[johnoh]

I posted a few weeks ago but have made some progress on my problem.

Car is a 1993 Buick Century 3.3l, with a 4T60 (not 4t60-e) tranny.

When shift lever is in Drive, not overdrive, tranny is perfect

When in Overdrive, especially when cold, car will not downshift from 2nd to 1st at a stop, but instead goes straight to neutral. It will downshift from 2nd to 1st if you punch it (hard accelerate) when in 2nd. Only when stopping is there a problem. So I have been driving without overdrive for a few weeks now.

I read that the 4T60 has a engine vacuum line running to it but that the 3T60 (3T60 has no overdrive) does not, so it got me wondering if the reason overdrive was not working was an engine vacuum issue. My engine vac is 17hg when hot, 15hg when cold. Maybe that's normal to have a difference. Try as a might, I cannot find any leaks, and have replaced several vac lines just to be sure.

This week I noticed that when the engine (not talking about the tranny now) crankcase is 1 pint overfilled with oil, the downshift problem goes away when in overdrive. But when it is 1 pint underfilled, the problem is worse. Maybe the vacuum pressure goes up with more oil?

In any case just wanted to put this new info out there to see if anyone has a thought. Thanks

[brian0605]

does the engine run ok?? 15in of vac is a little low. It really ought to be up around 20 at idle. Brian

[johnoh]

Originally Posted by brian0605

does the engine run ok?? 15in of vac is a little low. It really ought to be up around 20 at idle. Brian

Yes it runs pretty weel - could idle just a bit smoother but with 140K its nothing to complain about.

What is frsutrating is that these sympton differences could just be attributable to the change in temp - the vac of 15 is when cold and 17 is when hot, but the tranny could just need to be hot itself and its not a vac issue.

[briscoe]

Johnoh I think there is no correlation between the vacuum situation and the present trans concern, here's why.

The input clutch is applied as soon as you start the engine. It remains on in first and second gear, in manual and overdrive position. It's even on in reverse.

For engine braking it comes on in manual 3rd gear.

The rubber on the input piston and input accumulator will wear badly because of this circuts function [ busy circut ] and other reasons as well.

The modulator is for shift feel, if you disconnect the vacuum line from it, the line pressure will go up, downshifts will be harsh, [ clunks ].

Because the rubber on your input pistons is probably compromised, you can get a neutral out condition, on coast downshifts, here's why.

If your shifter is in the overdrive position, the manual valve [in the valve body ] will be in the overdrive range.

In the overdrive position, when the 2-3 shift valve moves to the upshifted position, fluid pressure TO the input clutch piston is exhausted thru the D3 passage and the manual valve, which releases the input piston [ unapplied ]

With the shifter in the 3-2 or 1 position, the input clutch piston will remain applied, because of fluid pressure in the D3 passage.

This explains why there is no neutral out situation, until you put the shifter in the overdrive position. Two choices, keep driving in manual 3, or spend a lot of money.

Please note, this is not a flippant posture I take with your trans concern, but rather a matter of fact position, given the info you have expressed in your post.

I hope the info I posted will be of some help to you, and I wish you the best of luck!

[johnoh]

Originally Posted by briscoe

Johnoh I think there is no correlation between the vacuum situation and the present trans concern, here's why.

The input clutch is applied as soon as you start the engine. It remains on in first and second gear, in manual and overdrive position. It's even on in reverse.

For engine braking it comes on in manual 3rd gear.

The rubber on the input piston and input accumulator will wear badly because of this circuts function [ busy circut ] and other reasons as well.

The modulator is for shift feel, if you disconnect the vacuum line from it, the line pressure will go up, downshifts will be harsh, [ clunks ].

Because the rubber on your input pistons is probably compromised, you can get a neutral out condition, on coast downshifts, here's why.

If your shifter is in the overdrive position, the manual valve [in the valve body ] will be in the overdrive range.

In the overdrive position, when the 2-3 shift valve moves to the upshifted position, fluid pressure TO the input clutch piston is exhausted thru the D3 passage and the manual valve, which releases the input piston [ unapplied ]

With the shifter in the 3-2 or 1 position, the input clutch piston will remain applied, because of fluid pressure in the D3 passage.

This explains why there is no neutral out situation, until you put the shifter in the overdrive position. Two choices, keep driving in manual 3, or spend a lot of money.

Please note, this is not a flippant posture I take with your trans concern, but rather a matter of fact position, given the info you have expressed in your post.

I hope the info I posted will be of some help to you, and I wish you the best of luck!

Wow thanks a lot Briscoe. That is the most plausible and thorough explanation I've heard by far, and I am inclined to go with it as the final conclusion. But I do wonder why this problem comes and goes.

The problem started when I changed the fluid 6 weeks ago - after that overdrive would fall out of gear with every coasting 2-1 downshift.

But this week it is fine 90% of the time. First thing in the morning is about the only time it falls into neutral, though once in a while it will happen during the day as well, but not much.

And two weeks ago it went an entire week without one problem in overdrive.