Hello all, I'm looking for information regarding this "vacuum box" (see picture) which is connected to my Oldsmobile TH350. There is a screw and it seems that this one allows to adjust the gear ratio or something like that. If some of you have explanations... Thanks !
It is the vacuum modulator. That one is really old, and might be leaking? look to see if transmission fluid is present in the vacuum fitting. Do not adjust the screw. The modern replacements are much smaller, and have a screw inside the vacuum fitting hole. Usually no adjustment is needed. What it does, is tell the transmission how much load there is on the engine, so it can shift at the proper time. Open throttle, means more load, less vacuum, later shifts.
You have an old style vacuum modulator which is altitude compensated. There is an aneroid bellows inside that expands and contracts with changes in atmospheric pressure. While it has an adjustment, the factory adjustment works the best overall.
That vacuum port on the top should be connected to manifold vacuum. They originally had a metal tube for most of the length, just rubber at each end. Make sure if you do use a rubber hose over in France, it's not kinked or subject to heat. Now that you know the name, there are posts here about adjusting it, but that's like @squirrel said, don't mess with it for the street. Even for racing, it should be changed only if you understand what this can mean as far as messing things up.
My engine is from '66, so this modulator is 58 years old, not yet retired in France ! However, I saw new ones in Rockauto website and I think I'm going to buy one ! I thought to adjust it because it is from a Cutlass and goes to a 32 roadster, not really the same weight to move ! Thanks guys !
How long has it been since the transmission was last driven in a car? I would expect it to leak pretty soon, they usually lasted about 10-20 years when they were new.
The modulator controls transmission working pressure and shift points. It doesn't have anything to do with vehicle weight. However, shift points are determined by a balance between modulator pressure and governor pressure. If you look at 350 transmission specs you will find that the working pressure is the same regardless of the size of vehicle the transmission is in. GM did use different governors in different applications to tailor shift points to different vehicles. My guess is that the stock modulator and governor will work perfectly well in your application.
The transmission is not from 1966, since it's a TH350, and they were introduced in the 1969 model year. But the modulator is pretty old, GM quit making that style around the late 1970s, as I recall.
Yes, sorry, my engine is 66, the transmission is from 73 ! Many many years, I don’t know exactly ! I suppose it will not work well because of its age and a lot of years without moving !
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