My 91 SS's 4EAT was rebuilt some time ago by the PO.

Mileage 191k now

 

Fluid is fresh and smells fine.

 

It does this little dance upon upshifting warm.

 

Shifts are fine no slippage.

 

It just upshifts to third then

about 2 seconds later it downshifts to second.

(Seems like when the intake manifold goes positive i.e. boost starts, due to the load inpressed by third gear)

 

Then back up to third - this is somewhat controlable

if the throttle is regulated.

 

I'm wondering if a NA valve body/TCU was used

and it does not like seeing the ECU's mention of boost?

The valve body type should be similar, and as long as it was rebuilt correctly, that shouldn't be an issue. It's possible the TCU may be acting up. If you get the part # off of it, I can check it to see what application it's for.

 

You could try posting on the legacycentral bbs to see if other SS 4EAT owners exhibit similar issues.

Thanks for the offer Josh, the car is on loan to my sister for a while.

Will do on the legacy central post, good idea.