ATF change, now trans flares

[SL113fan]

Performed an auto trans fluid and filter change, incl torque converter.  Filled in the prescribed amount (4 litres) before firing it up, then added the remaining .75 litre or so.  Trans took several minutes before it could engage forward or reverse, despite being filled with correct fluid amount.  After it allowed me to reverse out of driveway, I took a spin around block and verified fluid level on level ground.

Now for the real test driving. While before the trans performed very well, it now (with fluid change) flares between the 3rd and 4th gear upshift (like someone shifted into neutral briefly).  It has done this several times, mainly when below full operating temp.  This is disconcerting because if not paying attention it could quickly overrev the engine.  The flaring occurs mainly with 75% throttle acceleration.  New fluid was basic Shell ATF Mercon / Dexron.  I'm disappointed that this preventative maintenance attempt has rendered the transmission less "idiot proof."

Could there still be air trapped in the gear band solenoids, or am I looking at a modulator pressure check and adjust?  Three way solenoid clicks and appears to move back, center, front.  Intake switch clicks just after tip in.

Thanks,

-Hans

[ja17]

Hello Hans,

Double check the fluid level, with the car running on level ground.  I always check the torque on the transmission valve body bolts when changing the filter in the trans. The bolts loosen over time, causing an internal pressure leak at the valve body gasket.

Changes in engine ignition timing can cause changes in engine vacuum which will change modulator pressure. Did you do any engine tuning recently to cause a change?

[SL113fan]

Thanks; I checked timing and it appears OK at ~ 8 ATDC.  But I will check that the main trans vacuum line actually pulls and holds vacuum from the manifold.

[dseretakis]

When did you last change the  transmission fluid - I mean before this change?  I heard that if the transmission has been neglected and then you change the fluid after 100,000 plus miles you could expedite its impending failure.  This statement could be completely fallacious - it's just something that I remember hearing.

[xcashewx]

my SL would do this from 3rd to 4th, and rarely off the line it would slip into nuetral then kick back in gear.
it ended up being the vacuum modulator was weathered out, not enough to allow lots ATF into the intake, but enough to affect the correct modulating pressure causing occaisional "funky" shifts.

[SL113fan]

Interesting.  One does "disturb" the vacuum line when dropping the pan, by removing the funnel tube on the right to which the vacuum line is attached.  But I wouldn't think that a sudden vacuum drop or intermittent leak could initiate a drive-neutral shift change.  A rough shift, for sure.

[ja17]

Hello SL 113,

Chances are it is something you may have disturbed. I would back track and check every step.