4L60E - weird things happening!

[Dman77]

Hi all. New to the forum, and hoping you can point me in the right direction.

My kid has a 96 Impala SS with some modifications that the prior owner added (more on this later) - and the transmission is acting up, badly. I'll start back 4 days ago. He called me up and told me the transmission was slipping suddenly. No warning; it was working flawlessly up to this point.

I went to meet him where he was broke down. I had him start the car so I could start by checking fluid. He did. Fluid was low, but still reading on the dipstick. I'll start by saying the fluid was more brown than red, but not the worst I have seen. Before doing anything else, I jumped in the vehicle, started it, shifted into drive, and everything felt normal again. So the issue (in my mind) was related to hot fluid.

I added about a half quart and that brought it back up to the warm safe range. Started driving home. Got about a half mile away and it started slipping again. Ok - had it towed home.

The next day, I dropped the pan and changed the filter. The pan had a very thin film of friction material at the bottom, but nothing I was alarmed by. I measured the fluid that I extracted and added that same amount. Put that back together. Then I opened up the trans cooler line and flushed the fluid a few quarts at a time - measure how much the transmission pumped out and adding that much back in via the dipstick tube. I kept doing this until the fluid was coming out red. Ok - tightened everything back up - and then went and drove it. I drove it hard trying to see if I could get it hot to act up. Everything seemed to work fine. So at that point, I assumed the fluid was old and the new fluid did the trick.

I told my kid to drive it gently for the next few weeks. He made it about 3 more days (and he assured me he was being nice on the car). Last night, on his way home, he told me the car started slipping again, in higher gears. I said if he can make it home driving slow with hazards, try and do so. He did. It brought him back to the house.

Here is where I am perplexed. I immediately got into the car and started it up. Checked the level. It is good. I put it in reverse, it shifted nicely - but then everything went to crap. Just idling and slowly moving backwards in reverse, the transmission felt like it was disengaging and re-engaging with a jerk and a bang. I had only moved about 6 feet, and that happened 3 or 4 times. I put it back into forward gear to re park in the garage - same thing.

I am not a transmission expert, but I am very comfortable taking most things automotive apart and rebuilding. But I don't know where to start. My first thought was a bad torque converter, but I assume there is probably more to the story.

Here is where my comment about modifications comes in. The transmission has an obvious shift kit, based on how it feels when shifting (normally). The catalytic converters have been removed by the prior owner. At the same time they did that, they reprogrammed the ECM. This is where things get difficult. The ECM no longer has a history in it - so it only shows codes at the moment something is faulting, and usually only when it is a direct misfire (with a flashing check engine light). Plugging into it provides no insights into what may be happening with the transmission.

I read the sticky for common failures for 4L60E, and nothing in that list really discussed off idle / slow movement issues. I am hoping someone here has experience with this issue or an idea of where to start.

Thanks in advance!

[Accelo]

Are both back tires worn the same? Or are they much worse than the front?
Third gear is the culprit and is the likely issue with it slipping.
My guess is, there is a 98% chance it will have to be rebuilt.

[Dman77]

Accelo,

The rears are more or less about the same as the front. Part of the modifications in this vehicle are a 9? Ford rear and with a True Trac limited slip, so if there was wear in the rear, my first thought would be due to that.

[Accelo]

The line pressure rises in reverse. Sounds like it?s low and causing the reverse issue.
I?m confident it?s should be removed and rebuilt.