Gotta do a tranny change for the G/F's dad tomorrow - fmx behind a 351.

He has a new tranny(reco'd), the old one took ages to select reverse and a was a general pig.

How do I ensure that the existing TC is okay? How can I test? There's no noises from the splines not meshing etc.... Just that the old one shifted like a pig and wouldn't hold 1st any more.

I'm adding a new pan gasket and output seal, as well as a Trans cooler as this one burnt the oil as well as leaked it!

Any help would be great!

did you ever replace the vacuum modulator on the FMX ? when older they can cause exactly what you describe and even worse if stuffed. I had FMX in my worked 2 door (over 280 at treads) and at 30 years old its still kicking a***.

So is that the item at the back of the box which the kickdown hose feeds?

We have a new trannie to go in, so the issue is more of making sure the T/c is okay to use again...

yes thats it and if the hose going to it is old it can sometimes be split or leaking through perished rubber. If the new tranny is not quite as smooth as you expected then renew the hose making sure there are no leaks and renew the modulator. From memory i think it was around $40 but prices could have changed since then.