Pontiac TransAm Transmission- No 4th Gear

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2001 Pontiac TransAm WS6 5.7L. Occasional TCS off. Transmission rebuilt twice. All new parts - core, cable harness, etc. PCM re-flashed by GM dealer.

Question:
I recently has trans rebuilt due to shift issues with shift kit. Afterwards noticed trans going into after driving for a short while - loose 3rd 4th. Starts out in 2nd and stays. Had trans shop look into this and had car for 2 months. The car now will loose 4th (overdrive) for no reason and occasionally the TCS will go off I can't turn it back on. I do have rear-end noise and need to have it rebuilt. Wondering if they are connected. Not sure where the ABS rear speed sensor is on this model. May be located on the drive train and this may be somehow related? When 4th goes out, I can shift through the other gears fine. I need to manually put the trans in 3rd when this happens so that I have torque lock or else when I remove throttle, the car will coast with no engine braking. Please advise

Answer:
You mentioned a shift kit. What parts were replaced to install this kit? How many miles on the kit?

Visitor Reply:
Thanks for the response. The original transmission had a shift kit, but it was all removed in the last rebuild. Currently I have all new parts, except that the core has about 30k miles on it according to the transmission shop. They have rebuilt this trans twice trying to figure out why it, at that time, was loosing 3rd & 4th gears and staying in 2nd (limp mode).

They had been talking to GM people and others about what it could be, because the trans was practically new. Finally they took it to a local GM dealer and they eventually re flashed the PCM. Seem to be OK when I got it back at first. The next day, just as it is now, I loose 4th for no apparent reason and I need to manually put it in 3rd so that I have engine braking when I decelerate. All the other gears shift fine. I'm trying to determine what all is involved in the down-shift - if it could be temperature related, because it seems that in the morning I can go about 3-4 miles from home before it looses 4th.

To make matters worse, I randomly get the TCS Off indication and I can't turn it back on. It will go back on when it decides to. Sorry for the long story, but trying to fill you in.

Answer:
This problem you are having is not electronic. If the 1-2 shift solenoid were to fail, you would loose 4th gear AND 1st gear. This sounds like a valve body problem. Either a valve is hanging up, there is debre in a passage, the valve body gasket is split, or the spacer plate has a bad spot in it where a check ball gets lodged and cannot move freely. Other possibilities is a cut in the 3/4 clutch piston seal, or input housing orifice cup plug problem.