On my truck I have a 400 motor with the c6. I put this thread on this subject because it wasn't getting any replies from anywhere else. So here it goes, in my motor I have a little bit bigger than stock cam. I have a Holley 2 barrel carb with the dizzy advance hooked to it. I have all the unused vacuum ports blocked/ plugged up. I have my EGR blocked off with a thick carb gasket. The problem I am having is I can the speed at 45-60 MPH and the transmission will start kicking down a gear, and back up, but so fast it doesn't do anything but make a clinging noise and jerk the truck. The modulator valve on it is the adjustable kind, I have already tried turning it 4 turns counter clockwise.

 

I would check the kickdown rod. But i honestly dont know much about automatics. I thought the modulator told the transmission when to shift, not when to downshift.

 

I have checked it all. I thought that was what the modulator was only for too, but I guess it could be for both. What would cause a sudden vacuum pressure drop? and all my hoses have been checked.

 

The kickdown,or should I say throttle valve,and the governor control the down shift timing. The modulator controls the firmness of the shift.
Sometimes there's a little spring on the kickdown at the carb to help pull it forward. I would try your truck with the kickdown unhooked. pull it up,and forward,and tie it that way.Then test drive it. If it wants to down shift funny you have Governor problems

 

where is the governor on a 400 at?

 

I'd look into the trans if the kickdown is ok.

 

what in the transmission would cause this though?

 

X2 on the kick down. I have seen the modulator do some strange stuff on c6's. Mine wouldn't downshift and the expertise from this site said to go through the governor, I did and problem persisted. I had tranny shops blame the tubes carrying oil to and from the governor for leaking, faulty governor, and so on. I turned the screw in about 4-5 turns on the modulator and works like a top. Keep playing with it and see where you land. The modulator regulates the line pressure and that in turn regulated the governor and the firmness of the shift. Ford actually specs in their 1979 truck shop manual to change rod length for the modulator to get desired operation. This is something I have never done, nor has any of the local tranny shops. They told me that there is one rod length and all adjustment is in the modulator itself. They clearly never read the factory specs for a C6!! Good luck.

 

hmm... I'll try to turn my modulator in tomorrow, if the ground is dry. and different rod lengths????

 

ford specs to change the rod length if necessary. My suggestion is unhook the kickdown and drive it. If problem persists unplug the modulator hose and plug the supply and drive it see what happens then. if problem persists I can send you ford's trouble shooting from my shop manual and the valve body break down. I hate to say this but you might have to pull the valve body and go through it. It might have a line pressure issue or a sticking valve. A sudden downshift from 3 to 2 might be the line pressure pushing too hard on the secondary governor or the governor not seating/moving right. Could be too much line pressure=a rod too long or a modulator spring too stiff. If this was the case the the shiftd should be firm, if the shifts are smooth then chances are the line pressure is not too high, but a faulty/sticking governor might cause this with lower line pressure. Good luck.

 

I have unplugged the modulator line, It doesn't shift at all. I can shift it into 2 manually, but after that it doesn't shift up any more. I hooked it back up and it shifted. How do I check the governors?

 

The governor is in the back of the transmission. To access it the tail housing/transfer case is removed, then shine a light into the back of the trans and there is a housing that is actually bolted onto the output shaft with 4 bolts (3/8 socket I believe) If you get inthere youll see it and it comes off out the back and check for sticking valves (theres 2) spring condition, the fluid orifices open and free or debris, all needed to disassemble it is a screw driver or plier to remove the clips that hold the springs. If it kicks down at those speeds it is probably the secondary governor, It has more to do with the higher speeds. It relies on fluid pressure to open it then relies on centrifugal force from the drive shaft spinning to close it. If the fluid pressure is insufficient, it would not open fully then might get eratic shifts at certain times.