Have a '69 with a C6 and it won't shift. I can manually shift all gears with no problem but it won't automatically. The trans has been rebuilt a year ago and was working fine until the last time out. I've only driven it around the block for the past year as I'm restoring it. My vacuum line comes off the base of the carb and showing 15 inches of vacuum. The hose off the vacuum modular is also showing 15 inches. I checked for leaks and found none. Any suggestions?

 

maybe the vacuum modulator? if that's not working then the gearbox will never see any vacuum and believe you are running WOT the whole time. probably preventing to shift away from 1st gear. if in auto, are you stuck in 3rd gear or in first ??

 

A new vacuum modulator is not that expensive, I'd replace it. If that's not it, I'm guessing you have some gunk in there that is holding up a valve or something.

But, I'm not that experienced with auto trannies, except for putting in shift kits.

 

It stays in 1st. When I manually shift and stop, it drops back to 1st. When I have it in 3rd, drop to second it shifts to 2nd, but unless I manually move the shifter it stays in 1st.

 

not a transmission guy myself but that's the first thing I'd try the modulator. check when buying. There's two different types. one is screw in, the other is push-in. and just as you said make sure the line from carb to transmission is not faulty (i think you checked that before). if it get's no vacuum (modulator leaking) the trans believes you are at WOT at all times and could cause stupid shifting. never experienced that, but that's my theory ..,. :-)

 

Maybe it's the timing. I know it was shifting good and I worked on the shaker and pulley system, I could have bumped the ditributor just enough to throw it off timing. If not I'll replace the modulator.

 

It is not the timing,,,,, can't say what it IS, but like the others I say change the modulator before anything else.

 

When you have modulator off use a small magnet to verify the internal modulator piston smoothly strokes. Sometimes bad modulator causes rough shifts or slow shifts. Skill level=Very easy.

Run pump pressure test. Skill level=Very easy.

Valve body removal is not a major job but care is required.
Look for exact diagram of your VB.
Verify all VB valves move under their own weight and all springs look good.
W/ VB out doesn't hurt to run tranny air checks. Skill level=If you are not meticulous then don't try this at home.

 

Maybe just get under there and adjust the screw in the modulator opening back and forth a little. It may just need to be adjusted. try it out.

 

I hooked up a vacuum gauge and adjusted the distributor and my vacuum went from 15 inches to 19. Test drove and it shifted but still needs a modulator adjustment as it was late in the first shift.

Thanks for the help.