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I am having a problem with the C4 in my 71 Bronco. It will shift from first to second normally, but it doesn't want to shift from second to third unless I let off the throttle. Then if I apply more than about 1/4 throttle, it shifts down to second angain then back up when I let off the throttle and back and forth like that as I try to apply the throttle.

I have a cable kickdown installed and I've adjusted the cable stop on the carb end way out so it isn't working at all to make sure that isn't the problem. Still same condition.

I had the whole tranny rebuilt not too many miles ago. Worked fine after that.

Fluid level is correct checked hot in park after driving for a while. Fluid looks brand new.

Vacuum Modulator looks new (replaced during rebuild). Vacuum hooked up.

Any guidance would be appreciated!
 

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I am having a problem with the C4 in my 71 Bronco. It will shift from first to second normally, but it doesn't want to shift from second to third unless I let off the throttle. Then if I apply more than about 1/4 throttle, it shifts down to second angain then back up when I let off the throttle and back and forth like that as I try to apply the throttle.

I have a cable kickdown installed and I've adjusted the cable stop on the carb end way out so it isn't working at all to make sure that isn't the problem. Still same condition.

I had the whole tranny rebuilt not too many miles ago. Worked fine after that.

Fluid level is correct checked hot in park after driving for a while. Fluid looks brand new.

Vacuum Modulator looks new (replaced during rebuild). Vacuum hooked up.

Any guidance would be appreciated!
Sounds like a low vacuum problem...

Do you have a vacuum gauge? How much vacuum does the engine pull at idle?

Tee the gauge into the vac line to the mod and monitor the vac as you are driving and the problem happening.
The vacuum must rise and fall immediately with throttle activity.
If it's low or acts lazy or sticks or flutters there is a problem.

If you don't have a gauge, you must make SURE that there are no vac leaks ANYWHERE in the system,
and the trans modulator line MUST go to full manifold vacuum, preferably right on the intake, not on the carb.

Also there must not be ANY restriction in the vac line to the mod.

They sometimes carbon up or get blocked with gunk.

If not a vacuum problem then it's time to look at the governor or valve body, or both.
 
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