C4 Vacuum Modulator question

[Chiclassics]

Hey C4 experts question

I have a dual port stock vacuum modulator on my 74 which I thought was starting to go bad. My shifts are happening too early like under 1 sec.. So decided to change it. I got one from TOMS (https://tomsoffroad.com/products/ford-c4-automatic-transmission-single-outlet-vacuum-modulator)

After I changed the vacuum modulator there is really not much difference and the shifts gotten worse(flare between 2-3) with original problem still remaining.

I noticed that when I changed the vacuum modulator the transmission fluid came out pouring not dripping. and Yes I have both the modulator and pin. Thats when I started doubting it may not be the modulator itself and something inside is missing. Is it supposed to drip or pour when you change vacuum modulator. If its pouring then am I missing a part inside?

Little history this C4 was serviced by a shop in a rush as the shop was closing 8 months ago, so I wont be surprised if they missed a part or put it wrong when servicing.

 

[BronCowie]

I changed mine out, I think from a red stripe to a green stripe (long time ago). I got a pretty good fluid bath on my arm when doing this. One important thing you may have missed is adjusting the tension on the unit. I had to take a small screw driver with me on a short cruise and stop periodically to tweak the vacuum tension. As I recall, clockwise inecreases the tension for later shifting. Counterclockwise loosens the tension for earlier shifting. After a few stops and tweaks, I finally nailed it.

Disclaimer, I am not a trany expert, rather, an educated hack.

Good luck!

 

[Chiclassics]

Unfortunately I already did the clockwise adjustment upto 2 full turns with no change in response.

 

[BronCowie]

Unfortunately I already did the clockwise adjustment upto 2 full turns with no change in response.

Do you have a vacuum gauge that you can use to check what you have there? 'May be a vacuum leak somewhere in the system.

 

[Chiclassics]

I did some more digging this is what I have inside the vacuum modulator - heard its called valve or piston? the one on right is the one where my pin was going in.

I am going to check the vacuum , how much should it be?

https://imgur.com/a/3BWJ9BJ

 

[BronCowie]

I am going to check the vacuum , how much should it be?

I can't say off the top of my head. I think what I did, some years ago dealing with my VM issues, was check my vacuum at the manifold/carb port where it originates. Then compared that to my readings at the VM. There are many splices/places along the way where leaks can occur. Let us all know what you find.

Another disclaimer... I am a shadetree mechanic whannabe/hack... get most of my knowledge from friends more versed in this stuff and, of course, this site.

 

[Bung72]

I did some more digging this is what I have inside the vacuum modulator - heard its called valve or piston? the one on right is the one where my pin was going in.

I am going to check the vacuum , how much should it be?

https://imgur.com/a/3BWJ9BJ

Hi, Chiclassics. Does this help you at all?

 

[Chiclassics]

I checked my Vacuum. its at 18 at idle and when under load (D) 14. I also checked vacuum both at carb manifold and at vacuum modulator to rule out any vacuum leaks in the line. So I know its getting 14 in driving conditions

Sounds normal?

I am probably going to try another new green stripe vacuum modulator and see if that changes anything. The one I got from TOMs has a yellow dot on it if that makes any difference? I heard about green stripe and yellow stripe ones.

 

[BronCowie]

Your vacuum readings sound normal to me...