A500/A518 4-Speed

Fig. 2 A-500/518 automatic transmission:

Both the A-500 and A-518 4 speed overdrive transmissions are two-section designs. They consist of a modified Chrysler three speed transmission with an overdrive unit or rear unit, which replaces the extension housing and provides a fourth gear ratio of 0.69 to 1, Fig. 2. The front section of A-500 overdrive unit is a modified A-999 three speed transmission. The front section A-518 overdrive unit is a modified A-727 three speed transmission.

Fig. 3 Valve body lower housing. A-500:

Fig. 4 Valve body lower housing. A-518:

The components in the front section of A-500 and A-518 are basically same as those in Chrysler LOADFLITE transmissions. However, different components are needed to attach the overdrive unit. The three speed transmission is equipped with an intermediate shaft instead of the output shaft, which serves as the front section output shaft and connects the overdrive unit to the front section. The standard LOADFLITE valve body is modified by adding several components to the lower housing, Figs. 3 and 4. These components include an overdrive solenoid, lock-up solenoid (A-500), 3-4 shift valve, 3-4 timing valve, 3-4 accumulator and 3-4 shuttle valve.
Fourth gear (overdrive) is electronically controlled and hydraulically activated. A variety of sensor inputs are supplied to the engine controller, which operates valve body solenoids. The overdrive control switch is located in the instrument panel. In On position, automatic shifts into fourth gear overdrive will occur. In Off position, the switch overrides electronic control module preventing a shift into fourth gear. Fault codes are employed to help diagnose electronic components that operate overdrive unit and lock-up converter.