My freshly rebuilt C6 with Transgo shift kit and Hughes 2500 stall converter won't downshift until I'm nearly at a dead stop. My Cruisomatic did the same thing in stock form. I roll up to an intersection and the light is green. I slow way down to negotiate the turn. I then throttle when headed straight and the tranny is still in Drive. It does the same thing when slowing way down for speed bumps. Could the shift kit or stall converter affect downshifts?

 

It's not the converter. It is either the shift kit or a problem with the governor.

 

Ok. After the rebuild; I believe the governor was sticking. The truck would keep taking off in Drive from a complete stop. I drove it for a few miles manually shifting it and it finally started downshifting to Low automatically at nearly a complete stop. It shifts up fine now. Would it upshift fine if governor is stuck?
Thanks!

 

Maybe, maybe not. It could be sticking coming down, but not going up.

 

Thanks Mark. Would the vacuum modulator adjustment affect downshifts?

 

Yes it will.

 

Mark;
Which way would I turn the modulator adjustment screw to bring in the downshifts sooner?
Thanks!

 

I don't know.

 

Screw the modulator adjustment out to have earlier upshifts.

Screwing it in may make the coasting downshift happen at a higher speed, earlier.

 

I thought I'd post an update.

Governor was sticking as well as the pressure regulator. All repaired and the tranny does the same. I can slow down to under 10mph over a speed bump and accelerate again and the tranny is still in Drive. Nobody can seem to tell me at what speed the tranny should shift into second or low. I have read that the C6 shifts down from Drive directly to Low when coming to a stop. Another post says that the C6 freewheels when slowing down for emissions sake?

P.S. Just read that the modulator valve must match the manifold vacuum range of my engine.

The engine has 16"hg of manifold vacuum at idle in Park. At idle in Drive the manifold vacuum is 13-14"hg.

 

Problem solved. The shift kit is causing the issue in combination with the '67 valve body. Somewhere around 70-72 the manual valve was changed from the detent style to a floating style with a spring and roller in the case instead. At some point after 72 the valve body was the same except for the pressure regulator. The early style had a plate to retain it where the newer style used a small spring clip. In 76 or 77 they stopped putting a check ball in the low/rev piston and installed it in the valve body instead. Chrysler went through the same change with their a518 tranny.