Montana :: 1997 Venture - No 3rd Gear After Shift Solenoid Replacement

97 Venture, 4T60e trans, 197,000 mi. No shift or performance problems ever, solid as a rock. Well taken care of. Then a few months back it throws a P0758 code for Shift Solenoid B, and intermittently seems stuck in 3rd or 4th gear when starting from a stop. Wife drives w/ kids so immediately I park it to investigate and drive it myself and observe. Problem would happen usually after warming up and driving a couple miles, but would sometimes go away after shutting it off for a bit and going again.

After lots of research I decided it was probably a stuck solenoid and tried Sea Foam Trans Tune to see if it would clean it up, and it WORKED! For about 4 weeks that is, then came back. So I took the proper time and steps to replace both shift solenoids myself, and of course the fluid & filter (since you have to drain it to open the side cover anyway). My work is meticulous and clean, so I know nothing was done shoddy.

When removing shift solenoid A, however, a spring-loaded spool came partway out of that port in the valve body, didn't fall out, and I pushed it back in with the new solenoid. Everything else went back together flawlessly. On the first drive, it shifted to 1st as expected so all seemed well. Then after a several miles of driving I noticed something felt wrong - it was skipping 3rd gear altogether (1-2-4), and it would NOT downshift from 4th to 3rd. However if shifter was in "3" it would shift from 2nd to 3rd just fine, then I could put it in "D" and would hit 4th, but could only downshift to 3rd manually.

I am stumped, and it's been parked for months because something in the solenoid change has caused a major problem. I don't believe it could be mechanical gear/clutch failure as it shifts to 3rd w/ shifter in "3", and there was never a mechanical problem before. And it also still intermittently starts out in a high gear though quite seldom (3rd or 4th, not sure which but it barely moves) but putting shifter in "2" or "1" makes it start in 2nd. And never a SES light. Checked solenoid resistance at trans connector and they're perfect.

Tranny shop checked computer and said it's good, and also said "it's shifting just fine" which is absolutely not true. So much for a "free" diagnostic check... Could I have just gotten a bad solenoid? Could the spring-loaded spool that came part way out gotten hung up or the spring not reseated properly? Could it be the vacuum modulator (it got slightly bent upward when lowering engine)?