FMX will not shift out of low....

[Taylors69]

New question. 351W installed into my son's 69 and got it tried last evening. We set the timing tonight and then took it for a drive to fill it up with gas. The tranny stays in low and won't shift up. I checked the vacuum connection and it is good. Is the vacuum modulator what is giving me the latest heartburn? The help and tips I have received from members so far have been very beneficial and is greatly appreciated. This father son project would not be this far long without your help. Scott

[Mach428]

I had the same problem with my recently restored Mustang with an FMX tranny. The vacuum line came off of the modulator valve and the trans couldn't shift. You may have either a vacuum problem at the valve or a bad valve.
Either one is an easy fix.

[Taylors69]

Well.....replaced the modulator....checked to make sure I had good vacuum going to the module and the tranny still will not shift automatically. We were able to shift it manually but as soon as we let off the gas it will not downshift automatically either and feels like a gutless wonder. Help??? Suggestions? Thoughts?

[BobbyFord]

Sounds like the governor may be hanging up.

[Taylors69]

Governor was hanging up and had a bad vacuum modulator as well. While out driving and working the tranny between drive and low, we blew the pump seal out. Tranny is currently in the repair shop. The cost to fix will be $670. We are definitely learning alot about taking this baby apart and putting her back together. I just want to be able to drive it once. Soon enough, I guess, soon enough.