Technical Too low of vacumn to modulator valve ?

Three C-4s in my Sunliner and all three have third gear issues,just installed a used trans this weekend and its not shifting right. I could have gotten a bad one but this one acts like the modulator valve is not hooked up and has delayed and harsh shifts until its driven some and then its still not right,disconnect line at trans and the idles does not change much but disconnect it at the engine and its very noticable. The only part used on all three trans is the line and hoses to the vacumn advance,used 1/4 in brake line and thinking stepping it up to 5/16 and see what happens.

 

Delayed and harsh shifts, low vacuum at the modulator would be about 95% of what I would be looking for. Try a vacuum gauge at the engine then try it again at the modulator hose end. You should have at least 15in of vacuum to make it shift correctly. If vacuum is less at the bottom run a rubber hose from a good vacuum source to the modulator for a temporary test. Quick and easy. Also, if you have a downshift linkage you could disconnect it and drive it without it to see if that makes a difference. A C4 does not need the linkage to the trans to make it upshift correctly, don't know but if it was stuck in the downshift position would make the trans shift very late.

 

Make sure you have adequate volume of vacuum. These were usually connected to a manifold source, some carb connections don't offer enough volume. Make sure the hose isn't collapsing also; not all hose is suitable for vacuum.

 

"The only part used on all three trans is the line and hoses to the vacumn advance,"
Maybe just a typo but what does the vacuum advance have to do with it?

 

Your description points to a vacuum line problem. The fact that it makes a difference if you disconnect it at the manifold, but not at the modulator points to a problem in the hose. Disconnecting it at the modulator should make the same difference as when you disconnect it at the manifold. Fix that and you should fix the shifting problem.

 

Oversizing the line will do nothing for you except maybe make it more difficult to connect at the ends without adaptors

 

As others have mentioned, if disconnecting the vac line at the modulator doesn't affect the way the engine is running then the line going up to the engine is probably clogged or crushed. Temporarily bypass the line with a rubber line and retest.

Verify the modulator is getting MANIFOLD vacuum. Ideally it should have it's own source, usually connected to the vac tree on the back of the intake. Modulator itself is also adjustable for fine tuning inside the vac nipple. Also there are slightly different types, look for any color bands painted on it.
Combined with the governor these two devices fine tune and dictate shift points.

Check the kickdown linkage, it should be free to move. Verify the kickdown lever on the side of the transmission itself is free to move, it is spring loaded and should freely return to at rest position.
For normal driving and testing it is not really needed. It's more of a convenience device.

 

Its getting vacumn from all 8 cylinders from the carb spacer,this engine has never idled as smooth as I thought it should since day one and the vacumn line to the modulator has been on it from day one. It also has a Lokar cable kick down that I never really liked so I will disconnect it too.

 

A fluid suction line is not an air vacuum line. different dynamics..

 

Sounds like there’s a problem with the steel line like mentioned above. If you originally made it using a tubing cutter sometimes the cut metal blocks the opening. A knife blade can check for clear. I use a 3/16” line and all works great. A 1/8” line is on a powerglide and they are good too. I use only a short piece of rubber tubing on each end no more than 2” and all bends are sweeping or made with a tubing bender.
It would be a pain but buy 2-3’ of new vacuum hose and eliminate the metal tubing for a test. Good luck.

 

Probably will be tomorrows project,first I will put a gauge on it at the trans and see how much its getting and then on the engine. It usually was getting close to 18 but that was a few years ago since I last checked,I am going to be pissed if that was the problem the whole time and the tubing was new when I put the car togather and its not like it gets driven in the salt.

 

I’ve had factory lines that looked good have a hidden bent place from a rock or something hitting them and almost closing them off.

 

Will find out tomorrow after work.

 

Here’s a question for you guys about location of the vacuum supply for the C4.

I have a fitting that goes into the manifold right between the front 2 Strombergs on my Edelbrock 573.

One side is 1/2” for the PCV , the other side is 3/8”.

I have the PCV hose attached to that location.

Is it better to have a different source for the modulator vacuum or will the source shared with the PCV be adequate?

 

17 inches of vacumn at the modulator valve and hooked the vacumn pump on the line at the carb with the modulator hooked up and no major leak so it looks like I got a iffy trans,it looks like the old trans is going to get rebuilt again.

 

Have it upgraded with some beefier parts rather than a straight rebuild. This why Ford didn't use the C4 behind the bigger motors...

 

What all needs to be upgraded,I do not know if I can find anyone in my area who would know what to do.

 

Use a 1970 and up trans.It has a larger input shaft.you can get extra clutches in the high drum,use a (C) servo for the intermediate band and a shift kit.Unless your racing U should b good.

 

They should have their own source

Sent from my SM-G973U using The H.A.M.B. mobile app

 

Not sure if this relates to your issue or not, but years back, I had some shifting issues with the C4 in my 47, I adjusted the length on the rod that the modulator reacts on. Friend at a transmission shop, told me about it. Can't remember what the length was supposed to be, but I remember trimming a slight amount off, and it shifted better. Sorry, I can't add more; it's been 25 years or so.

 

I’ve had to adjust rod length more than once when I was building a lot of C4 trannys .I never modified the original rod.I always used a piece of rod the same diameter and made a new one.Shorter for earlier shifts.Longer for later shifts.Don’t go crazy .060 will make a difference if the trans is in good shape.

 

I am going to adjust the modulator valve and see if I can get it working better and found someone who will guide me through the rebuild of the trans I just took out that I got hot on a trip.

 

So Where would you suggest the vacuum source be taken from given that there are no vacuum ports on the Strombergs?

 

Place a spacer with vacuum ports on each or one carburater. I still have the one I bought for my Stromberg.
Or you can get spacer without a vacuum port and tap it.

 

Yes, now I remember, used a piece of same diameter welding rod, lightly changed in length as suggested.

 

Having been a transmission and rear end mechanic at a Ford dealer years ago. C-4 transmissions have a very finicky valve body. A minor amount of varnish or a small amount of dirt will cause them not to shift at all or not correctly.
It was a policy of the dealership I worked at to install a Ford rebuilt valve body in all of the C-4 rebuilds. It was a policy before I got there and after I left. The valve bodies came in a sealed bag to ensure no trash got in them. We would use the old valve body as a core.
You probably have a good transmission and may just have a varnished up valve body causing late and sluggish shifts. I would drop the pan and check it out before I took the transmission out.

 

Make sure you have tubing running to the transmission. The factory usually crimped the modulator side to keep the vacuum steady..

 

Going to play with the modulator valve adjustment tonight,might grab some of that stuff to put in the fluid to see if it would clean out a sticky valve body if that is what the problem is. Found someone who is going to show me how to rebuild one and rebuild the one that I got hot over the winter so I will have a fresh trans but this time with better then stock clutches and bands.

 

I might bet getting somewhere with that trans,adjusted the modulator out some and installed a factory metal line and shortened up the hoses on the engine and trans and it seems to be better.

 

A little better but it still does not want to go into third until the fluid warms up some to time to rebuild the trans I took out.