I started out having shift problems, my tranny wouldnt shift out of first after a dead stop until after 40mph/3K-RPM, and once it did it would shift up and down between 35mph and 45mph, from 2 to 3 and 3 to 2 and 2 to 3, etc.. It wasnt a shudder, it took a few seconds between shifts. Then it started not wanting to shift into 3rd. I would have to manually shift to 2 and back to 3 and most of the time it would kick into 3rd.
Terrified it would cost me a mint, I posted a Q in the tech section at FSB. Got a few ideas, and meanwhile I dropped down to a Cottman's transmission shop for a free checkout. The tech said he removed the vaccuum line from the modulator and oil (transmission fluid) ran out, so the modulator was bad. The modulator simply has a diaphragm that is vaccuum actuated to adjust the shift points (tell the tranny when its okay to shift). If you pull the vaccuum line and fluid runs out, the diaphragm is busted and fluid is getting by. This means the vaccuum on the line is not pulling the diag. and hence the small shaft inside is not moving.
Remove the bracket bolt with a 14mm socket, pull the old modulator from the rear of the C-6 transmission. Be carefull not to loose the plunger, its about a 2 inch piece of metal with rounded ends. Put the plunger into the new modulator, slide it onto the transmission and bolt down the bracket. Remove the vaccuum line from the old modulator and plug up the new, but allow for any excess fluid to drain fron the line.
Failure to replace the modulator could result in bruning up the transmission. I noticed mine not "acting right" since I first put it on the road (2 months), but it was nothing major. Yesterday on my way to work, the tranny's shif went all to hell, nothing was right.
Below, please see a picture of the modulator on the C-6. There is also a before and after video. Notice the MPH and RPM on both the before and after. At one point before, I pushed 3K RPM at 45MPH and it never shifted out of 2ed.
With the new modulator, the tranny shifts like a war hawk, just like its supposed to! And, this may also, most likely be the source of my tranny fluid leak (As verified by the tech at Cottman's).
Video: Before C-6 modulator replacement
Video: After C-6 modulator replacement
Modulator was purchased at Advance Auto for $13.99. Cottman's wanted $140 (parts and labor) for the rapair.
I hope this helps someone out!