How a C4 Transmission Worksby Moss Strohem
Ford C4 Transmission
The C4 transmission is a three-speed automatic transmission used in Ford medium-duty, rear wheel-drive car applications. This transmission debuted with the Ford Mustang in 1964 and was used in most Ford automobiles through 1986. It replaced heavier, cast-iron automatic transmissions and featured a lighter-weight aluminum case and improved internal components. Because of their simplicity and strength, they are popular in high-performance applications. In rear wheel-drive vehicles, the engine is in the front of the car with the transmission bolted directly behind it. A drive shaft extends to the rear axle assembly, which drives the rear wheels. The C4 works the same as other automatic transmissions. After selecting a forward or reverse gear, simply apply power. There is no clutch or gears to shift. Shifting takes place automatically, through the automatic sequencing of internal actions based on the high-pressure flow of transmission fluid.
All automatic transmissions operate in a similar manner; they are driven by the engine and hydraulically transmit power to the wheels. The C4 is a relatively simple design. The transmission is coupled to the engine by a torque converter. The converter bolts to the engine flywheel (or flex plate); the bell housing of the transmission case fits around the converter, bolted directly to the engine block. The converter is not readily visible once the transmission is fitted to the engine. Torque converters are hollow, sealed, round steel parts the size of a half basketball with a snout on the rear and filled with fluid when the engine is operating. The snout fits into the front of the transmission case and drives an input shaft. A seal around the snout where it enters the front of the transmission prevents fluid leakage. The converter spins at the same speed as the engine, and fluid in the converter is routed by fins (turbines) at high speed into the transmission. A converter-driven pump at the very front of the transmission further pressurizes the transmission.
Once fluid is forced into the transmission by the converter and pump, it's routed through valves and passageways that apply pressure to "baskets" of rotating clutches that drive internal gears. Depending on which gear position is selected by the driver, different portions of the transmission are engaged. In "Park," pressure remains low, the internal clutches don't engage, and the output shaft at the rear of the transmission is locked so that the output shaft can't rotate. In "Neutral," there's still a lower amount of pressure so that no force is applied to the clutches to couple the input and output shafts, but the output shaft can rotate. In "Drive" gears--either forward or reverse--pressure is routed so that higher pressure is applied to the internal clutches and movement in the desired direction occurs. At "Idle," pressure is low and the car can be held stationary by using brakes. As power is applied, pressure increases to further lock up the drive clutches more securely and the power is driven to the wheels more forcefully. The pressure and speed of the rotating components of the transmission govern the movement into higher gears. When vehicle speed is sufficient, higher-drive clutches engage until the transmission couples to the output shaft's final drive ratio.