Help, car shifts from 4th to 3rd gear

I have a 95 supercharged riviera that is shifting from 4th gear down to 3rd after it warms up from about 20 minutes of driving. The car also seems to chugg a little at slower speeds when I'm accelerating lightly, like its not shifting up. The car threw a P0740 code, torque converter clutch circuit. I had the transmission flushed and filter replaced, lock out solenoid replaced, all the solenoids tested out okay electronically, but after all that I still have the same issue. The car drives fine except for this issue, please help.

Did you have it flushed before the problem started or after.?

after

are you sure it's shifting to 3rd, or are you just feeling the torque converter unlock, which puts you at a very similar RPM to 3rd. When you think it's bumped up to 3rd, try moving the shifter from OD to D or 3 (I forget what's there) and see if it shifts again...

anyhow, I have heard so many problems with TCC solenoids and Pressure Control solenoids gettings stuck from a tiny tiny piece of dirt from a flush. I strongly advise against flushing, only do a basic fluid/filter change. Flushing seems to have a bad habit of dislodging that little piece of dirt that horks your solenoid.

With luck I have heard of somebody with a TECH-II (GM's do-it-all computer tool) activating the solenoid seperately and dislodging the blockage, whatever it may be. You'd have to know a GM mechanic well to get your hands on one, though. A trans shop may or may not have access to such a device.

thanks for the help, I'll give your suggestions a try

ajohnson wrote:

The car also seems to chugg a little at slower speeds when I'm accelerating lightly, like its not shifting up.

This is a classic TCC solenoid not acting properly. It is not releasing when trying to accelerate at moderate throttle position, causing the "chugging' feeling and other times it is engaging too soon and/or too late.
I had similar issues with mine before my overhaul. All my solenoids tested good also.
But had a problem with fluid bypassing the solenoid plungers due to a scored bore. In my case they were able to install a kit that requires boring the bore oversize and installing a sleeve. That took care of the fluid leaking past the solenoid plunger on the high pressure solenoid.
The fix does not necessarily require an overhaul though, just need to gain access to the valve body and remove solenoids, inspect bores and repair if necessary. Not an easy job.