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Discussion Starter · #1 · (Edited)
Having hard shifts from 1st to 2nd, enough to jerk you back and forth... Seeing other post about testing the vacuum modulator, unplugged the hose at the modulator to test the pressure and transmission fluid came out.
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Also read the fluid is sucked through the engine, negatively affecting engine performance... How so ? Can someone elaborate how it negatively affects engine performance ?

Also read it could be the cause of my hard shifting issue...

I bought a new adjustable modulator, but has to be ordered to the parts store...

Liquid Camera accessory Font Fluid Cameras & optics
 

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Oil reduces fuel octane and has chemistry that when burnt leaves gums and ash behind. Essentially you risk detonation and preignition as well as building deposits on the valves, combustion chamber, and piston crown while the gums goo up the rings.

This can get ugly really fast.

Bogie
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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You read correctly that the vacuum modulator failure will cause a hard shift. The modulator receives a vacuum signal from the intake manifold (in most cases). It tells the transmission how much load is being placed on it. The pressure is increased in low vacuum conditions, i.e. full throttle. The diaphragm inside that modulator is obviously leaking and is not communicating the proper vacuum signal. Your shifts will be hard and most likely later than usual at part throttle from the input of the governor (under that round cover on the other side of the transmission, measuring "road speed"). A new unit will have it back to "normal." The red stripe modulator is adjustable. It should come with basic instruction regarding adjustment.
 

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Discussion Starter · #5 ·
You read correctly that the vacuum modulator failure will cause a hard shift. The modulator receives a vacuum signal from the intake manifold (in most cases). It tells the transmission how much load is being placed on it. The pressure is increased in low vacuum conditions, i.e. full throttle. The diaphragm inside that modulator is obviously leaking and is not communicating the proper vacuum signal. Your shifts will be hard and most likely later than usual at part throttle from the input of the governor (under that round cover on the other side of the transmission, measuring "road speed"). A new unit will have it back to "normal." The red stripe modulator is adjustable. It should come with basic instruction regarding adjustment.
Didn't come with adjustment instructions...

How far counter clockwise can I go ?

I'm already 2 turns out, and slightly see better results... Easier and quicker shifts, but I don't want to go out too much...
 

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Discussion Starter · #9 ·
I'd seat the adjustment screw, counts the turns to fully unseated, dived that by two and run it back in that many turns. Then adjust to suit from there.
I just turned it clockwise 1 more full turn, making it 3 full turns counter clockwise...

Had to do a rocker valve adjustment real quick to take out a lifter tick before I took it for another test drive to test the shift points... putting valve cover back on now and then I'll go test the shift points 3 full turns counter clockwise.
 

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Discussion Starter · #10 ·
I just did a filter, gasket, and fluid change about 2 months ago... So I know the first gear issue isn't any of those, and possibly be the vacuum modulator issue due to the findings so far...

I also read about "governors" are the main source of shift points and are also easy to check and replace...
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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Yes, they are. I'm not a transmission guy outside of my own vehicles but I have never had a governor fail. Folks mention failures but, I have either been very lucky or they aren't problematic very often.
 

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Discussion Starter · #15 ·
You wouldn’t. But it must have if the modulator isn’t doing its job. Governor controls when it shifts, not shift quality so don’t waste time playing with it.
So if I adjust the modulator both ways (in and out) and get no results, then there must be a shift kit installed ?

With in gear RPM @ 1400-1500 RPM, 1st to 2nd gear is slow and slightly rough acceleration and bucks when it shifts into 2nd gear @ 4000-4500 RPM
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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OK. First thing. Do you know where you are in your vacuum modulator adjustment? The last remark I saw was that you were at 3 full turns counterclockwise. I don't have a large sample size as I stick to my own transmissions and an occasional friend's car or truck here and there. I can't recall ever having settled on an adjustment that was counterclockwise to the factory preset. In fact, some of the builders I know give it a full turn clockwise before it even goes in the car. If you know where you are, return it to the factory preset or even one full turn clockwise and go from there. If all of your adjustment is happening in an overall counterclockwise from the factory preset, it could explain the lack of results with it.

Second. I can assume from your last post that the kickdown/detent linkage is connected. (That detent can be modified to stay in a mostly depressed position, but let us go with the far more likely scenario where it is connected) The vacuum modulator can give you a softer/earlier shift at part throttle, if it is adjusted that way. However, the detent linkage will override the modulator when you have a more aggressive throttle position. Don't get caught up in this idea just yet. One thing at a time. I was just making an example of what can happen.

Lastly. Sometimes there are a couple of things you can see with the pan off when a shift kit is likely to have been installed. The detent cable modification would strongly suggest a shift kit install. If you see a doubled up steel plate in place forward of the valve body, this is pretty much a guarantee. It allows the direct clutch (3rd gear) to be "dual fed" to improve the holding power in 3rd gear. It's a great modification. There are others. I just think this is what I have seen the most.

Some of this may seem like foreign language now. It won't always be that way. The last 2 points are that we aren't out of answers yet. Stick with it. You can and will get this figured out!!!

I am a "back yard" mechanic and by no means an actual transmission guy. There are A LOT of folks on this site that are experts. They can feel free to comment or correct anything I said. We are all in this together. We are all still learning. Please, keep us posted on what you're doing and how the car is responding.
 
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