Ford AOD Transmission

Ford AOD Transmission

With the introduction of the AOD transmission. Ford became the first manufacturer to offer an automatic transmission with a built-in overdrive. The AOD is a 4-speed transmission that uses the Ravigneaux gear train.

Another feature of the AOD is the split-torque-type torque converter. This arrangement consists of two input shafts driven by the converter. One of these shafts relays torque into the transmission through the normal hydraulic action of a converter and the other shaft provides a mechanical link from the engine to the gear train when the transmission is operating in third and fourth gears.

The AOD uses the common ring gear as the output member. Two sun gears are used, a small one called the forward sun gear and a larger one called the reverse sun gear. The planetary gearset has two sets of planetary pinions gears, three long and three short ones. The planetary pinion gears are able to rotate on their own shafts while sharing the common planetary carrier.

The AOD uses four multiple-disc clutches, two one-way clutches, and two bands to obtain the various gear ranges. The forward sun gear is driven by the turbine shaft when the forward clutch is applied. The reverse sun gear is driven by the turbine input shaft when the reverse clutch is applied and is held, during overdrive, by the overdrive band. The reverse sun gear also can be held by applying the intermediate clutch in second, third, and fourth gears.

The single planetary carrier assembly can be held by either the low/reverse band or the low one-way clutch. The long pinion gears are in constant mesh with the ring gear, the reverse sun gear, and the short pinion gears. The short pinion gears are in constant mesh with the forward sun gear and the long pinions.

The short pinions do not mesh with the ring gear but can drive it through the long pinion gears. The ring gear is splined to a flange on the output shaft and the ring gear is always the output member.

Input Devices The AOD uses a torque converter that splits power flow from the engine. The converter drives two input shafts to provide both a hydraulic and mechanical input into the transmission. A typical input shaft from the converter's turbine hydraulically drives the forward clutch drum. An additional input shaft linked to the converter cover mechanically links the output of the engine to the direct clutch. The amount of input from either or both sources varies according to the operating gear. When the transmission is operating in first, second, and reverse gears, 100 percent of the input is delivered hydraulically by the turbine shaft. When operating in third gear, the gear train receives approximately 40 percent of its input from the turbine shaft and 60 percent is delivered through the mechanical linkage. In fourth gear, all of the input to the gear train is through the mechanical linkage.

Regardless of the input into the transmission, the gear train receives power through the application of the forward, direct, or reverse clutches, which serve as the input devices for the gear train.

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