Question on the C4 Transmission

[dwmckenzie]

My 1977 Bronco 302, C4 transmission not shifting gears in drive. Reverse, drive, 2nd, 1st all works fine. Fluid is where it's supposed to be. I swapped out the modulator thinking that might be the cause and blew out the vacuum line and replaced the short 2 hoses connected to the metal line going up the vacuum tree. I noticed there is no vacuum at the tree on the intake. I put a vacuum gauge and nothing. I took the tree off and cleaned it up a bit and don't really understand how it works. So, the question is there has to be a vacuum at all times going to the modulator for transmission to shift?

 

[Speedrdr]

From my experience with a c4 in a car, if the engine is running, there should be vacuum. The modulator can be good, but if there’s no vacuum or partial vacuum it won’t shift correctly, if at all. Just what I’ve had happen.

 

[JeepGuy]

My 1977 Bronco 302, C4 transmission not shifting gears in drive. Reverse, drive, 2nd, 1st all works fine. Fluid is where it's supposed to be. I swapped out the modulator thinking that might be the cause and blew out the vacuum line and replaced the short 2 hoses connected to the metal line going up the vacuum tree. I noticed there is no vacuum at the tree on the intake. I put a vacuum gauge and nothing. I took the tree off and cleaned it up a bit and don't really understand how it works. So, the question is there has to be a vacuum at all times going to the modulator for transmission to shift?

Do you have a kick down cable or rod installed?

 

[dwmckenzie]

Do you have a kick down cable or rod installed?

Rod

 

[Broncobowsher]

The vacuum tree is really simple. It is a hole. If there is no vacuum on the branch when the engine is running, it is plugged up. It needs to breath freely to the intake runner.

Vacuum is used to rate load on the engine. Less vacuum, the more air in the intake manifold, the more torque the engine is making. More vacuum, less air going in, less load on the engine. If the modulator does not see any vacuum, it see that as the engine is under full load and delays the shifts to the point common with full throttle driving.
Vacuum does two things in a C4. Lowers shift points and lowers line pressure. The lower line pressure is less load and smoother shifts. Lack of vacuum, high line pressure, harsh gear engagement, late shifts like you are laying into the throttle.