C4 Experts...need some help

MrKwik

Founding Member
Jan 29, 2000
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Kansas City, Kansas
Recently did a C4 conversion on my 93 vert. Problem Im having is that when you have the stick in drive and take off from a light, its not starting off in first gear, its in second. Basically its like a 2 speed, just goes from 2 to 3. If you use the stick to manually shift, it gets 1-2-3 no problem so I know it will phisically go into gear and all that. The trans has a fresh rebuild on it. My nephew rebuilt it in his auto tech class up at the community college and he has spoken with his instructor but he doesnt seem to have any idea what could be causing it.

I was thinking maybe the modulator had issues and I read somewhere there needed to be like 11+ inches of vacuum at the modulator in idle. I put a guage on the modulator line at the trans and it has the same 14" that I have at the intake. I'm also not running a kickdown rod but most everybody I spoke to about it swears I donr need it. One guy I know that I think knows his **** said that it is required and that it acts like the TV cable on an AOD and wont have enough pressure on the bands or something if its not hooked up. I have never run a kickdown on my chevy automatics and have never had any issues. Any thoughts on this?
 
update:
I figured out the problem and got it working correctly. I started thinking about how when I did my first test drive I had forgotten to run the line for the modulator and it wouldn't shift out of 1st. So I figure no vacuum = no shift...too much vacuum = too much shift. I checked the modulator and there is a little set screw inside the vacuum port. It was all the way out so I screwed it in some thinking that would regulate the vacuum down. Bingo! Now it shifted but at like 1300rpm. Gave it a few more twists and now it shifts at about 2k. With the 2500 stall and 3.73's, thats still a bit early so I'm gonna give it a few more and see if I can get it closer to 2500. But now I know the tranny is fine and I'm happy.