Yet more good info above, thank you..
Not to sound stupid, but if I elect to experiment with a TCC toggle switch,
where do I locate the PCM'* command wire to the TCC..?
Where is it & what color(*) is it..?

Also, I know that on some transmissions, manual shifting can actually harm them.
Will manually up and/or down shifting the transaxles in our cars cause damage..?
Not just between 3rd and 4th (OD) as discussed above,
but also manually downshifting into 2nd for engine-braking on grades,
then upshifting back into 3rd and/or 4th..?
Is that harmful..?
I do not drive around constantly manually shifting the transaxle,
but I do hit some pretty long & steep grades on a daily basis,
and like to utilize engine-braking to help control speed on downgrades.
If it'* harmful in any way I won't do it - Brakes are cheaper to replace than a transaxle..

there are two TCC related circuits on the transaxle connector, one for TCC PWM, the other is just TCC. you'll want to "intercept" TCC. it should be a tan wire, black stripe, in position "D". if that circuit is open, the TCC will never apply. if closed, will only apply the TCC when the PCM wants it applied.

manually shifting between 3 and 4(or D/3 or D/OD depending on how your shifter is labeled) is no problem at pretty much any speed. manually dropping into 2nd is.... rough, to say the least, and i really wouldn't recommend it unless necessary. it may or may not be damaging to the transaxle, but it is definitely damaging to trans/engine mounts.

for maximum engine braking, i have my tune setup so that when i lift my foot off of the throttle, the TCC won't automatically unlock until it gets down to the MPH threshold to unlock in a 0% TPS situation, which is around 36MPH in 4th and 26 MPH in 3rd. TCC staying locked alone helps a little, but i also have DFCO allowed to happen in the same instance, combined they produce quite an effect.

also, interesting tidbit: 1st gear is the only gear the PCM can actually force in/out... due to the way the manual shaft is setup on the 4T60E, if the PCM is commanding 4th and the shifter is in 1st or 2nd, the highest gear it will actually achieve is 2nd. shifter in 3rd, highest achievable gear is 3rd.

anyways, that last part is just trivia really, but useful in certain situations, like if the trans loses all power to the shift solenoids for some reason, moving the shifter between 2 and 3 will get you 2 forward gears instead of just 3(3rd because when both solenoids are de-energized, 3rd gear is being selected).

I am new to this forum and also new to NE PA. I had a banging shift both upshift and downshift on the hills here.(there weren't any hills on Long Island where I used to live). A local shop found trans fluid in the modulator and replaced it with one of the adjustible ones. He also installed a vacume restrictor in the line. I have removed the restrictor and adjusted the modulator screw to 2 turns out but I still get harsh shifts above 40 mph. Should I get an original GM modulator @ $155? The trans was rebuilt 2 years ago for an intermittant loss of TCC after one hour driving at highway speed.