TH125C/3T40 Problems

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Jun 22, 2018
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#1
Make
Pontiac
Model
Sunbird
Year
1994
Miles
135000
Engine
3.1 v6, LH0
I have a TH125C/3T40 auto, model year 94. Attached to a 3.1L MPFI v6.

The TCC was unplugged when I bought the car, I replaced the TCC solenoid and reconnected it, it worked great for the 8k miles or so I drove the vehicle.

This was always an issue since I bought the car:
Placing the Shift lever in 1 while stopped and the car goes to 1.
Placing the lever in 2 while stopped, the car will usually start in 1 then shift to 2. However coming back to a stop, the car doesn't downshift to 1, and taking off it stays in 2, and sometimes suddenly slams into 1 instead after just starting forwards.
Manually down shifting from 3 to 2 while moving works fine, manually downshifting from 2 to 1 works fine while moving (at a safe slow speed).

This never bothered me as I only ever use D, never 2 or 1. And it always shifted 1-2-3 correctly when in D, and it downshifted 3-2-1 correctly when in D and stopping or when pressing the gas pedal far enough in D.

This started recently:
Another Issue that just started, I will place the vehicle in Drive or reverse, and it will not stay in drive or reverse, however sometimes it will. It falls between Neutral and whatever gear I have selected. Once it stays in drive it pulls great, flooring it and it downshifts properly, TCC works correctly, no slipping. Same with reverse, when it does stay in gear it works fine, but when it falls out of gear it just free revs like it is in neutral.

Park and Neutral works fine.

Things I checked:
TV cable connected and still functions as it should.
Shift cable is connected and functions as it should.
Fluid level correct, checked in park with engine running at idle on flat surface, at temperature.

Misc Info:
Back when I replaced the trans filter and did a fluid change there was some signs of use in the pan, nothing unusual as I've had several cars with this trans in the past.
However there was a Metal Piece about 8mm in each direction, like a mini cube that looks like it was broken off of something, I never thought much of it and finished the filter change and kept using the car. And it kept working fine, but now the above issue is present, the one that started recently.

I first noticed it when I went to leave a parking lot, I took my foot off the brake, pressed the gas like usual, and it acted like it fell to neutral, then it suddenly slammed into drive and went, part way home it did it again while driving about 40 mph.
Backing up in my driveway it kept acting like someone was moving it between R and N nonstop.
There were times it stayed in gear when moving the lever myself between R and D but not always.

I was hoping this could be an easy fix if someone knows what could be wrong, if not I'll either sell the car cheap if someone wants it or junk it.
 
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Joined
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#2
However there was a Metal Piece about 8mm in each direction, like a mini cube that looks like it was broken off of something
That may be a big clue. Perhaps some detent that was to hold trans in correct position??? I am NOT a trans expert but hang in there and you may get a response from someone here that is.
 
Joined
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#3
That was before this new issue though. Maybe whatever it broke off of eventually failed after 8k miles or so? Idk, I'm not a transmission expert.
I started the car today and tried to recreate the problem just moving the lever in the driveway for a couple minutes and no problem was showing up so I quit. It will just stay parked where it is unless it has to be moved, its out of the way so it can stay there till next year or whatever.
 

bp042665

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#4
ok i have had a issue like this at my shop and after adding a extra Quart of fluid it worked fine it will read over full on dip stick but that is fine will not hurt anything try this and let me know