I posted this in the drivetrain forum, but no one has responded hoping you guys can help me out.

I just put in a new(well rebuilt) c4 in my f100, doesnt like to shift from 1-2 until about 30 mph, my previous one shifted at about 10-15 mph. is there anything i can do to fix this? or is it the way the tranny was built, ive got my kickdown linkage screw all the way in so i dont think that is it, and ive tried it all the way out also. could it be the modulator valve? the previous tranny had one with a red stripe and this one has a green stripe, also the red one has hook ups for two vacum lines, this one has none???? ive heard that the modulator controls how hard it shifts not when, and the kickdown is for when? but not working that way...any ideas?

thanks
chris

 

There is an adjustment screw inside the Trans Modulator to chnage shift points.

Do you have a listing of the OEM shift point specs ?

 

im not sure, i have a haynes rebuild book on it? would they be in there?

 

There should be a section on how to adjust your modulator. If its not shifting correctly, are you sure the vacuum line is hooked up, and has vaccum going to it, and has no leaks?

 

could be that the vacum isnt hooked up right, that is one of the things im going to make sure of in a bit. but before i get started, to get the tranny to shift sooner i would turn the modulator CCW and CW to shift later. my manual dosent have a procedure for adjustment that i could find.

 

it was the vaccum line, i hooked the wrong one up, my previous tranny had two vaccum lines, for egr im assuming, this one only had one. i plugged the wrong one and hooked up the other one. but i swapped them and now its working right, engine still needs alot of work, but at least it drives right now.

thanks for the help

chris

 

You're welcome

THE HYDRAULIC CONTROL SYSTEM

The hydraulic pressure used to operate the servos comes from the main transmission oil pump. This fluid is channeled to the various servos through the shift valves. There is generally a manual shift valve which is operated by the transmission selector lever and an automatic shift valve for each automatic upshift the transmission provides: i.e., 2-speed automatics have a low/high shift valve, while 3-speeds have a 1-2 valve, and a 2-3 valve.

There are two pressures which effect the operation of these valves. One is the governor pressure which is affected by vehicle speed. The other is the modulator pressure which is affected by intake manifold vacuum or throttle position. Governor pressure rises with an increase in vehicle speed, and modulator pressure rises as the throttle is opened wider. By responding to these two pressures, the shift valves cause the upshift points to be delayed with increased throttle opening to make the best use of the engine's power output.

Most transmissions also make use of an auxiliary circuit for downshifting. This circuit may be actuated by the throttle linkage or the vacuum line which actuates the modulator, or by a cable or solenoid. It applies pressure to a special downshift surface on the shift valve or valves.

The transmission modulator also governs the line pressure, used to actuate the servos. In this way, the clutches and bands will be actuated with a force matching the torque output of the engine.