like the throttle position sensor, bad wiring in the tranny harness, possibly low signal from the transmission controller, worn valve body passages, problems with the old valve body plate, issues with the 1-2 accumulator piston, or maximum line pressure being commanded by the PCM because of slippage detection. Where does the pressure control solenoid come into play? My searches only seemed to reveal an "either it works or it doesn't" type of consensus.