Jim Smart
July 1, 2014
Photos By: The Manufacturers, George Reid

Ford’s hardy cable-modulated Automatic Overdrive (AOD) was produced from 1980 to 1991, then became an electronically controlled transmission known as the AOD-E in 1991. Later, the AOD-E became the 4R70W.

In the beginning, the AOD was your basic automatic transmission with a lock-up Overdrive feature, first available in the Mustang in 1984. The AOD was not intended to be used as a performance transmission, as evidenced by Ford’s use in everything from vans to Lincoln Town Cars.

Though the AOD was a new transmission in 1980, it was basically the FMX geartrain package in a fresh aluminum case with an Overdrive/lock-up unit incorporated into the existing gear-train. The FMX’s time-proven Ravigneaux planetary gear set, known for extreme durability, is the heart of all AOD, AOD-E, and 4R70W transmissions. The AOD’s weakest link is its 1.50-inch-wide Overdrive band and Reverse clutch drum, which didn’t improve until the new ’93 Lincoln Mark VIII, which got the wider 2.00-inch Overdrive band and Reverse clutch drum.

The other sticking point with the AOD is that pesky throttle-valve (TV) cable and the split-torque feature that annoys a lot of enthusiasts. The TV cable must be adjusted spot on with a pressure gauge along with a test drive and good gut shift feel in the seat of your pants or you risk burning up the AOD. The AOD’s TV cable, tied to throttle movement, determines line pressure at various throttle positions. The more cable tension (throttle opening), the more line pressure there is for firmer shifts. In other words, at open or wide-open throttle, we want greater line pressure at the clutches and bands for a firm shift without slippage. By the same token, at light or no throttle, we want a soft shift or downshift.

The TV cable does the work of both the vacuum modulator and kickdown linkage on C4, C6, and FMX transmissions. It eliminates the vacuum modulator and the kickdown linkage, which were often problematic. TV cable tension coupled with output shaft speed (governor) determines shift firmness and timing.

Ford’s Automatic Overdrive (AOD), introduced in 1980, is a rugged and dependable transmission you can retrofit into your classic Mustang using off-the-shelf components and kits from the aftermarket. We’re going to show you how to make the AOD better.
The AOD functions via two levers located on the lefthand side of the case. The short lever, indexed at 7 o’clock, is the manual shift lever. If it is down, it is for a floor shifter. If it points upward, it is for a column shift. The dog-leg lever is the factory throttle-valve (TV) control, which is controlled by a cable tied to the throttle body or carburetor.

Building the Perfect AOD

In your search for an AOD core, you will want a late ’80s production unit, though nearly all AOD units dating back to 1980 can be upgraded using the right internal parts. AOD main cases are all basically the same. It was the internal gear-train that improved over time. Early AODs had teething problems easily remedied with revised gear-train and valvebody parts. There are cast drums (early) and steel drums (later), which are lighter and stronger.

Mike Stewart at Mike’s Transmission in Southern California builds hundreds of AOD transmissions every year for customers around the world. He views the AOD, AOD-E, and 4R70W as hardy transmissions that could use improvement. He strongly suggests the wider AOD-E/4R70W Reverse drum and Overdrive band, which take the load better. He also suggests the “A” Overdrive servo for greater Overdrive band clamping pressure. And finally, he suggests the 4340 chrome-moly input shaft, which eliminates the stock input shaft breakage issues. Mike says that with the right parts, the AOD can take up to 800 hp. Some can be built to withstand 1,200 hp using aftermarket parts.

If you take a bare AOD case and fill it with the AOD-E or 4R70W geartrain, coupled with the 4340 input shaft, you will have a fiercely rugged and dependable AOD. The difference between the AOD-E and 4R70W is gear ratio. The 4R70W offers better gearing, which means hot acceleration for your small- or big-block Ford. The 4R70W is an improved AOD-E designed specifically for the 4.6L Modular V-8, which lacks the snappy low-end torque of a small- or big-block.

What bugs people most about the AOD are its two input shafts and that dysfunctional split-torque 60/40 function. The smaller removable secondary input shaft is tied directly to the torque converter’s shell and forward clutch. The larger hollow primary input shaft is driven by the torque converter’s turbine (torque multiplication) in First, Second, and Reverse gears.

Ford calls this split torquebecause 40 percent of the engine’s torque goes through the torque converter as torque multiplication (via the impeller, stator, and turbine), while 60 percent goes through the smaller input shaft in Third gear. When the transmission shifts into Overdrive or Fourth gear, 100-percent of the engine’s torque goes through the smaller secondary input shaft as straight drive and lock-up in the gear-train and clutch pack. Tip in the throttle and the AOD goes into Third gear with the split-torque pattern. At WOT, 100- percent of the torque goes through the torque converter and hollow primary input shaft.

This is the AOD’s secondary input shaft, which is splined directly to the torque converter shell and the engine’s flexplate for direct drive function in Third gear and Fourth gear/Overdrive. Your AOD build should include a 4340 chrome-moly secondary input shaft for durability. If performance is your only goal, you want the chrome-moly one-piece input shaft, which does the work of both the primary and secondary input shafts, and keeps you on the torque converter for constant torque multiplication.
This is the AOD’s manual and throttle valve linkages—a shaft within a shaft. The outer shaft is the manual shifter. The inner shaft is for the throttle valve. “A” is the manual shift linkage with a roller detent to hold the shifter in a specific gear; “B” is the throttle valve (TV) linkage, which modulates line pressure relevant to throttle position. The TV linkage is linear and without detents.
This is the AOD’s output shaft governor, which controls line pressure and shift timing based on vehicle speed. Shift valves are fly-weighted and spring-loaded. This governor was phased out on the AOD-E and 4R70W and replaced with an electronic speed sensor.
Here’s the AOD’s Reverse drum (often mistakenly called the Overdrive drum), which is this transmission’s weak link. Both the Reverse drum and Overdrive band are not wide enough to hold securely and typically fail under the strain. Beginning in 1993, Ford fitted the AOD-E with a wider Reverse drum and Overdrive band for the Lincoln Mark VIII. This is what you want for your AOD.

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The Differences

In your search for an AOD core, you will find a variety of AOD types from different applications, including the AOD-E and 4R70W. At a glance, they all look the same. Closer inspection reveals huge differences in the AOD, AOD-E, and 4R70W.

All of these transmissions are of the AOD family, but they are different. The AOD is a mechanically-modulated transmission with a throttle-valve cable designed to modulate line pressure, shift timing, and shift firmness. The AOD-E and 4R70W are computer-controlled transmissions void of the TV cable that make engine and transmission function cohesive. With this, function happens via multiplex plugs and the PCM (Powertrain Control Module).

There are aftermarket electronic shift control solutions for the AOD-E and 4R70W, so don’t count them out for vintage overdrive swaps either.

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Here’s an AOD with the one-piece input shaft, which does the work of both primary and secondary input shafts. It keeps the AOD on the torque converter in torque multiplication mode. The down side is inefficiency, higher operating temperatures, and reduced fuel economy. However, you get real performance from your AOD.
This is the AOD’s input shaft package. There appears to be three shafts but there are actually two. The smallest splined shaft is the secondary, which splines directly into the torque converter shell. It turns all the time, but it is not always engaged. It becomes engaged in Third and Fourth gear only. Otherwise, it’s just along for the ride. The next larger hollow input shaft is the primary, which is driven by the torque converter turbine. It does its job in First, Second, and Reverse gear. The largest shaft is the stator support, which sits inside the torque converter to support the stator.