Very Specific Transmission Slip

[(insert name here)]

Having an issue with an '89 Grand Marquis. Under light acceleration, there are no issues. Under moderate to heavy acceleration from a stop, after starting to move, the car will suddenly slip out of 1st for a few seconds before suddenly slamming back into gear. The issue is intermittent (more often not happening). All other shifts work exactly as expected with no sign of funkiness while accelerating or cruising.

The car did have a transmission fluid smell when it was purchased. I took a look and it was hopelessly overfilled, but color was fine. I drained it to the correct level, but the intermittent shift issue never went away.

Any ideas what's going on here? This problem seems weirdly specific, and I can't find any one else having the same issue when I try to Google it.

[gadget73]

only thing that comes to mind is a missing or damaged transmission filter gasket. If thats missing it can suck air between the filter and the valve body, possibly only when the fluid sloshes to the back though.

[FordMan77]

Are you checking the fluid level with the transmission fully hot, in park with the car idling on level ground? If not it won't read correctly on the dipstick.

Does it have a leak somewhere to give off that smell? I hope it's not burning clutches your smelling. Is the TV bushing still in place? Just thinking of common AOD issues.

[Tiggie]

Does sound like a low fluid level possibility. X2 on what FordMan said / be sure you're checking it in park, warm and level.

[(insert name here)]

Noted on the filter gasket. Than pan has not been dropped, so that'll be something to look for if we go that far with it.

Fluid level was checked a few times while hot, idling in park, on flat ground.

If I recall correctly (it's been awhile since the car was first acquired), the smell was coming from transmission fluid being puked up out of the dipstick tube once the transmission reach operating temperature (that's how overfull it was). I believe the smell is gone now that the level is correct.

As a side note, this is not my car. I'll take a look at the TV bushing in the near future when I'm at the owner's house. I generally only drive and work on carbureted cars, so I'm a bit out of my element with something this new. If I'm understanding correctly, the TV cable was a replacement for the vacuum-driven modulator valve that was used on older transmissions?

[Arquemann]

As a side note, this is not my car. I'll take a look at the TV bushing in the near future when I'm at the owner's house. I generally only drive and work on carbureted cars, so I'm a bit out of my element with something this new. If I'm understanding correctly, the TV cable was a replacement for the vacuum-driven modulator valve that was used on older transmissions?

Kinda yes, a TV (throttle valve) cable controls line pressure and shift points, and it does kickdown aswell.
Unlike the vacuum modulators on C4/C6 etc, if the TV cable/rod is not hooked up or is very loose, it will result in very sloppy shifts that let the trans slip way too much, causing fast wear.

[sly]

I would think that the seller didn't wanna mess with the transmission and just overfilled it to mask the issue. I'd also wager that it's the filter has a seal issue or has fallen down.