TH400 Shifting
Thread Starter
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
TH400 Shifting
I'm gradually addressing all the issues with my '72, 350cu, TH400 Vette (had it for four years). The tranny has always shifted "hard", slamming into gear like the tranny has a shift kit installed, the PO says it doesn't. I've replaced the vacuum modulator, and while doing so, noticed the metal vacuum line has been replaced with a rubber hose (from the manifold to the tranny). Is the vacuum (13 - 16 lbs) from the manifold enough to collapse this hose? Is it critical to have a steel line here? Also, the modulator is the adjustable type but I'm not sure which way to adjust - clockwise = hard shift, counter/clockwise = softer shift? Any help appreciated.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Yes. The hose will collapse and no vacuum will get to the modulator. Buy some 3/16" stainless fuel/brake line at the auto parts store and bend it up to fit your system. You just want a short piece of hose running from each end to its connection point. If the modulator is still good, that will solve your problem.
Safety Car
Joined: Oct 2004
Posts: 3,927
Likes: 740
From: Youngstown Ohio
Adjusting the modulator will change the shift point, not the shift firmness. Wide open throttle shift point can be varied about 500 rpm up or down. (I forgot which way, clockwise/counter, governs what.)
Thread Starter
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
I fixed everything. Installed a steel vacuum line - checked vacuum levels. Replaced modulator, kickdown solonoid and tranny case connector, and all associated O-rings. Car shifts great, and, finally has a kickdown function!
