Yes, we all need humor, otherwise we had to jump from a chair with a looped rope around the neck, twice a day 🤣.

The codes #26 #39 #48 #64 #65 are always present. If you put the positive wire off from the battery for a few minutes to reset the codes and put the battery back on, they're gone. You just need to drive one mile, without driving fast (e.g. on a highway) the codes are back.

Sometimes the SES light comes ON now, during my road tests. If it's ON and I put my foot away from the acceleratior pedal it went OFF, if I accelerate just a bit more, it also went OFF for a while. Seems to be a random thing.
When I stop the car, turn the ignition OFF and back to position 1 the SES light don't comes ON (all the other warning lights comes ON for the bulb check as usual). If I sit and wait for 15 to 30 minutes, the SES light suddenly comes ON.

By the way, today 'Little Bonnie' had a good day, the TCC locked up very well, when I drove to work and back (I hate random faults ☹️).

Just to give you an overview of the parts I had thrown on the car the past 3 month: camshaft-, crankshaft-sensor with balancer, ICM, coil pack, spark plugs and wires, fuel pump, fuel-, air-, oil-, transmission filter, fuel pressure valve, canister purge solenoid, MAF sensor, IAC valve, TPS sensor, both coolant temp sensors at intake, fan relais, several vacuum lines, all motor and transmission mounts, brake disks and pads, ECU, drive belt.

Parts thrown 2-3 years ago:
Battery (75 Ah), steering, tie rods and steering pump, sway bar bushings, drive trains, lower control arms with bushings and ball joints, shocks, struts, starter, A/C compressor 134a all A/C tubes/lines/hoses/solenoids/dryer/orifice, A/C and coolant radiators, brake drums and shoes, transmission overhaul, timing chain kit.

Parts thrown 5 years+ ago:
Alternator, brake discs and pads, tie rods, EGR-valve, IAC, hood- and brake release cable, O2-sensor, tail light clear plastic covers, (mabey some other stuff, I don't remember...).

...and please don't ask me for: How old are the WIPER BLADES 🤣???
Don't worry, the regular maintenance stuff is done twice a year, the absolutely old original factory parts are parts like e.g. the torque converter, transmission solenoids, wiper motor, fuel injectors.

Known faulty parts at the moment: A set of four new tires, belt tensioner (it has a rattle noise). New EGR and O2-sensor are ordered and will arrive hopefully soon.

For further questions, feel free to ask. If you like to see a well maintained 1991 Pontiac Bonneville, visit me in Germany. - Exorcists always welcome 😜.

Thanks for helping me with your experience and knowledge.

See what codes set with KOEO after doing the reset.

Just 3x #12 - short break - 3x #12 (means no codes set, code reading procedure sucessful).

Should I fire it up and run it for a minute and check for codes coming up without driving? - Just to see, if code #39 is set without shifting out of park position.

Will try, just give me a few minutes.

Test #1:
Okay, done: No SES light comes ON, no trouble codes stored, car runing in park position for a minute, don't touched the acceleratior pedal, don't touched anything during that test. - Idle speed was at 1500 rpms, sometimes it went down to 1400 rpms for a few seconds, but went back up. - Nothing unusual.

Test #2:
Engine runing, gave her some acceleratior pedal hits up to 5000 rpms, sometimes a fast short hit, sometimes some slow acceleration up to 5000 rpms. If 4000+ rpms are reached it sounds like a stumble or a smooth missfire, but the car is not shaking or poping loud like from a shot gun.
Result: No SES light ON during test procedure, no trouble codes stored.

Test #3:
Engine runing, foot on the brake pedal, shift lever to overdrive position without any acceleration. Shift lever moved several times from park position to overdrive position.
Result: No SES light ON during test procedure, no trouble codes stored, rattling sound from belt tensioner.

SUMMARY: No codes, but my wire clamp looks worn 😪.

[QUOTE=Bonnilover;1645350]Test #1:
Okay, done: No SES light comes ON, no trouble codes stored, car runing in park position for a minute, don't touched the acceleratior pedal, don't touched anything during that test. - Idle speed was at 1500 rpms, sometimes it went down to 1400 rpms for a few seconds, but went back up. - Nothing unusual.

Test #2:
Engine runing, gave her some acceleratior pedal hits up to 5000 rpms, sometimes a fast short hit, sometimes some slow acceleration up to 5000 rpms. If 4000+ rpms are reached it sounds like a stumble or a smooth missfire, but the car is not shaking or poping loud like from a shot gun.
Result: No SES light ON during test procedure, no trouble codes stored.

Test #3:
Engine runing, foot on the brake pedal, shift lever to overdrive position without any acceleration. Shift lever moved several times from park position to overdrive position.
Result: No SES light ON during test procedure, no trouble codes stored, rattling sound from belt tensioner.

SUMMARY: No codes, but my wire clamp looks worn 😪.

Test #4:
Another road test. - Distance: 3 miles. Accelerated up to 60 mph in overdrive position, but no TCC lock up (tried several times), also no SES light ON during road test. - Stopped the car, turned key to OFF position and back to position 1. Again, all safety lights in the instrument cluster comes ON, but no SES lights up -> code reading impossible.
Sit and wait for SES back ON to read codes...
By the way, with the wire clamp jumping A and B port of the OBD1 connector, the malfunction just affects the SES light. The coolant fan turns ON to indicate test procedure has been started.

Yesterday, it wasn't possible to pull the codes after my short ride, because the SES light won't come up.
Today, before I went to work, I could pull them and got the codes #26 and #39. I drove to work and back and the codes were still present, but no other codes were stored.

The torque converter didn't lock up the whole day.

I don't think you measured the TCC correctly. One ohm is not right.

Yes, I agree. But I did it right. Right the same way, as I did with the other pins at the plug.
However, at the moment it doesn't feel like a solenoid fault.
Yesterday everything, I visited a friend, and drove a distance of 12 miles each direction. Code #26 and #39 were stored in the ECU from the day before. - I didn't reset the ECU. The TCC locked up this time as it should. When I arrived, and turned the engine OFF, I turned the key back to position 1 to check if the SES light comes ON. It don't lighted up.
Three hours later, when I startet driving home, the SES light came ON with key at position 1 and went OFF at idle as usual. I drove home, on the highway the TCC again worked well. At home I checked the SES light again with key in position 1, again not coming ON.

This shows me, that the sometimes not working TCC has nothing to do with the sometimes not working SES light.
It just guees, the SES light issue is a heat related issue, regarding to the temperature inside the car. At cold temperatures, when the car sit for a few hours it works and when the passenger compartment has heated up, it doesn't seem to work anymore.

I will check this later and will not use the heater and open the windows during my next road test.

Those codes will not go away until you fix the TCC. I'm done here.

I'm sorry, but it seems you're wrong. I pulled the vacuum line from the transmission vacuum modulator and my overdrive is back to normal. Sure, it locked up very hard now and when I come to a stop, I also get a little kick from the transmission.
I guess, if there's an issue with the TCC solenoid, it won't lock up, with or without the vacuum modulator connected.
I will order a new 'adjustable' vacuum modulator and post the results.