View Full Version : C-4 Transmission not shifting

03/13/06, 04:41 PM
I have a 1977 Bronco with a 302 automatic. I recently upgraded the intake manifold and carb. Prior to the upgrade the transmission would shift, but not always smoothly. Since the upgrade however, it is first gear all the time. I checked the fluid level and color both seemed normal. I then checked the vaccum line and it did not seem to be cracked or leaking. I've been told that it could be the modulator. Is there any truth to this?

Thank You

03/13/06, 05:58 PM
What carb did you use? The modulator may not be connected to the correct vacuum port.

03/13/06, 06:02 PM
What carb did you use? The modulator may not be connected to the correct vacuum port.

What he said...and also check the vacumm line at the modulator on the transmission. It could have worked loose when you moved it around while changing the intake.

03/14/06, 07:31 AM
The vacuum line is connected to a tree coming off the back of the intake manifold. The tree also supplies the line for the power brake booster. I upgraded to an edelbrock performer series manifold and carb. Went from a 2bbl. to a 4bbl.

I checked the line at the modulator and the connection seemed fine.

Any thoughts

Thank You

03/14/06, 09:18 AM
Pull the hose off the modulator, start the engine and feel for vacuum at the modulator.

If you have a strong vacuum, fix the kickdown rod. It's probably pushed down and the trans ignores the vacuum signal and keeps you in passing mode.

That's about it for external stuff related to carb and intake swap. Next stop is the trans shop.

03/14/06, 09:36 AM
Yep, all the above plus if you have a vaccuum gauge you can check the vaccuum signal at the modulator. you need 14-20 in.Hg. (inches of mercury) @ idle to make the C-4 shift normal.

03/14/06, 01:53 PM
Thank you I will take a look at that tonight.

Any ideas as to what it should cost to rebuild a C-4??


03/14/06, 02:26 PM
had to pull mine out myself, it cost me 550 w/ a new torq conv. ;)

03/14/06, 03:11 PM
Thanks for the info.

03/15/06, 10:22 AM
I crawled underneith last night only to find that the kick down cable is missing. Atleast, I could not find any cable coming off of it. Only the shift linkage. Could this be why the transmission is not shifting??
Where should the kick down cable connect at each end??

03/15/06, 10:57 AM
There is a kickdown rod that connects to the carb linkage and the tranny. It pushes the lever on the tranny back under acceleration to make it kick down. You might try pulling the lever on the tranny forward and tie it in that position. If it shifts then you know what your problem was.

Of course you will have to manually shift the tranny to make it kick down at speed if you do that.

Good Luck!!!
8-) :) 8-)

03/16/06, 10:22 AM
Thank You

I'll give it a try