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  Overflowing the automatic transmission in your Ford automobile can aggravate design flaws in your vehicle's components. Symptoms associated with an overflowed transmission can vary from diminished engine performance to significant damage to the transmission itself. To prevent this condition always fill your transmission only to the reservoir fill line and never over it.
  
  

Leakage Problems

• An overfilled Ford automatic transmission can begin to have leaks in its hose connections, dipstick and its transmission vent. This is especially true of Ford models using the AX4S transmission, which has a high fail rate due to a weak forward clutch piston that cracks and causes component-wide leaks of pressure, according to automotive diagnosis and repair website AA1 Car. The AX4S was used in Ford's Taurus line of automobiles as well as the Ford Windstar and the Mercury Sable.
  
  

Slipping Belts and Gears

• An overfilled Ford transmission may cause fluid to back up into the component's geartrain assembly, which can cause air bubbles to develop in the transmission fluid. Once air bubbles happen the gears and belts that maintain the functionality of the transmission can slip. This manifests in your driving experience as an inability to shift or a tendency for your transmission to slip its gears spontaneously and drop to a lower gear. If the condition is allowed to go on for an extended period of time significant transmission damage can occur.
  
  

Other System Problems

• When transmission leaks occur the fluid has to go somewhere. Because of the proximity of a Ford transmission to the vehicle's engine, leaking fluid can often interact with its components. This can cause diminished engine performance, degrade fuel economy, and in severe cases the fluid can mix with engine oil, resulting in damage to pistons and other integral engine components.