The 1973 OPEC oil embargo had a massive impact on the auto industry. In short order, fuel economy suddenly moved to the forefront of deciding factors for new car shoppers. While fuel prices started to level out in later 70s, it had enough impact to change the American auto industry to focus on more fuel-efficient vehicles. Some of this was forced on them through the US Government passing the Energy and Conservation Act which introduced CAFE regulations that came into force for the 1978 model year. Ultimately though, when fuel prices spiked again due to the 1979 Oil Shock, it became clear that the enormous V8 gas guzzling dinosaurs of the early and mid-1970s were headed for permanent extinction.
To help improve fuel economy, initially automakers used band-aid solutions, such as very steep rear axle ratios, to increase fuel economy. As time went on, newer more efficient American cars were designed that focused on being smaller and lighter. Cars such as the GM’s 1977 B-body or Ford’s 1978 Fox bodies were radical departures from the past and offered significant improvements in fuel efficiency. During this time, engineers were looking at all areas of a vehicle to increase fuel economy, including things like low drag brakes, low rolling resistant tires, aerodynamics, and more efficient transmissions with less rotating mass and lockup torque converters.
When it came to automatic transmissions, the standard formula from the 1960s to the late 1970s was a three-speed automatic with a non-lockup torque converter. While there were some efficiencies gained with newer light weight transmissions, like GMs ill-fated TH200, ultimately what was really needed was an overdrive. Borg-Warner built an auxiliary mechanical overdrive for manual transmissions which was first offered by Chrysler in the mid-1930s. In the 1940s and 1950s other manufactures also offered these auxiliary overdrive units for manual transmissions. As fuel prices dropped, engines became more powerful and the automatic transmission became the primary choice of Americans, the overdrive option faded into the past during the 1960s for the most part.
It was during the 1960s that Ford had started to experiment with an overdrive equipped automatic transmission called the XT-LOD (Extension Lockup Overdrive). Ford envisioned an automatic transmission that featured split torque power delivery to help improve efficiency. However, the relatively low fuel prices meant that Ford ultimately decided that there would be little interest from the public. The project was shelved around 1966 until the oil embargo in late 1973 brought the project back to life.
Development continued through the 1970s for a 1980 model year release. The newly developed transmission was called FIOD (Ford Integrated Overdrive), however, it was renamed AOD (Automatic OverDrive) once it was released to the marketplace in late 1979. The AOD was considerably larger and heavier than Ford’s smaller C-4. It weighed in approximately 195 lbs with a torque converter, which was comparable to the heavy-duty C-6. It was also fairly close in width to C-6, but a bit shorter in length being about the same length as the smaller C4 and FMX. This meant an AOD could be utilized in most RWD Fords that used one of the aforementioned older transmissions.
This video shows a Simpson gearset. It only allows for three forward speeds.
At that time, Ford’s most commonly used modern transmissions were the C-4 and C-6. Both of these were 3-speed automatics that used the Simpson gearset as their basis. This was also the case with many torque converter equipped three-speed automatics of that era, including the Torqueflite 727 and the Turbo Hydramatic 350/400. While the Simpson gearset has some advantages like simplicity of design, compact size and ease of manufacture, it is limited to only 3 forward speeds. To add a 4th overdrive ratio, an additional gearset is required.
This video shows the Ravigneaux gearset. It allows for four forward speeds, one of which is overdrive.
Although Ford had introduced its Simpson based transmissions in 1964 and 1966 respectively, it also introduced the FMX transmission in 1968. Rather than being a clean slate design like the C-4 and C-6, the FMX evolved from the MX/FX transmissions, which evolved from the old Borg-Warner based Cruise-O-Matic transmission. Even in 1968, the FMX seemed to be a somewhat old fashion stop gap that only really only existed to help with production shortages. It used a heavy cast iron case and continued to use the Ravigneaux gearset of its predecessors. While the Ravigneaux gearset was only used for 3 speeds in the FMX, this gearset could actually be reworked to have four forward speeds, one of which being overdrive. So, when Ford started the development work on a 4-speed overdrive transmission, using the familiar Ravigneaux gearset provide a simple and cheap solution.
