mkhargrove

My 84 auto refuses to downshift. I've tested vacuum to the transmission and it's good up to the banjo bolt on the front of the intake manifold. The cable has been tightened to the point that I think it's actually putting a pull on the throttle. Fluid level is good. I have no clue how well the fluid was maintained before I bought the car. It leaked at every connection, so I was constantly adding fluid until I fixed the hoses and installed them with new crush washers. Along the way, I changed torque tube bearings, so quite a bit of fluid was replaced in that process. I don't know how old the filter is. Is it realistically for a filter to clog to the extent that it would cause this problem? Also, can old/dirty fluid cause issues in the shifting valve? If so, would adding some thinning cleaner (i.e. Seafoam Trans Tune) for a few hundred miles be helpful in rinsing out any accumulations of bad stuff from the shifter valve? (drive it with the additive, then do a full flush, replace filter)

of note, I have not checked vacuum pull at the transmission with the motor running. I have not manually pulled vacuum to the valve while driving. I have not tried any adjustments to the vacuum valve. I have not tested or adjusted pressure.

GregBBRD

Is it a possibility that the kickdown cable is so tight that the transmission won't upshift and thus can't downshift?

At any rate, adjust the kickdown cable properly.
Confirm that the modulator holds the vacuum that is getting to it.
Make sure that the electrical kickdown is working properly.

I'd be really careful about adding "fit it" products to these transmissions.
If you don't know the history of the transmission filter,
pull the pan and change the filter/see what's in the bottom of the pan.

Landseer

I've had that little spring break.

The Forgotten On

The trans can shift without the vacuum modulator so you can ignore vacuum and the valve until you find the real issue.

Without the modulator working it will shift extremely firmly, but still shift.

Once you get the trans shifting again then you can diagnose any issues it has with the modulator.

I would do as recommended above and adjust the cable to factory spec and verify the electronic kickdown works.

GregBBRD

Is there some advantage to not checking and fixing the vacuum modulator, while fixing the other functions, that I'm not aware of?