Transmission shift at high RPM’s

[White Knight]

Have C-4 automatic ..... got in today to drive to car show. Once in drive, transmission shifting at high RPM’s hard. What should I look for?

 

[Viperwolf1]

Check vacuum to modulator.

 

[Broncobowsher]

Check that the vacuum hose to the modulator is still in place. Loss of vacuum signal.

Now if it fell off, why? If you worked on something just before this happened, that explains it. But if it just fell off, it's probably soaked in ATF. Because the modulator is blown and the engine vacuum is sucking ATF through the line and the hoses are now soaked and swollen.

If all that looks good, check the modulator hoses for cracks. The rubber gets old and when it splits you loose the vacuum signal to the transmission. And ATF soaking can let them split as well.

Last up, how is the engine running? Put a vacuum gauge on it. If the engine is running poorly and has no vacuum, there is no signal to the transmission. The loss of vacuum to the transmission is a default signal that you are at full load. Transmission wise it is a fail safe. Full line pressure so the clutches don't slip but it hits into gear hard.

 

[White Knight]

Transmission question

Thanks guys!

The hose to the modulator was torn .... replaced it and all good

Again... thanks for the feedback!