Weird 4->3 downshift issue.

[King_V]

All,

Ok, I hope this isn't my trans beginning a slow decline into death - the symptom is a little strange and confined to a single, very specific behavior.

ALL upshifts are absolutely normal.

All downshifts are normal except for 4->3 downshift.

The 4->3 downshift is always normal, EXCEPT for in one very specific circumstance.

If the car is in 4th gear, and I'm slowing down, with no throttle, the 4->3 downshift occurs around 30-31 MPH (I have 2.73 gears).

If I happen to give the car gas at the exact moment that it's starting to downshift on its own, the car will rev momentarily as if it's in neutral, then 3rd gear catches.

Now, I don't go out of my way to do it, mind you, but it just sometimes happens that I have to speed up at that moment.


What's going on? Am I looking at a major systemic failure, or something to the effect of a broken/mangled/missing check-ball or spring, or a single passage clogged in the valve body.

The fact that that particular symptom is the ONLY thing that happens and that everything else is absolutely normal is just baffling to me.....

AOD gurus, can you tell me what's happening inside my trans?

[gadget73]

My Mark VII does something very similar, though mine always neutrals on a 4-3 downshift, no matter what the circumstances. Consensus is dirt in the valve body or some other likely minor vb issue, but I've made no effort to look into it beyond asking questions.

[Blaze86Vic]

It's actually normal. The reason that your car does this is because of how the AODs work. In 3rd and 4th gear they do not use the torque converter, (so it's a solid connection from the engine to the rear wheels just like a manual).

A torque converter is not a solid connection, it's using a fan to spin another fan, when one spins the other does, but they aren't physically connected. A torque converted does this between the engine and the trans. So when you shift from 1st to 2nd or 3rd to second you don't notice it because it's using the converter normally so you expect that the engine can rev a little. However, in 3rd and 4th, this isn't being used, so the revs do not change with throttle like in 1st and 2nd, but instead only change with vehicle speed. So to help smooth out the shift, the AOD disengages all clutches before smoothly engaging the clutches the for next gear (just like you would do on a manual). On acceleration this happens pretty fast because the revs are up and there is plenty of flow from the pump in the trans to make things happen fast. But on deceleration, the revs are low so the pump isn't spinning as fast, so it can take slightly longer to complete the transfer of pressure from one clutch pack to the next. Hence, for a split second it can seem like it's in neutral.

So King_V,nothing to worry about there, it's all normal. However, you should try and avoid hitting the gas at that exact moment, as it is speeding up the wear on your clutches. Since you know exactly when it will happen, it's not that hard to avoid.

[King_V]

Hrm, but it didn't USED to do that!

I think I might've made a discovery, though. I finally ordered the bronze TV bushing and installed it yesterday.

Whoa! This thing's factory rubber bushing was disintigrating - the TV bracket hadn't fallen out, but there was some play while it was inside the bushing - I suppose maybe enough to make the trans act as if the setting was one click looser? Still holding, but the rubber was mushy and disintegrating.

In any case, I put the bronze bushing in. So far it feels SLIGHTLY different - and I haven't gone out of my way to avoid the particular behavior I describe (nor to cause it, either!). Hasn't happened yet in about 50 miles of driving - I hope this is a good sign!

[gadget73]

Hence, for a split second it can seem like it's in neutral.

Mine neutrals for a full three seconds every time it does a 4-3 downshift. I know thats not normal in my case. The AOD in the Towncar does have that split-second neutral on a 4-3 but in the Mark its so bad the engine will tach up an easy 1000 rpm before it grabs third, at which time it basically slams into gear and accelerates. Its very disconcerting, and it does the same thing if I manually pull it from OD to D, though the rpm increase and the gear slam aren't as noticeable since I'm not adding throttle while its not engaged.

Lower than normal TV pressure definitely can cause some wierd symptoms. Good thing you caught it before the bushing totally failed.

[Blaze86Vic]

Yeah, that's a little more extreme than I was talking about. I guess I should clarify that the gap I'm talking about would be barely noticeable, not something that were obvious.

And definitely good catch on the bushing.

[King_V]

Yeah, mine went from absolutely didn't happen at all (or at least so little that I didn't notice it), to taking maybe 1/4 or 1/2 a second, engine would come up a few hundred RPMs when it'd happen.

Maybe, depending on how things feel, I'll tighten the TV cable by one click.... I figure that slight amount more of pressure couldn't hurt, and might even help longevity.

[King_V]

Update - so far so good - that weird 4->3 slip has never happened since.

Also, my 3->4 upshift happens ever so slightly later. Not much, but it's noticeable to me because I'm looking for it. I never realized that a worn, somewhat squishy bushing with only that slight amount of flattening could actually have a noticeable effect on the trans, compared to a new bushing.

[gadget73]

yeah, the cable movement is pretty critical. Even a slight change in position is enough to affect how the trans behaves.