I agree. I think there is a lot of rumor about this, and I've never personally experienced any harm from doing so either.

Being an auto obviously gives you the option to use full automatic mode (which is the point, for most people most of the time), but that doesn't necessarily mean that it was designed ONLY for automatic gear selection. If this was the case, then there would be no option or ability to select anything other than OD or D while at cruise or even from a stop. For example, it will not allow a manual downshift to a lower gear when doing so would potentially cause driveline damage due to an RPM spike outside of designed parameters; it could just as easily have been setup to not allow any manual downshifting at all if such was not part of its designed abilities.

Looking at the GM service manual for the TH350 (obviously not the stock trans for these cars, but still an auto nonetheless, and the only manual I have handy right now), it states that manual selection of 2nd or 1st gear is fine at any speed, but that 1st gear may not occur until/unless vehicle speed is below a certain point, depending on gear ratio. It is also mentioned that these lower gears can be used for engine braking.

Similar experience here. I've been using the ratchet shifter for manual operation of my TH350 car on regular occasion for many years now. It's difficult to get the detent downshifts and governor controlled WOT upshifts to be right where I want them, so for anything other than cruising I always handle the shifting manually. No issues at all. I don't do this nearly as often with the 4L60E because the automatic shift points are accurate, and the stock shifter isn't nearly as precise as a ratchet shifter (nor does the trans respond as quickly to manual shifts.)

 

In an electronic trans, you aren't shifting a damn thing. You are manually commanding the computer to shift the trans. Some builders can key you into some differences when you manually select a gear but the bottom line is the computer has the final say in commanding the solenoids. You moving the shifter isn't doing much other then telling the computer you'd like for it to follow your command. In reality, you really aren't manually shifting per se.

 

I downshift mine all the time. The last time the trans was apart (3-4 clutch wear, busted converter hub/ fluid pump) everything, including the overrun clutches, looked brand new except the parts I was replacing.

I reused everything but the 3-4 clutch set, the band (had some minor chunks out of it), and the pump.

This is with the Trans Go HD2 kit if that makes a difference.

 

Umm, there is no way in hell I would be downshifting AND applying gas. Trying to rev-match an auto? Not gonna do it.

 

You are in fact shifting the transmission when you manually shift, even an electronic transmission. Depending on the trans and modifications, the computer may or may not have say over the shift.

Manually downshifting some full manual VB's can cause issues. In a TH350 or TH400 without engine braking, if you downshift (or shift to neutral) at speed you can create an over-run condition and cause the transmission to explode.

Automatic transmissions are designed to upshift under throttle/load. They are not as strong on a downshift in many cases. It's easier to build a drag racing transmission than a street/strip transmission because the drag racing unit never has to downshift.

 

By downshifting what you are really saying is downshifting under throttle correct? So by that rationale, if someone wants to go to a lower gear, say 4th to 2nd, it would be "easier" on the transmission for the user to select 2nd gear manually, then apply wot as opposed to pressing wot first and letting an "automatic" downshift occur under power correct? Prime example of how not letting an auto trans auto shift IS beneficial, though its been disputed further up in this thread.

 

i have had several automatic transmissions over the years and i would manually down shift them all from time 2 time...never had any issues...If it was really that dangerous I doubt that they would even give u the option of manually shifting the gear...dont u think?

 

This is probably the best way for the average driver to think about shifting an automatic trans - you aren't really shifting, but rather telling the computer what your preference is. This is especially true with more modern cars than these.

However, as Jake hinted, there is more to it than that. Moving the shift lever moves the "manual valve" in the trans which controls the hydraulic possibilities. For example, when you move the shifter to [D2], the manual valve closes the "D3" and "D4" hydraulic circuits and there is no way the the 3/4 clutch could then stay engaged.

The PCM in pre '08 4L60E does not read the shifter position directly, but rather via the hydraulic circuits that the manual valve activates; it uses the "pressure manifold switch" for this.
Curiously around '09 GM changed to a physical switch on the linkage, in other words, it is reading the shifter position directly.

In short, while not technically correct, you should think of the shifter position as a "preference" and not an absolute.

 

For some reason I assumed the selector switch near the valve body was electronic in nature....learn something new everyday.

 

I don't exactly apply gas (or at least not additional gas), I just don't fully release the throttle, basically keeping it at the same point that it was to maintain cruising speed in the current gear prior to the downshift. It doesn't need to be as specific as a "rev match" (and I don't think it exactly ever could be with an auto), just enough to maintain at bit of load. Not sure that it's necessary, but it does make the transition smoother.

I'm not worried about automatic downshifts either, as this is another thing that the transmission allows because it was in fact designed to do it. If this was a "bad" thing that produced failure in short order, then it wouldn't be part of regular operation as designed by the manufacturer.

Looking in the owner's manual under transmission operation, the only specific caution mentioned about manually downshifting is to not shift into 2nd at speeds above 65mph. If manually downshifting was overly harmful in general, it would advise against it altogether, and/or it would have been setup from the factory to prevent manual downshifting.

 

When you move the lever to some position you are simply selecting a range of gears... the PCM figures out what particular gear (in this range of gears) should be selected based on the shift tables (which have axes for TPS and VSS)... you simply go along for the ride (altho you have some control by manipulating the throttle).