long story short when i swapped in a new cam and fixed the broken vacuum tree on my intake, it seems now the vacuum modulator on the transmission does not work. before i did all the repairs it shifted fine with no problem. now i have to manually shift it in order to change gears. i hooked the modulator vacuum line back where it was on the vacuum tree on the 390 intake. i replaced all the vacuum lines with new rubber so it isnt leaking anywhere. could my modulator just of went bad just like that? the truck was only down for 3 days.


thanks

 

Check the vacuum at the modulator it should have about 15 to 20 inches of vacuum at idle.

 

It sounds like you have no vacuum at the modulator or the modulator is bad or the line to the modulator is bad.
With engine at idle get under and pull the vacuum pipe off the modulator at the trans. There should be a noticable change in engine note. If no change put your finger over the end of the rubber pipe and listen for a change in note and you should feel a strong vacuum. If there is a change in engine note then and you do feel a vacuum then the modulator is bad.
If still no change and no vacuum then plug pipe back on and investigate why there is no manifold vacuum connected to the pipe at the engine end.

 

dont have a guage, but when i pulled the vacuum line off the modulator it doesnt have any vacuum to it when i put my finger over the end. so im asuming that the vac. tree on the intake doesnt work. the only other ported vacuum source i have is 1 port on the carb that my distributor vac. advance is hooked too. could i run it off that to make sure it works on that port?

 

Don`t see why not.

 

It needs to be manifold vacuum, ported vacuum wont work the modulator correctly.

The vacuum needs to drop when you press the accelerator.

 

ok i see. could i install a T-fitting on the back of my carb spacer since it ties with the brake booster line?

 

I have never installed it there, as long as it has vacuum at a idle and drops during acceleration it should work. Try it you can always undo it.

 

ok thanks i'll give a shot. worse case it dont work lol.

 

just got back from the drive. shifts like its supposed to. thanks for the help guys

 

The dist. port is below the throttle butterfly so should meet those conditions. However it is small and so modulator response may be slow, not sure.