Auto box fault question

[GalantOnly]

Hi Guys

I'm not an expert in auto boxes, so I need to hear what you think about that.

The friend of mine with N/A V6 auto. His mechanic filled Dextron II, because he didn't know better /Hissy
After a month the car begun to rev up and down slightly (100-200 rpm) at 70 mph, like TC is slipping or something. It's only happening at that speed...
He changed ATF to SP3 right away then, but the problem is still there

What could be wrong?

Thanks...

[AlanDITD]

did you flsuh the whole lot, or just drop the sump?

[Kieran]

Did he disconnect the battery following the change of fluid? Sounds like the T/C isn't locking up properly.

[eyeballprawn]

It's probably still half full of the dextron.

[GalantOnly]

Well... Everyone got a very good point here

ECU can still remember Dextron.
There can still be lot of Dextron in the system, if they just dropped the sump...

Thanks /yes