Understand Automatic Transmissions, Mustang C4, C6, and FMX
The ins and outs of the simple transmissions
The FMX is a standalone cast-iron three-speed automatic that was in production in various forms beginning in 1950. There was the large-case MX Ford-O-Matic and the small-case FX transmissions prior to '66. When Ford introduced the C6 in '66, these old ironsides were dropped in favor of the newer, lighter C4 and C6 transmissions.
When C4 and C6 production shortfalls created challenges in the late 1960s, Ford turned to the tried and proven FMX with beefy MX internals inside the smaller FX case for the 351 engines that arrived in '69. It was a rugged and dependable three-speed automatic. Some companies, such as Trans Go, still offer rebuild and performance kits for the FMX today.
What makes the FMX different than the C4 and C6 is its Ravineaux twin-planetary gearset design, which was also employed in the later AOD, AODE, and 4R70W automatic overdrives. The Ravineaux planetary design consisted of fore and aft planetary gear sets in a single package, which is what makes it different than the separate Simpson planetaries used in the C4 and C6. Aside from this difference, the FMX operates much the same way as a C4. Band adjustment is similar, with a setscrew and locknut like C4 and C6. FMX intermediate and low-reverse servos are located inside the case.
Throttle Valve/Vacuum Modulator
The C4, C6, and FMX operate off the same basic principle of throttle valve operation. The throttle valve, also known as the vacuum modulator, gets its signal from intake manifold vacuum, which is controlled by engine load and throttle position. With the accelerator pressed, manifold vacuum is low. With your foot off the throttle, manifold vacuum becomes high. Low manifold vacuum (open throttle) causes the throttle valve to modulate high transmission control pressure. High manifold vacuum (closed throttle) causes low control pressure.
Pressing the accelerator pedal causes the engine to rev and manifold vacuum to drop. This is when the transmission's clutches and bands need high application pressure for better power transfer. Nailing the accelerator does two things in your transmission—high control pressure and delayed upshift. Let up on the accelerator and manifold vacuum increases, which causes reduced control pressure and a smoother, more immediate upshift. By the same token, when you're slowing down, you want reduced control pressure, which causes smoother downshifts.
As a rule, throttle valves are factory adjusted and don't require adjustment. In fact, original factory-installed throttle valves cannot be adjusted. However, service replacements are adjustable. Just follow your Ford Shop Manual instructions. One full turn clockwise increases control pressure 2-3 psi. One full turn counterclockwise does the opposite. Make your adjustments in baby steps (quarter and half turns) and take a test drive.
Think of your transmission's hydraulic control system like a finely tuned watch. We want aggressive upshifts during wide-open throttle, yet smooth and unnoticed during deceleration to a stop. Throttle valve operation works hand in hand with the governor and kick-down system to get shift points close to where they need to be. It takes patience and a feel for shift points when you're adjusting the throttle valve. We suggest the use of a pressure gauge for best results.