aod slow down shift.

[84merc951]

my 1984 aod trany has a hard time down shifting out of overdrive while coming to a stop. only does it during the first 15 mins of driving. the aod is original and has 240k on her. what would cause this to happen. do the bands need adjustment or could it be a tv issue. this has been going on for a little over a year and is driven every day. let me know what you guys think thanks matt

[86VickyLX]

Only while coming to a stop?

[BerniniCaCO3]

I've only recently been educating myself on how these transmissions work;
But as I understand it, the valve body is a sort of hydraulic computer, where at, I'm making this number up, say 60 psi, you'll be in overdrive. At the made-up 60psi, the overdrive band will be applied, and --I'd have to get out the book-- some clutches will be applied, other released, etc. At 60psi, that pressure pushes on pistons against springs, they slide over, and line pressure passes through to apply servos pushing against bands, and apply pistons against clutches; or is blocked, and servos and pistons are not applied.
The tv cable is one input affecting governed pressure, the governor is the other. The governor as it spins, centripetal force forces a valve outwards, raising line pressure causing upshifts at higher and higher rpms. The tv cable, I think, resists this; and at a certain point when it counters the governor, will drop line pressure, and you downshift again (kickdown).
There are probably gaps in my specific understanding.

Point being, if you're not downshifting at lower speeds/ deceleration, if it's hanging in overdrive, it might be a sticking governor or a sticking valve in the valve body, or sticking OD servo/ fatiguing OD servo return spring. Experience, which I lack, would advise what to look at first. I think my rebuilt transmission has a sticking governor, because it will try and start in 2nd gear for just a second before dropping into 1st gear with a minor shudder. This is the valve in the governor being stuck at an extended, read, higher rpm, position; so the hydraulic computer "thinks" the car is going say 15mph when it's in fact at a stop. If your governor has a valve sticking at a higher position, or a weakened internal return spring, such that it doesn't fall back when you decelerate, that is just one viable explanation for hanging in overdrive: like mine hangs in 2nd gear.
I shouldn't think it would be the tv cable, because if that sticks in an extended position, it would only promote downshifts not oppose them. I think.

No one ever really adjusts bands on an AOD; and it seems to be a dubious practice even for those transmissions on which it's most successful.


As for options;
I suppose if you found out it's in the valve body, or it's the governor, either of those is still cheaper and easier than dropping and rebuilding the whole transmission. Indeed, you could do either yourself.
But a rebuild AOD, installed, is still an inexpensive affair. You can find working AODs from mustang guys, transitioning to manuals, for like $300; with more like 100k miles.
At 240k it might be most effective simply to rebuild or replace than try and diagnose one cause and fix it, only to have it not fixed, or have something else go later.

[84merc951]

yes only while coming to a stop for the first 10 to 15 mins untill the drivetrain is completly warmed up and then its fine till the next morning or later in the evening. it almost feels like the engine will stall because its stuck in od till about 10 mph or so then it will down shift. for the past 6 months or so i just down shift manualy into 3rd if i have to come to a stop. i do that till its warmed up.

[BerniniCaCO3]

fluid is thicker when it's cold.
Sounds like something's sticking.

Fluid is clean; not dirty or even gritty with clutch material?

[84merc951]

yes the fluid is a little red in color and with some grit. not a perfect pinkish color. thanks

[84merc951]

the fluid was changed maybe 20k ago. i had also put a bottle of lucas tranny oil in but this was maybe 2 years ago and the down shifting issue came about a year ago i would say.

[84merc951]

i just took the merc out and the slow od down shift while stopping lasts for around 15 mins if im driving around town but if i get on the interstate for like 10 mins then exit of it has no problem down shifting problably because it warms up faster on the interstate. but like i said if i know that the tranny is cold and i feel that its going to stick then i will manualy put it in 3rd and its fine. the tranny shifts fine besides this. what cure if any should i check out. thanks matt.

[BerniniCaCO3]

OK, so it has no trouble downshifting manually,
and if you hit the gas hard, it downshifts (kicks down) for passing gear, also without delay?

The only time it sticks in overdrive is when you're slowing down the car with brakes/ coasting to a stop, and it won't downshift?

