727TF shift problem

[Dave_1972]

My 77' M880 is experiencing problems when shifting under heavy load.

The symptoms:

When gently accelerating up to 55-60mph the tranny will shift normally at all three gears.

When rapidly accelerating up to 55-60mph it will shift late and sometimes doesn't want to shift at all. In fact, I have found that when I feel the truck is past it's natural shift point and the RPM's start to get hairy I can ever-so-lightly ease up on the pedal and the tranny will immediately shift. Sometimes it will happen at the 1st-2nd shift (but it is always an abnormally late and awkward shift under these conditions) but it will happen every time on the 2nd-3rd shift.

I am reading conflicting info on the net about how this works. Should the M880 have a Throttle Valve that controls the shift point and is affected by vacuum?

If so - what is the procedure to inspect/replace/rebuild it?

Thanks for the help.

 

[NDT]

Sounds like the linkage between the accelerator pedal and the transmission is not adjusted properly, and full throttle is being mistaken for kickdown. Disconnect the linkage and see if you get normal shifting.

 

[M543A2]

There are two levers on the transmission valve body. One connects to the shift linkage, the other to the carb linkage to sense throttle position. The one that senses throttle position adjusts transmission pressure upwards to prevent slippage at higher loads. It also controls shift points. By adjusting the linkage to it to move it not as far rearward, you lower the shift points. By adjusting the linkage so it moves further to the rear as you apply more throttle, you raise the shift points. They can be somewhat sensitive. It is adjusted by in your case shortening the control rod a bit, then trying it. Adjust until you are happy with it, and still have kick-down.
Regards Marti