Transmission only working without vacuum
 

My automatic transmission has been getting progressively worse and with the cold now flares too badly to reliably make the shift from 3 to 4 and even 2 to 3. However, when I disconnect the vaccum hose, it has no shifting issues except of course that it shifts very hard up and down. Vacuum leaks are not bad, doors, trunk locks work, etc. Any advice? Thanks. Also, the transmission fluid is burnt, but I'm afraid to change it given the state of things. Should I?

Have to change fluid and filter. If it kills it, then it was time.

See how it performs with new fluid........but........I doubt it will improve dramatically. It would benefit from increased modulator pressure to reduce the 2-3 and 3-4 flaring somewhat.

It's not a magical elixir, however. If the trans is near the end.........not much can be done about it.

Thanks. I will have to bite the bullet and see what happens. However, I have been reading many of your posts about replacing the vacuum modulator, plugging the EGR, etc. and I had hoped some of this might help, although admittedly this is a bit over my head. Should I at least replace the modulator and see if a decrease in vacuum helps flaring? Thanks much.

No, I don't believe you'll need to replace the modulator.

It appears that your vacuum levels are a bit too high and it might be possible to tweak the vacuum to reduce the level to the modulator and cut the flaring a bit.

You'll need a Mityvac for this and it takes a bit of effort to set it up and measure what the modulator is currently receiving for vacuum. If you can adjust the vacuum downward, the flaring condition will be alleviated.

Additionally, the older unit would probably benefit from increased modulator pressure. It offers us a wider range to tweak.

Thanksm, Brian. I did change the transmission fluid and my mechanic changed the modulator while he had the car up -- the old was cracked (unless the nose cracked off while he was removing it). But, unfortunately, no improvement. Are there any posts I can look up on how to reduce the vacuum with a Mityvac as you suggest? I'm a relative novice and my mechanic doesn't seem to know how to do this well enough. Thanks again!

With the engine not running, connect the Mityvac to the T that is directly above the vacuum control valve. One side of the T goes to the valve........one goes to the transmission.........the third will be your Mityvac.

Pump up the Mityvac and get a reading. Make sure it holds vacuum.

Report back with result.