Still sounds like a governor issue then.
let me run it by my transmission guy as a theory!
Because if all the "machinery" in the valve body, and the OD servo itself, are working a-ok, as evidenced by manually shifting to Drive (3rd); and the piston itself must be releasing by the same logic,
then it can only be that the transmission doesnt' "know" that it needs to be downshifting unless directed to do so by the shift lever or tv cable. And it gets that que, from the governor.

If it is the governor, I can probably find you an spare and ship it for $5.
Changing it out, you need a 12mm 12 point socket and preferably impact (or a torch, and leave the car in gear to hold the driveshaft) to remove the 4 driveshaft bolts,
drop the driveshaft,
remove the transmission mount. You can remove the whole bar --18mm socket and 21mm wrench-- or you can remove the two nuts in the middle --15mm socket-- and with a jack push the transmission up and out of the rear mount.

Remove the speed gear. 10mm nut or something stupid simple like that.

Remove 4 bolts holding on the housing.

And that rectangular bar is the governor.
Forget how it was held on; nothing daunting though. There's a ball in a detent that acts as a key, don't lose it! It'll fall out when you pull the governor.

Plan for a saturday morning at least, anyway. But nothing horrendous.

[84merc951]

OK, so it has no trouble downshifting manually,
and if you hit the gas hard, it downshifts (kicks down) for passing gear, also without delay?

The only time it sticks in overdrive is when you're slowing down the car with brakes/ coasting to a stop, and it won't downshift?

Still sounds like a governor issue then.
let me run it by my transmission guy as a theory!
Because if all the "machinery" in the valve body, and the OD servo itself, are working a-ok, as evidenced by manually shifting to Drive (3rd); and the piston itself must be releasing by the same logic,
then it can only be that the transmission doesnt' "know" that it needs to be downshifting unless directed to do so by the shift lever or tv cable. And it gets that que, from the governor.

If it is the governor, I can probably find you an spare and ship it for $5.
Changing it out, you need a 12mm 12 point socket and preferably impact (or a torch, and leave the car in gear to hold the driveshaft) to remove the 4 driveshaft bolts,
drop the driveshaft,
remove the transmission mount. You can remove the whole bar --18mm socket and 21mm wrench-- or you can remove the two nuts in the middle --15mm socket-- and with a jack push the transmission up and out of the rear mount.

Remove the speed gear. 10mm nut or something stupid simple like that.

Remove 4 bolts holding on the housing.

And that rectangular bar is the governor.
Forget how it was held on; nothing daunting though. There's a ball in a detent that acts as a key, don't lose it! It'll fall out when you pull the governor.

Plan for a saturday morning at least, anyway. But nothing horrendous.

yes thank you. it has no problem slaming down into low gear or going into passing gear with no delay. and the only time it sticks is while the tranny is cold and coming to a stop. it does it worse if i have to stop real fast like at a red light stop compared to if it was coasting.

[84merc951]

so if it my govenor that means it is sticking. is there any way to unstick it or is disasembly of the tail shaft required. thanks

[86VickyLX]

so if it my govenor that means it is sticking. is there any way to unstick it or is disasembly of the tail shaft required. thanks

If it was sticking, I would think it would affect more than just overdrive. And yes, you'd need to pull the tail shaft off.

[BerniniCaCO3]

The valve in the governor could be sticking at a spot in its travel.
That is what my transmission guy is telling me about my 2-1 bump; that it's sticking towards one end of its bore (towards the output shaft), maybe yours is sticking at a spot away from the output shaft and not returning as you slow down.

And it seems to make sense, if it goes out of overdrive under all other circumstances (forced into Drive, kickdown via tv cable), that it wouldn't be in the valve body or the OD servo.

But, I'd get a professional opinion. Find an independent transmission shop run by an older guy who dealt with these, and have him test drive it. You can waste a lot of your time and parts chasing ghosts....

[gadget73]

I'd probably go for a fluid and filter change first if the fluid is gritty and dark red.

[84merc951]

I'd probably go for a fluid and filter change first if the fluid is gritty and dark red.

my brother was telling me that sense my tranny has so many miles and that the fluid turnned dark with in 2 years that should just leave it alone as draining it and putting new fluid in could make the trans slip becaus old trannys like dirty fluid. is this true.