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stevebfl · #6 03-18-2007, 12:54 PM

I have a hard time deciding whether a real knowledge of vacuum will be of any help to your quest. It is more likely that just accepting a couple issues as fact and proceeding to suggested solutions might give greater results.

But, I always liked knowing what was going on, so let me give you a little basic knowledge. Vacuum occurs in a gas motor (some gas motors, it doesn't exactly occur in my BMW 545 and I'll leave it up to the group to explain why) because the pistons are moving down and air pressure is trying to fill the space behind. This can get complicated if one uses the absolute pressure description. I'll just stay with the atmosperic modle that includes concept such as sucking.

So the piston goes down and sucks air in. If one wishes to control the speed of the motor one restricts the flow of air into it. This is absolutely different in a diesel and may be causing this quantity of vacuum concept in your mind.

Idle speed is the lowest controlled speed. At that speed the throttle is basically closed, the piston's suck is greatest and thus one gets the highest vacuum. Actually one can achieve a higher vacuum by raising the engine speed to about 2000rpms and holding it. Anyway as the throttle opens the vacuum goes down. It would help here to use the absolute pressure description but dream of it as less suck because the restriction (throttle is being removed.

To simplify this now think about full throttle. There is no difference in the manifold or the atmosphere so this is no vacuum. Maybe an example would be better. Think of a straw with a hole in the side. Suck! No fluid can be lifted. Put a finger over the hole and gradually cover the hole. If done slowly one can actually view the difference in vacuum quantity created by watching how high the fluid rises. If you want real numbers try placing a 30 foot straw into a 30 foot deep well. Nothing but a perfect vacuum will rise the water.

Anyway, the engineers understand that vacuum is a function of throttle position and load. Thus it is used through a vacuum modulator to modulate the system pressure of the fluid used to apply clutches and bands in the trans. (It is more complicated than that but lets just leave it at that).

So, by changing the shift point with a cable adjustment one now has the shifting they expect happening at a different throttle position (or different vacuum position). This isn't exactly right or wrong! But if results cause issues as is the case for you, then one must modify the conditions to solve the symptoms. One other point is that usually this all occurs because there is wear in the trans of either seals or clutches that changes the shift timing cushion (an even longer story, I'm not getting into).

So, the solution is to get the shift to occur before you let off the gas (increasing vacuum) or to increase the pressure the vacuum causes (modulator adjustment. Since the logic on your story is that the cable has a different adjustment I would start there.

Make the car shift earlier!