With my 69 delta 88 i can force downshifts.
By using the gaspedal,
Corect me if i'm wrong becouse i do not have the ownersmanual beside me to assist.

It works under 50 m/h with half throttle and faster with full throttle.
And is forced by vacuum to shift down a gear.

Now i'm not confinced if it workes how it suposed to be, it sometimes work and sometimes don't.
And it is hard to learn that point where it does shift.

And if a friend will overtake me i'm missing that fast downshifts as you would with a manual gearbox
And my car is the biggest and more in weight as my friends have.

I do not have a real agressive cam that limits the vacuum and checked for leaks.
So is there more to look at or adjust?

I read also that it should have a downshift thing close to the gas pedal?
So how does it that vacuum forced shift work and what are it components

 

You can adjust the downshift switch on the throttle linkage, if it's a T-400.
If a T-350, there's an adjustable modulator available - tailor to your liking!
With both, you can manually shift to a lower gear!

 

You don't state what trans you have. Without getting into the internals, which I think is outside the scope of your question, this should give you what you are after. If the trans is TH-350 then the kickdown is entirely controlled with a cable that goes from the throttle linkage to the trans. If the trans is a TH-400 then the "partial throttle kickdown" is vacuum controlled with a vacuum modulator located on the trans. This should have manifold vacuum to it. The full throttle kickdown is controlled via a solenoid in the trans - this is what the full throttle switch is for. The switch can be located at the carb or at the gas pedal. You can adjust some modulators with an allen wrench through the vacuum opening - this will adjust the vacuum level at which a part throttle downshift occurs. BTW - a lack of vacuum would lead to sooner downshift or held off upshifts. I suspect from your description you have a TH400 and the full throttle kickdown is not functioning.

 

It is a OF type automatic TH400.
This is the original gearbox a TH400 original mounted behind a 350 cid engine.
But i did put a 455 bigblock in it.
It works better with the 455 as it did with the 350

And the downshift does work, but not when i want it to kick in.
And i'm messing around with the position of the gaspedal to force a downshift.
In mean time i will be over taken by a other car,

The thing is, it works with half part throttle below 50m/h and works at full throttle faster than 50m/h. ( or 70m/h shall look at the ownersmanual tomorow)

I noticed the vacuum canister on the gearbox that is coupled with a vacuum hose to the carb.
could it be the sealing off the canister? and can i just bolt it of and on with a new sealing ring?

 

I am not sure if I was clear so I want to re-iterate (please don't take offense to this). The vacuum modulator will cause downshifts, however it is intended to base the downshift on a difference in hydraulic pressure within the trans. One hydraulic pressure is determined by the governor speed (vehicle speed dependant) and the other pressure is determined with the position of the valve which is shuttled with the vacuum modulator. This is why it requires more pedal to make the downshift occur at higher vehicle speed.

The electrical kickdown is a full throttle kickdown and is intended to force the trans into the lowest gear that it can safely obtain at a given speed (also governor speed dependent but much faster acting than the vacuum modulator and with higher priority than the vacuum modulator).

Check to ensure that the electrical full throttle kickdown is functional. From your description the vacuum modulator is working properly. There should be an electrical connector on the left side of the trans with one terminal. 12V applied to this should cause a downshift.

If I am not understanding your question properly, please feel free to point it out.

P.S. the O-ring on the modulator keeps trans fluid from leaking out. If the diaphram inside the modulator is bad (about the only place you could be leaking vacuum), the inside of the vacuum hose will be wet when removed from the modulator. In addition, the lack of vacuum is what causes the downshift - so a vacuum leak should cause a faster downshift and hold off the upshifts a little longer. To prove this you can just remove the vacuum line, plug it and drive the car a little. your upshifts hsould be delayed and very harsh compared to normal (unless the trans has been modified for a firm shift).

 

In addition some vacuum modulator adjustments are for a small flat tipped screwdiver, also located in the vacuum port fo the modulator.

 

Thanks,

as soon as my garagelift is free again i gonna check the 12 volt wire at first.
I asume it does work if set on contacdt with pedal fully pressed.
Without starting my car.

 

Also, as has been mentioned, don't forget that you can just slam the TH400 down into whatever gear you want whenever you want to. It will not shift into a gear that will over-rev the engine, but will shift down immediately, no matter what position the gas pedal is in. Back in the good old days, we used to drive 10,000 lb ambulances around NYC with 454s and TH400s - we shifted them constantly, and under all conditions, and they never hiccuped.

- Eric

 

hahahahahahah. MDchanic is correct and reminded me of the first time I experienced this. I was a teenager and reading some light literature - a chiltons manual (I think) on Automatic transmissions - more specifically the TH400 as that is what I had just converted my 71 GMC truck to. It said pretty much that same statement except it was talking about downshifting for a declining grade and that you could put it in first and it would downshift sequentially as the vehicle speed slowed. So.....being a teenager I thought "what better way to check this than to do it :-)"

Long story short - I let off the gas and put the shifter into 1 at about 60 mph while coming down a local pass to see what would happen. I had my seatbelt on (as always) and did it when I was the only one on the road so if things went wrong it would only be my dumb self involved. It did not downshift into first until it had slowed down "enough" (it went into 2 immediately), which was still a little high in the revs for me but nowhere near popping the engine. I nearly had to clean my drawers and pull my face from the windshield, but the bottom line is that the manual is correct - the governor will prevent the trans from going into too low of a gear (assuming all works correctly :-)).

To answer D Appledorn: ignition on and engine off should be all you need to check for 12V at the trans. It should be 12V at wide open throttle and 0V at all other times.