Recently purchased 76 El Camino shifts from 1st to 2nd but does not shift into 3rd . Vacuum modulator hose is connected to left passenger side port which reading says the line should not be connected to carburetor . Only intake fitting I see is in the rear which is connected to brake booster . If I tee that off with transmission vacuum would that possible fix no 3rd gear or use back or carb for brake booster and get a new fitting for transmission Vacuum ?

 

Welcome to the forum. Moving your post to the section of the Forum where you will get more views.

 

Update
Moved brake booster to back or carb , connected trans vacuum line straight to manifold as a temporary check I got a vacuum line to connect modulator to intake to eliminate any leaks from previous line but same results still not going into 3rd . Trans level is fine , fluid is in good condition not black or burnt odor . I did notice I slight leak from the governor housing but I don't know much about it

 

What is the modulator history? Some of those are adjustable. I can't see there being an issue with where ported, or non ported vacuum would matter. look inside the valve where the vacuum line goes in, if there is an allen head screw it is adjustable. I don't know how to adjust one, but it could be this style, and all it needs.

 

I replaced the modulator with a new one since I didn't know the history of the old one myself , it does have a screw you can turn to adjust shifting I did rotate it a full turn but no change . I had some Autozone credits I figured I'd drop the pan change the filter check the linkage possibly remove the governor make sure value is no sticking etc ... worse case could be still no 3rd then to a trans shop it goes haha

 

Don't forget to move your power brake booster line back to intake manifold which is what it uses. 🙂

 

Dropped the pan took out the governor , seems to be in working order springs weights and value . Replaced ghe filter , notice one of the value body bolts was not tight so torqued down all bolts to spec installed pan with fluid but have not test drove yet . I did notice this electric connection was missing ... I'm guessing it's for the electric kickdown ??? But have no clue where wire is , there is a connection on the gas pedal I'm guessing for kickdown



Missing connection

 

Like you said, I would suspect that the electrical connection is for the kick-down switch located up by the accelerator pedal. Is there a wire connected to that switch?

 

I've not seen an electric operated kick down, or at least not in modern times. That could be your issue with it not hooked up, not allowing it to shift into hi.

 

Now that I've thought about it, I'm not sure the kickdown switch would really have any effect on shifting into third gear. It's purpose was to downshift when you floored the accelerator.