I can’t seem to figure out if I really have an issue with my auto transmission swap or not.

Sorry the message in long, but I wanted eliminate/reduce all the “what about this” response and have an informative post.

I have a stock 88 Fiero GT with the original 2.8L. It had the original TH125C (which I still posses), was swapped out for a 440T4/4T60 a few months ago. I was a rebuilt 4T60 that was installed and the work was done professionally as I didn’t want to screw something up.

The Fiero runs great… Upshifts are smooth. While the engine is still cold, the shifts hang on a little bit longer than when the engine is completely warmed up. Basically I’m talking about going into OD. But that is not what really concerns me. It’s the downshifting.

When the vehicle slow down to make a turn or the traffic light suddenly turns green on the approach, you begin to accelerate it refuses to downshift to a lower gear, say from 3rd to 2nd. It just wants to stay in 3rd unless I keep pressing the accelerator and releasing it until it engages. I’m usually moving around 20-30 mph. There’s no power trying to accelerate while in 3rd gear… the engine just bogs down. It doesn’t happen everytime, maybe only about 5 to 10% of the time and only once the engine is warmed up. Doesn’t occur with a cold engine.

Downshifts from OD to 3rd, 2nd and 1st are smooth. Doesn’t “feel” like there’s an issue other than problem mentioned in the previous paragraph.

If I come to a complete stop or moving say 10mph or less, the transmission downshift into 1st gear just fine. Acceleration is normal.

The mechanic who installed it said there is plenty of vacuum and no leaks found. Everytime I take him in the car, it refuses to act up. He is certified ASE mechanic and a former Fiero owner himself. He had the same transmission installed in his Fiero.

I’ve checked the trans fluid and it is full and clean.

I’ve only put about 1000 miles on the 4T60 since it was installed.

Can anyone provide any clues as to what may be going on with the transmission and why it refuses to downshift?

"Bad" downshift affects only 1 gear?
Likely need a trans shop. Valve body or a servo is binding the 3-2 downshift.

Vac modulator, governor, and TV cable usually affects all shifting.

Need gauges to read line pressure etc to see TV, and maybe modulator, are set right.

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Easiest thing to check/play with is TV cable adjustment.
I had to do that when I had the 4T60 in my Indy.

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[This message has been edited by fierosound (edited 04-04-2014).]