Slipping 3T40 (THM125)

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Aug 15, 2006
Central Washington
On my old Cavalier, it has a 3T40 3-speed auto. I just changed the fluid. I used Castrol Dex/Mer I got on clearance. I always install 4 quarts after dropping the pan and filter as it always comes out perfect. Thing is I don't know the actual level of fluid in the transmission. (Transmission dipstick, meet positive post.)
After the change out once it gets hot, it shifts like dog poop, when it does. Sometimes it will decide to stay in 1st or 2nd for awhile. Sometimes, depending on circumstances, it will slip hard coming off the line. Feels like a neutral drop. Now, I had been running Type F in it for awhile because it would slip a little in 2nd, but this is so much worse..
I added another 1/3 quart or so in case it was low initally, but its still doing the same thing.
I had this trans in a cutlass ciera behind a 3.3 (beefiest motor they put behind it) and it was shuddering at 101k but lived to 153k when I junked the car due to rust.

Took it to a "good" tranny guy and he said the o-rings were worn and it wasn't making pressure as quickly. Told me to just drive it.

Another thing you can do is adjust the TV cable, it does stretch. Typically you tighten it "a lot" then mash the accellerator pedal and it clicks back to the correct position.

If your clutches are good and line pressure low type F won't help anything and might hurt it. Maybe maxlife dexron with its thicker base would help.

These trannies are still durable and you could switch in a junkyard one and probably have good luck.
hey colt,

you know how I was asking you about adjusting the TV cable on mine? the day before I was going to adjust it, my 3t40 started shifting normally again. I think something is sticking and got freed up. But I'm using Maxlife ATF/lubegard red. I think this trans holds 7 qts, and a pan drop is a 4 qt change. I wasn't sure if to use Maxlife, Dex IV, a multi-vehicle fluid, or put redline D4. maxlife is doing great in mine.
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Could the filter not be seated properly, causing air to be drawn in/cavitation? I've heard of that happening on other ATs when the filter was removed and replaced. IMO, after a while, you're best off leaving the window screen of a filter in place and just concentrate on the ATF.

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I'd go to the junkyard and pick up a dipstick, you can't properly diagnose this without confirming the fluid level is correct.
is that AT computer-controlled? If so you could try resetting the ECU and it might help. Also check all the electrical connections on the trans to make sure they aren't loose. and make sure your kickdown cable is adjusted right. If all that doesn't help, and your fluid level is correct, it may be time for a new tranny. Also you should consider a different ATF such as maxlife dex/merc. type-F can't be right for a GM transmission.
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It is not (mostly). It only has computer controlled TC lockup, thats it. I used type F before because it was in vegas and it was slipping a little. (Did not slip with F.) Its common to do with this trans since its not computer controlled.
Its possible the filter is not installed correct, but I throughly checked it..
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