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Yep, that tracks. The 7727/7730 confusion trips up a lot of people because the guts and calibration family overlap, but the packaging and mounting are what matter in the W-body. For CuttyBawl’s case I’d still verify the MEM-CAL broadcast code before trusting any ADX/XDF combo, especially with a reman ECM sticker involved. For scanning only, the right data stream/mask matters more than the XDF, so if it’s truly a 1990 LH0 he should be looking at $88 stuff first, not just grabbing A109 because it “sort of connects.” For tuning, yeah, starting from $A1 makes sense if you know what you’re changing, but I wouldn’t send a first-timer down that rabbit hole until the idle fault is sorted mechanically. OBD1 will happily let you chase ghosts all weekend, ask me how I know.

Thanks Black92GS, I’ll get a wiring diagram and start tracing anything I can.   Today I had an hour to check and even cleaned all fuse connections in both boxes.   Nothing looked out of the ordinary.   After I hooked the battery up again I tried a start and had crank for 1 second and then completely dead again.   Locks windows lights etc all work normally.  So I put my head under the hood and hit the remote start to listen.   Everything seems to power up and then after a couple seconds you hear the drop in voltage when the starter would engage but nothing.   Then as it cycles and tries again I hear the little clicks and chirps coming from the transmission.   It tries three times and nothing.      shift selector still says wrong gear (mostly 3) and there’s some more random codes my reader can’t detail.   Says something like only available with add on from GM.     tried one more time with the remote start while wiggling any harnesses I could get at from above and no changes or surprises.      I'm close to letting a professional look at it but I’m afraid they are going to take weeks to find the gremlins.   Pretty discouraging feeling today so I’m going to the mountains for 4 days in the suburban to listen to a bunch of bluegrass and drink myself happy.   I’ll keep this updated as it progresses next week.     IMG_0430.mov

After seeing the code list, my thoughts are now that you're either going to be dealing with a bad sensor somewhere on the 5 volt reference B side possibly shorting out internally and taking down everything else, a chafed wire or corroded connector somewhere.....or even a failed PCM. I would ignore the harsh shifting and random no-start at this time, since those are likely just symptoms of the overall issue.   5 volt sensors generally work based on a voltage output to represent a particular value.  Take your range sensor.  It would receive a 5 volt signal, and would output a different voltage to represent PRND321. If the 5 volt signal going into the sensor is being pulled down slightly and randomly by something else, the output voltage will change, and the PCM can interpret those changes as different reported shifter positions. If the vehicle is off and its reporting something other than park or neutral....it won't start. It may also begin shifting funny as it no longer knows which position the shifter is in.  If it were me...I'd be starting my troubleshooting on that 5 volt reference branch first. 

My scanner is a WiFi iPhone scanner.  It is mostly read only.  I could dig deeper into its live reading but all I’ve ever monitored was misfires and temps and very simple PIDs.  Unfortunately that level of scanner is the only one in my budget.  I agree live data would be helpful.     Last week after the first check engine light that included p0651 I cleared all codes and forgot about it and hoped it was a fluke (I can dream).   And then when it started doing the shifting problem I had no CEL and only read codes for the sake of it.  I cleared the second set of codes and hoped it was the heavy rain that day.  (Seriously a big dreamer here) that was the P0742 the first time I believe the last set of codes was after checking the connections at the PCM and DIC and cleaning up the battery connections for good measure.   There was still no CEL on the dash.  We drove it up the frontage road and back as soon as it shifted hard between 1-2   thanks for the ongoing help.   I’ll have much better info when I can get in there more.   Currently not able to work on it due to work.         

Let's start here..........what are you using as a scanner? Is it dedicated OBD2 scanner such as a Matco/Snap-on/OTC? Is it a laptop with a scanner program loaded onto the laptop? You want to clear all of those codes, start & run the car in *real time* (the laptop/scanner connected) & drive until those codes reappear. They're going to be either soft codes or hard codes. Hard codes will not clear until the problem(s) have been solved. Soft codes usually clear after a number of ignition key/engine run cycles & the PCM corrects.