The Ravigneaux gearset used in the AOD had gears with the same tooth count as the FMX. This meant that the first three ratios of the two transmissions were identical. This gearset produced an overdrive ratio of 0.67:1, which was considerably steeper than most overdrives used on manual transmissions. Add-on external overdrives typically had ratios in the 0.7:1 to 0.82:1 range. The AOD overdrive was steep enough to reduce the final drive ratio of a 4.10:1 rear axle to 2.75:1! The reality of the times was Ford was seeking best fuel economy numbers possible, so this transmission used with far steeper rear axle ratios, typically in the 2.73:1 to 3.08:1 range. While this returned good EPA numbers, in the real world it made for some very over geared cars.
In addition to the overdrive, Ford engineers used other design improvements in the AOD to improve efficiency. Torque converters are very good at multiplying torque at low speeds, but are not overly efficient at high-speed cruise; some torque converters can slip as much as 10% at highway speeds. During the late 1970s and early 1980s, manufacturers started to add electro-mechanical torque converter lockup clutches. When the torque converter was locked-up, there was effectively a direct mechanical connection from the engine to the transmission eliminating all torque converter slippage.
Ford incorporated a lockup mechanism in the AOD, but it didn’t used the electro-hydraulically activated clutch mechanism like most other transmissions. It used a more unusual solution that was undoubtedly also cheaper. Instead of a lockup clutch, the AOD “lockup” was direct drive connection to the engine that bypassed the torque converter. To make this work the AOD had dual input shafts. One input shaft was connected to the torque converter turbine, while the other was a direct mechanical connection to the engine flexplate. Since there is no cushioning effect from the torque converter with the direct drive mechanical connection, Ford engineers incorporated a damper to help reduce noise and vibration. The transmission could use either the input from the torque converter, the direct drive connection, or a combination of both.
So how did exactly did this function? In the first two speeds, the transmission operates using the input shaft connected to the torque converter turbine. This allows for torque multiplication under acceleration, just like the other non-lockup transmissions. Once the transmission shifts into third, things get a little more complicated. Here the transmission input is split between the two input shafts, hence the split torque name. Approximately 60% of the input to the transmission comes from the direct drive connection, while the remaining 40% comes from the torque converter turbine. The method by which this is achieved is explained in great detail on Ate Up With Motor’s article.
The end result is third gear in the AOD does not have a true lockup, but the split torque input significantly reduces the torque converter slippage, increasing efficiency. The disadvantage was that if the driver wanted heavy to moderate acceleration in the third gear there was minimal torque multiplication available.
When the AOD shifted into 4th gear, the input was solely made by the direct mechanical connection to the engine. This was the AOD’s “lockup.” The reality was that it was just simply completely bypassing the torque converter in 4th gear. Like the third gear, this methodology worked but came at a disadvantage. The AOD generally upshifted from third to fourth around 40 mph to maximize fuel economy.
Unlike other overdrive transmissions with a lockup clutch mechanism, the transmission had no ability to run the torque converter unlocked in overdrive. So, when accelerating and the transmission shifts from third to fourth, the AOD tended to lug the engine as it immediately went into overdrive with full lock-up. Competitors transmissions would shift into 4th with the torque converter unlocked which reduced the RPM drop and would typically only lock-up once the driver have settled into a cruise.
Furthermore, if the driver needed moderate to heavy acceleration while in overdrive, the engine would lug down since the converter could not unlock. Once enough throttle input was used, a downshift to third would occur with a significant RPM gain. In comparison, most other 4-speed overdrive transmissions would unlock their torque converter first. This would increase engine RPM and performance through the additional torque multiplication; it almost acted like having an additional gear between third and fourth. If further power was requested, the transmission would downshift to third, but with the torque converter unlocked the RPM change would not be as dramatic as the AOD.
Ford engineers also adopted a different method to control the transmissions shifts with the AOD. Unlike previous Ford transmission which utilized a vacuum modulator and a kickdown rod to control the part throttle shifts, the AOD adopted a throttle valve cable (TV cable) or rod. Ford elected to use the TV Cable due to the inadequate vacuum signal in overdrive and less sensitivity to operating at different altitudes. This cable was connected directly the throttle and the transmission utilized this input to determine not only the shift speeds, but also the transmission line pressure. Thus, adjustment of the TV cable was critical to proper transmission operation and an improperly adjusted cable could lead to transmission failure. Ford also designed the valve body on the AOD to permit manual selection of only three of the four speeds. Gear selectors for AODs read OD-3-1, with no 2nd gear selection available.
