Transmission shifting problem

[Kansasminers]

1976 bronco 302 c4 automatic. Problem: transmission will not shift out of low. I have replaced the vacuum modulator and hoses at either end of the steel line. I am not sure I have the vacuum source at the engine in the correct place. Does the transmission require a constant vacuum source? If so, that is how I have it connected. I have it connected to the tree on the rear of the intake manifold. Any ideas or suggestions? Thanks in advance!

 

[Broncobowsher]

Transmission gets full vacuum. That tree on the back works perfect.
Did you loose the pin that goes between the modulator and the valve body? Very common.
Do you have vacuum at the hose at the modulator? With the engine running pull the hose off and stick your finger on it, you should feel a good vacuum. Steel lines have rusted in the past.
Does it shift into 2nd if you manully shift into "2"? You may have a band issue.
What is the history leading up to this problem? Is this a new transmission swap? Did you do any other work and now this? Everything was fine and it suddenly did this one day? You bought it this way and have no history?

 

[Kansasminers]

Thanks for your reply. I double checked the vacuum at the modulator and it is good. When you manually put the tranny in low and second, it works fine. Reverse is also good. This bronco had been sitting for a long time. I decided to replace the engine with a new remanufactured one. Prior to this the transmission seemed to shift ok. However, that was only on a short test drive up and down my driveway. Also, the pin at the modulator is present.

 

[BwoncoHowie]

make sure the passing gear linkage isnt jammed or stuck in the downshift position.

 

[Kansasminers]

Thanks. It seems to be working as it should but transmission still doesn't shift out of low.

 

[Viperwolf1]

One more possibility, the throttle valve under the modulator and pin could be installed backwards or stuck. The end with the lands goes in first.

 

[Kansasminers]

Thanks, Viperwolf1. I did double check the valve and it was correctly installed, but appeared to be stuck. I was able to free it up, remove it, cleaned it good, and reinstalled. I decided to go ahead and remove the pan and replace the filter. I also drained all of the old transmission fluid and then put everything back together. After topping off the fluid, I took it for a little test drive and I am sure that it shifted. YEA! I could not go for a long test drive because it was now dark out and I have no lights yet. Thanks to everyone for their help! I think the problem is solved!