Strange gear change issue with TH350

[NaturalSnapple]

Hello Everyone,

After a top-end rebuild on a 1971 C10 (250 and TH350), when under full throttle, the transmission will alternate between 2nd and 3rd gears. For instance it'll downshift to 2 and then shift to 3 at the correct moment, but then it'll downshift back to 2, and then back to 3, all with the gas pedal held down without lifting.

It does not have a detent cable, and only has a vacuum source going to the manifold. I have very little knowledge of transmissions so thought I would reach out here to see if someone has had a similar issue.

[cleszkie]

I'm no expert. But every TH350 I've owned ran the detent cable. I think you need to install the detent cable to keep it in 3rd when the pedal is depressed.

[AussieinNC]

You will need to install and correctly adjust the detent cable to stop this cycling...

Also, make sure the vacuum modulator is not leaking trans fluid...into the vacuum stream...

[oneperfectrun]

The governor in the transmission can cause similar issues. I had a ‘79 Z28 back in the 80’s that was doing the same thing. Dropping from third to second, back to third then repeat. Can’t remember if it was at full throttle or normal driving though. Case was worn where governor shaft rode.

[NaturalSnapple]

Thanks all. I'll check out doing the detent cable first as that's not too bad and will go from there. Hopefully it isn't something internal to the transmission.