Help me understand the Vacuum to the Transmission
 

I am trying to understand how the vacuum signal to the transmission works. :dizzy: My car currently has a manual transmission and no emissions controls so my vacuum system has been quite simple, but I am planning to use the vacuum signal intended for the auto transmission to help control my VNT.

From reading the "It's CRITICAL..." thread here is my understanding:

The vacuum to the transmission is high (around 15") when the throttle is closed and decreases to zero when the throttle is wide open. The vacuum control valve on the injection pump controls this transition, and the orifice dampens the vacuum signal if the throttle position changes rapidly. Please correct me if I am wrong.

I plan to follow the attached vacuum diagram when hooking up the vacuum system (with the signal going to the turbo and not the transmission). Because I do not have any emissions controls I will simply plug the connections where the two red lines are drawn. Will this work, or does the signal to the transmission depend on those lines in some way?

Thank you.

What you must realize about the VCV is that it's a device that bleeds air. If the consumer, downstream of the VCV, is not properly matched to the capability of the VCV, your results will be markedly different than the vacuum that normally goes to the transmission.

Additionally, the flow restrictor is critical to allow the VCV a proper flow rate. Without a restrictor, the VCV cannot bleed sufficient air and the vacuum will be exceptionally high.

Some VCV's are adjustable in terms of the starting point at idle. You can drop the setpoint from 15" down to 6" if you so desire.

What "receives” the vacuum signal in the transmission? Some sort of diaphragm? The actuator on the turbo is a 3-inch diaphragm with a travel of less than one inch, so there is not much of a volume change. I know I will have to play with the different orifices and the vacuum control valve so that I get 15” or so at low throttle and 0” at WOT to the turbo, but it should be doable I think.

Yep, with some experimentation of orifice size, you can get what you need.

I suspect my VCV is bad. I hooked it up tonight and get 12" or so at the signal line (normally to the transmission) with the VCV hooked up at idle and 0" at WOT. So far so good, but when I shut off the car it took about 5 seconds to shut down which it did not used to do. I then replaced the VCV with a plug and was able to get 22" at the signal line and normal shutoff behavior. I tried playing with the adjustment nut under the cap but was never able to get higher than 12" at idle, only lower. For controlling the VNT the more vacuum at idle the better to lower the boost pressure when cruising.

Any ideas? :confused:

If your VCV holds 12" for a reasonable length of time, AND it bleeds down to zero -- it works as intended.

Its also adjustable.

I just spent a month studying, experimenting, and tuning my 722.117 on a 616. <No bowden cable, no EGR, no lock circuits, no climate circuits>.

Here are a few observations:
1) the "control" loop for the tranny, in which the governor diaphragm pulls a rod that controls the shifting pressure and ITS TIME OF CHANGE ACTUATION, can be "improved" or "destroyed" by adjusting the diaphragm stops; adjusting the VCV valve; or re-matching of the "snubber" orifice AND/OR "parallel configuration" that controls how quickly the vacuum ramps from setting to setting, i.e. WOT to idle.

2) IF you "use" too much vacuum on the transmission circuit; by improper setting of the VCV "range", OR poor selection of snubber; you can "drain" vacuum needed to pull the rack for shutoff. Hence, poor shutoff performance.
ALL vacuum switching is done by "controlled leakage", after all. Make one leak big enough, something else has to give.

3) As the system <vacuum pump, tranny, VCV,climate, locks> wears, the settings used for "new" REQUIRE some fudging. As well, if the system pressure on the tranny has been modified -- which is very easy to do -- all settings might need to be recalibrated!

4) The VCV "head", which is the difference between its idle and WOT settings that worked best for MY state of wear and tranny system pressure, was 8". The gave me extended, aggressive shift points at high throttle settings, while still invoking short shifts that have no overrun at low throttle. Shutoff is perfect. Its truly a mercedes engineering marvel!

5) During the messing about with orifices, vcv settings, and vacuum configuration <which lines used on which 'main' orifices, and in what parallel configurations> I managed to INDUCE FOUR "major" system problems; they are easily RE-INDUCED by choosing a configuration that is poorly optimized:
a) poor to NO shutoff; IOW, the IP wasn't getting enough vacuum to "close" the rack!
b) NO shifting past second gear!
c) 2-3 and 3-4 overrun on shifting
d) Shift "THUNKING" -- shifts so hard that the entire driveline would shock, even at speed; even with brand new motor mounts!

As always, YMMV. I concur, now that I've been through the "learning curve"; some time spent with a full set of orifices, a mightyVac, and some time to think about/play with "how vacuum works" -- especially where the use of the "rise time" -- or ramp -- from one vacuum level to another is REQUIRED to get best performance out of the system. And, was truly worth the wait.

I would expect, in trying to modify the system to work with a different diaphragm, and where the "system load" is different because of your special configuration; that the diagram you have posted is more or less, USELESS -- except as a starting point.

You are essentially choosing to design your own custom vacuum loops, ramps, and set of interactions ANYTIME you change configurations! Some, like EGR removal, can be done with little change variance -- on some years' configurations.

THIS is why people are sometimes confused about "controlling the transmission through the EGR loop." Removal alters the system "ramp" characteristics in a subtle manner; which, depending on wear, system pressure, optional equipment -- can make a difference, or might make no difference whatsoever.

Have some fun with it. I did. I can't wait to hack my spare transmission into a second vehicle....

--frankb

Thanks for the info. My VCV will hold a steady 12" at idle if I have the adjustment Maxed out, but will not hold any vacuum if I hook my mity-vac to it and pump away. So far I have not even hooked it up to the turbo, I have only hooked my vacuum gauge to the signal port. I think the controlled leak in mine may be out of control. :D I will mess with a second one soon.

Thats the way mine is working. My vacuum system would make 15" before I put the VCV in the system.

Does your car turn off?

There is about a 1/2 second delay AND my shutoff actuator leaks but yes, it does shut off.

for the vac. on my sd's tranny I have one line from the main vacuum pipe to the vac. modulator, then one line from that to the tranny. No dampener, and the tranny shifts perfect. I guell ill try it with the dampener from my other car to see what it does.