C4/C6 Vacuum modulators

[c9zx]

I have wasted a lot of time trying to ACCURATELY determine what the different color stripes on modulators really means. After wasting a lot of time searching the misinformation internet I have no definitive answer to that question. Is it diaphragm size, spring pressure, adjustment position from the manufacturer, pin length to be used with, or just a cruel joke to take people like me down a deep rabbit hole. Any and all input will be welcomed. Thanks, Chuck

 

[Big Red Mach 1]

I've heard all of those same things with no difinitive answer either. Car came with the purple one, To me was a bit "lazy" when shifting no matter how I adjusted it. Read some information/mis-information (who really knows) and tried the black one. Was better, but not great. Have the green one now and it shifts just like I'd like it to. Pretty sure the screw is run all the way in or dang near. But I do get nice crisp shifts and can chirp them nicely on the 2-3 shift. One thing not mentioned is the vacuum the car has. I'm 11 on a good day. Does that play into the formula of which one to pick? Maybe.

 

[Kilgon]

Attached is a guide for what one you should use in your tranny. As for what will work best is another question. For low vacuum, you will want one that has a bigger diaphragm. Spring pressure also comes into play as far as setting the shifting point and firmness. Attached is a chart from Rostra on their vacuum modulators. If you look at the C6 for example, you will that the black stripe listed for a 72 and up is the same as the purple for a 75 and up. Some have the same spring pressure and diaphragm size but different color stripe. You do need to note if it is a screw in or push in. Make sure you read the technical data at bottom of the last page. You can also call them with any questions you might have.

 

[c9zx]

Kilgon and Big Red, Thank you for the replies. The charts are very helpful. I'll be calling the tech support number today. It may just be a trial and error exercise as the engine and trans are far from stock. It may also be a fluid temp based issue as there is large trans cooler without a thermostat for the cooler. My knowledge of auto transmissions is limited since I've been a 3 pedal guy since 1971. Thanks, Chuck

 

[basstrix]

In my testing, on an engine with 11-13" vacuum, I found black stripe to shift the latest, then green. I believe I also tested a magenta stripe, but can't say for certain...whatever color it was, it shifted sooner than the green or black. None of them made a dramatic difference.

 

[c9zx]

Basstrix thanks for the live testing information. Also about 11 in/hg here. The issue I'm looking into is if I set the modulator to shift properly when cold, it shifts too soon when warm. If set to shift properly when warm, it doesn't want to upshift into 3rd gear until it warms up a bit (about 2.5 miles). I believe the modulator has a green stripe. I assume the pin length is right, or close to it, because it will upshift. Thanks, Chuck

 

[Big Red Mach 1]

Larger diaphram (green 1.7) with low vac. seems to jive what I only found through trial and error. Interesting that Black worked better for Basstrix. Like everything, each car, especially modfied, will all probably like something a little different. I never had one shift differently at different temps. Low fluid level is the only thing I've ever dealt with that ever caused anything like that. Mine has been a bit picky on fluid levels. A little too low and won't shift cold. A bit too much and it pukes it out the dipstick tube when hot. And this is the only car I've ever had where I just cannot get a decent reading that I trust on the dipstick.

 

[basstrix]

Basstrix thanks for the live testing information. Also about 11 in/hg here. The issue I'm looking into is if I set the modulator to shift properly when cold, it shifts too soon when warm. If set to shift properly when warm, it doesn't want to upshift into 3rd gear until it warms up a bit (about 2.5 miles). I believe the modulator has a green stripe. I assume the pin length is right, or close to it, because it will upshift. Thanks, Chuck

Mine shifts later when cold vs hot. I actually prefer the cold shift behavior. The trans likely needs a rebuild. My research indicates the hot/cold difference may be due to worn governor piston/bore and/or springs.

 

[c9zx]

Basstrix, Thanks for the information about the governor, I hadn't thought about that. Chuck

 

[Bentworker]

I had to buy a few of the modulators for my FMX. I bench tested them all with a mighty vac hand pump and logged what vacuum they started to move, and what vacuum was required for full travel. Just drop a pin in the end with a pen mark on it. don’t recall what color I went with, but I put in the “lightest” one I could buy (lowest vacuum needed to move). I have a fairly healthy Cleveland so it was the right choice to match my cam.

A mighty vac is an inexpensive tool and if you have one you’ll find other uses for it.

Somewhere in the forum is a ford tech book I uploaded about valve bodies, modulators and governors of FMX, C4 and C6 platforms. (Found it - see below)

In my experience the modulator alters the shift firmness from light throttle to full throttle. You don’t want it bang shifting under light throttle or when coasting to a stop.

The Governor changes WOT shift point.


https://7173mustangs.com/threads/fmx-valve-body-info.39371/
https://7173mustangs.com/threads/general-automatic-transmission-valve-body-info-c4-c6.39375/

 

[c9zx]

I had to buy a few of the modulators for my FMX. I bench tested them all with a mighty vac hand pump and logged what vacuum they started to move, and what vacuum was required for full travel. Just drop a pin in the end with a pen mark on it. don’t recall what color I went with, but I put in the “lightest” one I could buy (lowest vacuum needed to move). I have a fairly healthy Cleveland so it was the right choice to match my cam.

A mighty vac is an inexpensive tool and if you have one you’ll find other uses for it.

Somewhere in the forum is a ford tech book I uploaded about valve bodies, modulators and governors of FMX, C4 and C6 platforms. (Found it - see below)

In my experience the modulator alters the shift firmness from light throttle to full throttle. You don’t want it bang shifting under light throttle or when coasting to a stop.

The Governor changes WOT shift point.


https://7173mustangs.com/threads/fmx-valve-body-info.39371/
https://7173mustangs.com/threads/general-automatic-transmission-valve-body-info-c4-c6.39375/

Thanks for posting the information. And yes, the mighty vac has served me well for a LONG time. Chuck