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Old 01-26-2009, 07:17 PM   #1 (permalink)
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Cruise control Mod?

I was reading about vacuum operated CC systems and found this.

Cruise Control Basics

" it sends pulses to a vacuum controlled diaphragm connected to the accelerator linkage. The pulses it sends regulates the amount of vacuum the diaphragm receives. The more pulses, the more vacuum and the more vacuum the more force on the accelerator linkage. The system continues to add vacuum force until the set point speed is reached. At that point the system modulates the amount of vacuum the diaphragm receives in an effort to maintain the number of pulses coming from the speed sensor as close to the stored value as possible."


That sounds like the CC 'brain' is turning an electrically controlled valve in the vacuum line on and off, to keep the throttle cable tension just right..

What if, I cut the vacuum hose in half and installed a Joiner,

But, the hole inside the Joiner would be very tiny.
It would greatly restrict the air flow.

Would this vacuum 'restrictor' slow down the throttle function of the CC??
Keep it from instantly using more fuel, the instant you drive up any little hill in the road..?..

Or, would it just make the CC Brain go completely insane?

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Old 01-26-2009, 09:31 PM   #2 (permalink)
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You might be on to something there.

Anyone with vacuum-based CC want to try it out? It makes intuitive sense, so unless an engineer wants to chime in, I would be interested to hear what happens if someone tries this idea out.
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Old 01-26-2009, 10:35 PM   #3 (permalink)
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It wouldn't do much of anything... you're not dealing with airflow, you're dealing with a line that has no air in it. The vacuum doesn't "travel" anywhere.

If you want to test the theory, remove any vacuum line on your car, and test how long it takes for vacuum in the line to change with throttle differences. Remove a smaller line, then a larger line, and you'll see the same result. Statistical variances occur, but not enough to create any real data changes.

Since the diaphragm (and the throttle linkage) have springs in them, which create a certain tension, the vacuum diaphragm must have enough vacuum to create more pull than the equivalent of both springs. Since the entire system is already under vacuum, it doesn't require moving air, therefore restriction doesn't apply. (Except restriction that would not allow flow at all... a different story altogether.)

With that said, maybe you should try it anyhow... maybe I'm wrong! It's happened before.. better yet, there may be something I'm not taking into account. Even more likely to happen.

I did manage to find out yesterday that if you stick the brake booster hose into a metal SeaFoam can, it will instantly crush the can's sides. Which means that as a vacuum pump, your engine is pretty strong.
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Old 01-26-2009, 11:24 PM   #4 (permalink)
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How does the CC decelerate (slow down, like when you are going downhill)?

It almost seems like it would have to release some of the vacuum inside the
diaphragm, so pull on the throttle is relaxed a little.
So, do they let a little air into the diaphragm chamber?

Or, does pulsing the line with the electrically controlled valve with less vacuum behind it relax the diaphragm?
(I guess I need to see a video of a vac-CC in action)..

--------
I think my logic might be faulty. Since it's a feed-back system, the pulse rate will most likely be increased by the Brain, if the VSS pulses don't start changing quickly enough.?.

I'm starting to think the best way to Mod the CC is to automate the inputs to the manual up-down speed buttons.

Monitor the TPS voltage and when it is increased by the CC, pulse the Down button line into the CC..
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Last edited by Xringer; 01-26-2009 at 11:38 PM.. Reason: Did more Reasoning.
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Old 01-26-2009, 11:38 PM   #5 (permalink)
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Yeah it lets a little air into the diaphragm... but it's not how you're thinking.

If it's under vacuum, and you let some air into it, it's just under less vacuum.

In order for airflow to affect the situation, it would have to be under no vacuum.

Remember, there is a spring that applies tension to the throttle cable.

Lets say that spring is 3lbs of force (twisting). There must be enough vacuum maintained to allow the diaphragm to defeat that 3lbs of force, before any throttle actuation occurs.

How does this affect the outcome? The diaphragm can still be under vacuum, but not have enough pull via that vacuum to actuate the throttle. To ensure this, there is another spring (a "check spring") in the diaphragm, which further increases the energy necessary to actuate the throttle.

Now, what if your diaphragm has only enough kinetic energy (due to vacuum) to pull 2lbs? You're going nowhere. Up the vacuum a little more, so that it can pull 4 lbs, and now you're getting somewhere... you've opened the throttle (but just barely.. remember, springs aren't linear).

Now lets say the "brain" of the CC realizes that you're in an overspeed condition:
Release 2 lbs worth of your vacuum, and you're back to the 2lb pull that isn't doing anything.

The "check spring" I mentioned earlier is there to ensure that the diaphragm incurs more work than your foot does, so that response and pedal feel isn't changed while the CC is active. It makes it so that the diaphragm (which maintains vacuum while it's active) has to pull the throttle harder than your foot would. The diaphragm maintains enough kinetic energy via vacuum to slightly depress the "check spring", but not apply pressure on the cam spring for the throttle. This means that your foot feels exactly the same pressure regardless of the active state of the CC system.
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Old 01-26-2009, 11:48 PM   #6 (permalink)
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I see what you mean about the springs.
And, I see a "Vent" solenoid too. Must be were a bit of air can be let into the chamber.?.
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Old 01-27-2009, 12:02 AM   #7 (permalink)
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Yessir.

Now, given that I've spewed all this information over your idea.. it can only cost a dollar or so to try it out. So try it out! Once again, there might be something I'm not taking into account, and I'm far from an expert, so you shouldn't just take my word for it.

I do recommend that you check the voltage on that vacuum pump and vent solenoid during their operation and resting periods though... you might be able to modify the voltage so that there is less vacuum modulation during deceleration. (Make the vent solenoid open further to allow less vacuum to be applied to the diaphragm, or limit the voltage to the vacuum pump itself.)
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Old 01-27-2009, 02:47 PM   #8 (permalink)
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Ok Χριστός

I have some 1/2" Delrin rod, small drill bits and a Chinese mini-lathe.
So all I need to do is find out what size to make the Joiner and get to work.

Maybe I'll do it during the next snow storm (starting tomorrow).

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