Previa transmission slip... very occasionally

Hi all. I have been driving my 1992 Toyota Previa alltrac (automatic) for several years now, having previously owned the same vehicle with a stick shift.

Every so often, on the highway, my tranny slips out of fifth (i.e. overdrive) and into what sounds like third. I’m usually pushing the vehicle a little, at fast highway speeds, maybe passing a big rig. As soon as I let off the gas it goes back into the appropriate gear.

(The vehicle has 155K miles, and runs like a top, fwiw.)

Is this worrisome? I have, umm, killed a few manual transmissions over the years, and this is my first vehicle with an automatic transmission. And when I describe its symptoms as a “slip” it’s because this is the only term I know… it might (?) be that at fast fast pedal-down speeds it’s simply downshifting, albeit dramatically.

“Very occasionally” is too often! I would get it to a good independent transmission shop (not a chain) right away. Also, check the fluid level, with the engine warm and idling in park.

The engine downshifting, especially in cruise control going uphil is normal.

At the mileage you are at, you should already have done 3 or 4 transmission fluid and filter changes. If not, you may be driving on borrowed time!

Have you checked your transmission fluid? I have driven a few vehicles with AWD but not an alltrac equipped one. When you really get on the accelerator to pass they seem to shift to third gear which would be downshifting past OD and fourth gear. It may not be so but it seems like it. That may be what’s happening with your Previa although I am just guessing here based on my experience. If you have a 4 cylinder the odds are this is what’s happening.

I agree that the OP is likely experiencing downshifting of the transmission. Whether the transmission downshifts at a given speed is determined by factors such as how hard he pushes the gas pedal and also whether the vehicle is climbing a hill at that point.

However, Doc makes a very good point, as usual.
An 18 year-old vehicle should have had its transmission fluid (and filter, if so equipped) changed about SIX times at this point in its existence. If this has not been done, the trans is undoubtedly at the end of its life-span now. In fact, it is sort of miraculous that it has made it to 18 yrs/155k miles if it has not been serviced on that type of schedule.

OP–You have nothing to lose by trying a fluid and filter change (NOT a flush).
These Hail Mary Passes are frequently a case of too little, too late, so don’t be surprised if it is to no avail.
But, it may extend the life of the trans a bit, so give it a try.

Thanks everyone. The tranny has been serviced several times, and flushed soon after I purchased the vehicle 4 years ago. Fluid appears to be clean and at the appropriate level.

Regarding its “sudden” downshifting, this really only happens when I’m pushing it…

And yes, for an eighteen year old car, the thing is in great shape! I consider myself lucky to have found one in good repair and with proper upkeep. (Thank you Craigslist)

“Regarding its “sudden” downshifting, this really only happens when I’m pushing it…”

Ummmm…That’s essentially when it is supposed to downshift.
If it was downshifting while you cruised at a steady speed on a level road, that would be unusual and would be cause for concern.

Is this the first automatic transmission vehicle that you have owned?