C4 Transmission Doesnt Shift to Third!!

I have a 66 c4 green dot that refuses to engage third gear. Ive replaced the modulator, oil and adjusted everything but i still have no progress. Its rather frustrating and id like to know if anyone had any idea what this could be. Thanks so much for your help.

 

So it could be the gasket between the carb spacer and the intake log?

 

check for a split in the modulator vac. line.
i also had one that the vac. line had become soft and was collasping shut under vac.

......

 

Actually the hose connecting the steel line to the modulator is a soft one that seems easy to push in. Is it supposed to be a stiffer, thicker hose?

 

Should look something like this. http://www.karmustang.com/product.php?productid=3381

 

That is precisely what it looks like. So could it be the deteriorating line connecting intake manifold to the steel line? It is rather burnt up but im getting a pretty good vacuum reading at the manifold. Somewhere in the 19's and 20's

 

Is there a way i could check if the transmission would shift to third otherwise?

 

check your kick down, make sure it returns back up when you push it down

 

Checked the kick down and it definitely returns.

 

Check the vacuum at the modulator - it has to be 10 or above.
Could be a bad hose at either end of the metal line or a cracked line to the modulator.
If you have reverse then it might be a stck 2-3 shift valve if the vacuum is above 10 at the modulator.
It would help to know what speeds the transmission shifts from 1 to 2 also.
Paul

 

where is the vaccuum mod hose?

where is the vaccuum mod hose?

 

It connects to the modulator located at the passenger (right) rear corner of the main case - just above the trans pan. Test it there with the engine idling in park.
It should connect to the manifold vacuum either at the rear of the carb mount on the manifold or the vacuum tree on the firewall.