Do you own an older Ford vehicle from the 1980s or early 1990s? It probably has an automatic overdrive (AOD) transmission. AOD introduced groundbreaking features that we take for granted on modern vehicles, the most important of which is the overdrive gear. While it does have drawbacks, the Ford AOD transmission can still deliver good performance.
Overdrive is a higher gear that drives the output (the driveshaft) faster than the engine RPM. But for it to work properly on an automatic transmission or transaxle, a lockup converter is necessary to directly connect the transmission input directly to the engine, which, on the AOD, happens in 3rd gear and lockup remains in 4th gear (overdrive) as well.
Overdrive is a higher gear that drives the output (the driveshaft) faster than the engine RPM.
– Richard McCuistian, ASE Certified Master Automobile Technician
The AOD transmission is the first four-speed automatic overdrive transmission in Ford vehicles. It entered service in 1979-1980 and replaced numerous older Ford transmissions, including the C4, C5, and FMX.
The interesting element of overdrive transmissions is that they became popular (even on manual transmission vehicles, particularly) in the 1960s but disappeared for nearly 10 years. The AOD transmission reappeared in 1979 on the Crown Victoria, elevating the fuel economy of those vehicles to about 25 mpg.
Development of what would eventually become the AOD began in 1962. Ford sought an automatic transmission that was easy to operate and consumed less fuel.
Engineers started with the Ford X automatic transmission, an older design that dated back to the 1950s. They took the X transmission’s reliable Ravigneaux planetary gearset and built the new system around its gears.
The new transmission added a split-torque application to its third gear that led to a fourth gear, the overdrive gear. It also featured a lockup torque converter to improve fuel economy while maintaining acceleration. However, the torque converter didn’t have a damper that would reduce vibrations generated by the running engine. This came later with modulated torque converters, which, as mentioned, became common on most transmissions by the late 1980s.
Ford assigned the name Extension Lock-Up Overdrive (XT-LOD) to the new transmission. However, the company halted development of the XT-LOD transmission in 1966, four years after starting the project. It was the Swinging Sixties, the time of full-sized cars and cheap oil prices, and no one was interested in fuel-efficient vehicles.
Everything changed when the 1973 oil crisis struck. Fuel prices skyrocketed as supplies dwindled, forcing car manufacturers like Ford to find ways to reduce fuel consumption.
One approach was to downsize full-sized cars. Smaller, lighter vehicles generally used less fuel.
Ford also needed a new transmission for these downsized cars. The new system must also consume less fuel.
In 1974, Ford dusted off the XT-LOD transmission. Its engineers updated the unfinished design by adding the damper to the torque converter. The XT-LOD was renamed the Ford Integrated Overdrive (FIOD) and then the Ford AOD transmission before release.
The AOD transmission was put into various Ford models built between 1980 and 1992. Other car manufacturers followed suit with their own automatic transmissions with overdrive.
No, the AOD is a mechanical transmission that uses hydraulic fluid. However, it did serve as the basis for the 4R70W, which is the electronic version.
In 1992, Ford revamped the AOD transmission to use electronic controls instead of a mechanical system. The resulting AOD-E design shares many parts with AOD, but the two systems aren’t 100% interchangeable. Their different control systems preclude a total replacement.
Ford would continue refining the AOD-E transmission into more efficient 4R70W and 4R75W systems. It’s also possible to modify them for even better performance.
The Ford AOD transmission borrows a lot from the older three-speed C4 transmission, which also traces its ancestry to the X transmission. AOD retained the C4’s drive ratios in its first three gears.
A twin input-shaft system transmits engine torque to the AOD transmission. The outer shaft uses the torque converter to control reverse, first, and second gear clutches. Meanwhile, the inner shaft runs the overdrive gear clutch and directly draws torque from the engine.
The third gear’s split-torque application divided the torque between different routes. Splitting torque reduces the contact force on the gear teeth, allowing smaller, lighter gears to save space and weight.
In the AOD transmission, third gear is split between the inner and outer shafts. The inner shaft handles 60 percent of the engine torque. It’s also the smaller of the two shafts, so it’s more likely to wear out or break down.
A lockup torque converter features a clutch that can lock the engine and the transmission input shaft together. When the driver engages the clutch, it produces a 1:1 drive ratio, making it a direct drive ratio. That means the transmission input shaft rotates at the same speed as the engine’s revolutions per minute (RPM).
However, the transmission must perform better than 1:1 to save fuel. That’s where overdrive comes in. The overdrive gear has a drive ratio of 0.67:1. It needs less engine RPM to spin the wheels faster for increased acceleration.
The AOD transmission relies on a throttle valve linkage or cable assembly to manage the gear shifts.
The Ford AOD was the first automatic transmission that introduced overdrive as an integral capability. Earlier automatics either lacked the overdrive gear or added it on later.
You can find the neutral safety switch of the AOD transmission over the valve body on the driver’s side. Similarly, the speedometer drive cable hooks up to the vehicle speed sensor on the driver’s side.
The AOD transmission is heavier than the older C4 transmission, weighing150 pounds compared to the C4’s 110 pounds.
The AOD often gets compared to the GM TH400. While the GM design is more popular, the Ford transmission does enjoy some advantages.
The first advantage of the Ford AOD transmission is its lower weight. It uses an aluminum valve body, and its center support section saves more weight. The AOD also has a lighter rotating mass. The combined weight savings reduce the required engine load to move the vehicle, which saves fuel.
Second, the AOD transmission embeds both sets of its planetary gears into a single carrier. In certain scenarios, this simpler design enjoys advantages over those that require two carriers, such as the TH400.
Last but not least, the AOD has a thinner pump gear. The thin gear reduces drag for line pressure in certain applications.
It’s possible to modify the Ford AOD to make it a “bulletproof” transmission. However, such customization requires extensive mechanical knowledge.
Many Ford models manufactured between 1980 and 1992 used the AOD transmission. Its AOD-E version and successors continued to see use in select Ford vehicles until the early 2000s.
The original AOD transmission was used on the following Ford models:
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