In the fall of 1979 Ford introduced the AOD transmission to the marketplace making it the first automatic overdrive transmission introduced by an American manufacturer. It was standard equipment in the all-new downsized 1980 Lincoln Continental and Mark VI. It was available optional equipment on Ford’s LTD and Thunderbird as well as Mercury’s Marquis and Cougar. The big advantage of the AOD was the significant boost in fuel economy, but AOD equipped cars also had numerically higher rear axle ratios which helped to increase acceleration. The 1980 Ford LTD used a 2.29:1 rear axle ratio with a 3-speed automatic, but used a 3.08:1 rear axle ratio with the AOD. Ford claimed the AOD provided a 1.5 mpg boost to the overall EPA rating and a 4-mpg improvement in highway use along with an approximate 1 second 0-60 improvement.
Ford having used the Ravigneaux gearset continuously through the late 1970s certainly gave it a leg up on developing a new overdrive transmission. In comparison, GM’s first overdrive transmission was the TH200-4R, which was introduced a year after Ford. GM’s TH200-4R transmission used a more complex design which likely cost more to develop. Unlike the AOD, the TH200-4R utilized a conventional electro-mechanical torque converter lockup clutch. It also used a Simpson gearset which meant it required an added overdrive carrier/single gearset to add the 4th overdrive ratio. GM also developed the clean slate TH700-R4 overdrive transmission, which used a unique Simpson split-compound gearset, but it wasn’t released until 1982.
Automatic Comparison Chart
|Name||Manufacturer||Gearset||1st Gear||2nd Gear||3rd Gear||4th Gear||Reverse|
|E4OD||Ford||Simpson + extra OD gearset||2.71:1||1.54:1||1:1||0.71:1||2.18:1|
|TH200-4R||GM||Simpson + extra OD gearset||2.74:1||1.57:1||1:1||0.67:1||2.07:1|
|TH700-R4||GM||Simpson split compound||3.06:1||1.63:1||1:1||0.70:1||2.29:1|
Despite the fact that they all used different design concepts, the AOD, TH200-4R and TH700-R4 automatics all had significant teething problems that lead to unreliability for the earliest iterations. The AOD was continuously improved throughout the 1980s until it became a reasonably reliable transmission. Ford’s engines improved during this time too, which made the shortcomings of the AODs split torque operation less obtrusive. The AOD was utilized in many RWD Ford cars, including the Panther and Fox platform cars, and also was utilized in light duty trucks and vans.
For the 1992 model year, Ford introduced the AOD’s replacement, the AOD-E. The AOD-E was an updated version of the AOD, utilizing an electronically controlled valve body and a conventional electro-mechanical lockup clutch mechanism. This meant that the dual input shafts were gone and the AOD-E used a conventional single input shaft. The AOD was phased out of production after the 1993 model year. The AOD-E was eventually updated to the 4R70-W which use a revised Ravigneaux gearset. This new gearset created lower 1st and end gear ratios (2.84:1 first and 1.55:1 second) but a slightly higher overdrive ratio of 0.70:1. This wider ratio spread was better suited to the modular V8 engine’s power band.
Today, the AOD is often not remembered fondly, as is commonly reiterated on Curbside in the comment section. The AODs primary goal was fuel economy improvement, but it was used in some performance applications such has the high-output 5.0 Mustangs. That said, most enthusiasts of that time preferred the 5-speed manual due to the significant performance disadvantage of the AOD.
Ironically today, the AOD has received a bit of a reprieve in the aftermarket performance world. While in stock form they are not particularly strong or well suited for performance applications, an AOD can be built to be very strong good performance transmission with modern aftermarket parts. The strongest aftermarket versions eliminate the weak dual input shafts, split torque and lockup mechanism, making it a 4-speed non-lockup OD transmission. Specially built high performance AOD transmissions are even sometimes used in non-Ford applications, which is something that doesn’t happen with Ford’s other transmissions. There may not be too many of the 1980s Fords originally equipped AODs likely to become collector cars. But with overdrive swaps into old American cars being an ever-popular upgrade, I expect that many AODs will live on for a long time yet.