I have a 70 Mach 1, FMX auto transmission and there is a violent jolt when I go from Park into drive or reverse, it's worse when the car is warm, ie it doesn't happen when i'm reversing the car out of my garage, apart from that the gears shift great when driving, there is no knocking or delays when going through the gears, i've currently got the car booked in with a Mustang specialist as I had to have the carb rebuilt, but any ideas what the problem could be, do you think the transmission could need rebuilding? I sure hope not, we've got a new modulator valve on order as that is the cheapest option at the moment, thanks for any help

 

FMX's have a problem with the pressure valve sticking in the pump. It's due from lack of fluid changes. If all else is ok, you can drive it that way but it will need a rebuild soon.

 

if your idle is set too high that will contribute.

if there is excess wear in the diff that will also cause a bit of a bang as the box spins the driveshaft and it takes up the slack. you can get an idea of this by jacking the rear and moving a wheel while the car is in park to see how much play is present. if you have an open diff you'll need to leave one wheel on the ground or chock it.

 

Ok thanks for the replies