C4 Trans Modulator / 2 Vacuum Lines


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Heavy Metal

Well-known member
May 22, 2018
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My Car
1973 Mustang Convertible
302 C4


Looking for alittle help on my C4 Trans Modulator. It has 2 vacuum lines and I need to adjust the shifting points.
The engine has been highly modified, so just using vacuum for carb, power brakes and ignition advance on distributor.
Can I go to a single line Modulator and adjust it thru the hose port?

Thank you,

Heavy Metal
Florence, SC
What is your transmission doing? Shifting early, shifting late, shifting hard, soft? Shifting hard or soft at full throttle, hard or soft at light throttle?

Not all of the modulators are adjustable, and even if they are you likely need to get one that operates at a lower vacuum if you have decent sized to aggressive sized camshaft.
Thank you, and it did help. Looks like the 2 vac lines, 1 is for the EGR that is not used in my case. Still looking how to adjust the unit.
Usually with a small thin flat screwdriver. Turn it in to shift later, out to shift sooner. However, the modulator engages a small diameter "pin". If it is not there it will never shift properly. The length is also within a narrow range on length. A very faded memory want to say about 1.625". Please verify before proceeding. Chuck

EDIT: I found this: https://7173mustangs.com/threads/va...modulator valve,shorter pin does the opposite.
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There have been excellents posts in this thread already. I have little or nothing more to add that would be of any value. Many thanks for the links provided for more info. I am now beginning to fill out that part of my little library of into.
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