My 1981 vette with a TH350 (all original) doest not shift from 1-2 or 2-3 at wide open throttle. If the throttle is lifted to about 3/4, it will shift. Going WOT will cause a downshift. If I am patient and let the RPMs come up a few hundred, I can go WOT to the 2-3 shift point, and again it wont shift.

It shifts very nice during all other driving condition. Could the Detent cable being misadjusted cause this?? Is there a way to address this without a transmition rebuild

 

Has this tranny been rebuilt? Does it have a shift-kit added? Or, is it just a stock TH-350 transmission? If it is a stock unit with some age on it, you probably have a plugged-up governor filter screen. This is a little metal-screen 'sock' that is fit into the governor pressure line in the valve body area. When that gets plugged, it limits governor pressure...which, in turn, makes all the upshifts occur "later" (or not at all). If this is the case, the valve body/channel plate will have to be removed and the screen removed and cleaned. But, if it is that plugged-up, there is more than 'normal' debris circulating in the oil system and the unit likely needs to be rebuilt, anyway.

If this unit has recently been rebuilt and modified, you will need to change the shift schedule on the unit by exchanging or modifying the governor to produce more pressure/rpm. This is done by increasing mass of the flyweights and/or reducing the tension in the springs that try to limit their movement. You can buy kits for doing such work and all you need to do is remove the governor from the transmission housing. (Sometimes, that is no small task.)

Another thought: When you are driving normally, are the upshifts much later and harder than you have experienced previously? If so, check the vacuum line and the vacuum modulator to which that line runs. The vacuum level of the engine is used to modify the shift quality: low vacuum (WOT) causes later/harder shifts; high vacuum (light throttle) causes earlier/softer shifts. The vacuum line might have been disconnected, the line plugged with something so that vacuum does not reach the modulator, or the modulator could have failed. All of these items are very easy to resolve.

 

This is a stock transmission -- It has been in storage for about 10 years, but it did exactly the same thing when I put it in storage, I just lived with it. Now I am a little older, "wiser", and wealthier I am looking to fix it.

As for the modulator -- not to long ago I accidentally disconnected the vacuum and it didn't shift at all. At that time I sorted out the vacuum issues and replaced the modulator -- Now it shifts fine (except at WOT).

Thanks

 

You might just have some sticky (or stuck) valves in the valve body. Run it a while longer and see if it frees up. I might mention that I am NOT a fan of sticking any "additive" in an automatic tranny for any reason (unsticking valves or sealing leaks).

 

Most of the so-called helpful additives have a touch of solvent of some kind in them to "free" up sticky valves- problem is, at the same time they're getting your sticky valves unstuck, they're also dissolving your clutches and seals.

 

Well I finally got some time to work on it. I adjusted the Detent Cable. It was very particular. In just the right place it shifts at about 4500 rpm. One click to far and it will not down shift at WOT, and it will perform all shifts at 2500 RPM . One click to short and it will shift normally except at WOT -- doesnt shift at all at WOT.

Are all of the TH350's this sensitive or is mine special?