C4 transmission problems! help!

[elizabeth68]

I have a 68 mustang with a 302 and a c4. My problem... I noticed the other day that my tranny was acting funny in first, It was shifting late. Then not shifting, then it didn't want even change gears once from park. It was acting weak in first, I would give it gas it would rev up but no power really it would move slow. 2nd and 3rd were fine.

So I changed the transmission fluid (not torque converter) and filter. Filled her up. It drove better. Wouldn't take as long to shift from park to drive. All the other gears shift fine. Its only every now and then that it takes a pause to shift to drive. The only problem now is its shifting late from 1st to second. Got to get her up to about 3000 rpm and about 30-35 miles for it to shift to 2nd.

I checked the vacuum line to the modulator, no cracks. What I did then was plug the hose line from the intake manifold to the vaccum modulator, and the car drove better! It was shifting, okay it wasn't weak feeling. So it actually drove better, almost like no problem aside from the actual fact that the vacuum modulator wasn't plugged in. When I pulled the hose out there was no atf fluid coming out from the modulator nor in the hose. No weird color smoke from pipes.

I have no idea how old the tranny is and what not so. Don't when was the last time the bands were adjusted.

I find it weird that it runs better plugged off from the vacuum modulator.

If anyone can help me out that would be terrific!

[elizabeth68]

Any idea anyone?

[67Mustang Coupe]

Some suggestions for you:
Check your manifold vacuum should be around 18
Check to make sure the pin is in the transmission just behind the modulator
Check to make sure your timing is set correctly and the your vacuum advance is working, if its not set correct you wont build up vacuum and the trans wont shift right
Check you band adjustment
Check your kickdown linkage. Make sure its adjusted correctly and hooked up to your carb.

-Chris

[elizabeth68]

Thanks I shall! Question were is the pin by the modulator valve?

[88cougargt]

Maybe try a new modulator. They are cheap.
Does the fluid smell burned?

[Lee12609]

Sounds like low line pressure. No vacuum to the modulator just increases the pressure during shifts. I'd get it to a shop.

[67Mustang Coupe]

When you unscrew the modulator, there should be a pin sticking out of the modulator, its very short, so try not to lose it.

-Chris

[sdmustangrider]

If you get stuck, there's a shop in El Cajon called Valley Transmission. They're suppose to be really good with C4's and could probably give you a hint what the issue is if you get no luck. Give them a ring.