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How to Build a Fiercely Reliable AOD-E/4R70W Transmission
Things you need to know about Ford’s AOD, AOD-E, and 4R70W transmissions
Few things tend to confuse us more than the AOD family of automatic overdrive transmissions. Interchangeability. Swapping this and that. Should you go with a mechanically modulated AOD or electronically controlled AOD-E or 4R70W? What’s the difference between an AOD-E and the 4R70W? What is a 4R75W?
The original AOD (Automatic Overdrive) transmission was basically the old cast iron Ford FMX reborn as a lighter more fuel efficient overdrive slush box in 1980. The AOD had the FMX’s super rugged Ravigneaux planetary gear set with the addition of an overdrive unit in front of the all-new aluminum case. The AOD was developed as an all-inclusive automatic overdrive transmission with the overdrive designed into the unit instead of being an afterthought like they did at GM and Chrysler. Ford integrated overdrive into the FMX’s existing gear set with the same gear ratios throughout. Overdrive was 0.67:1.
Instead of a conventional locking torque converter like Ford had in its C5 three-speed automatic (C4 with a locking torque converter), lock-up would be via what was known as Extension Lock-Up Overdrive or “XT-LOD”. The AOD’s development began many years before it entered production. At a time when overdrive was very uncommon, the early 1960s, Ford was working on an automatic overdrive transmission. Development went on for years with different variations being developed and tested. It would be 18 years before a suitable example would be ready for production.
The AOD transmission employed a throttle valve control cable instead of the more conventional vacuum modulator and kick-down linkage associated with the C4, C6, and FMX transmissions. The throttle valve (TV) cable and modulator did the work of both the vacuum modulator and kick-down linkage. What makes the TV cable tricky is finite adjustment. It must be adjusted to the letter per your Ford Shop Manual and a pressure gauge or transmission failure is inevitable.
Early on, the AOD wasn’t much on durability. They suffered from significant failure issues especially in performance applications. The weak link was the overdrive band and really screwed up shift program that made this thing nearly impossible to race. It was via the tenacity of serious racers we learned how to live with the AOD. The AOD became a more user-friendly transmission in the middle of the 1980s when the Mustang took on more 5.0L muscle and Ford was under the gun to build a more durable automatic. Despite improvements in the AOD shift-quality issues remained for performance enthusiasts. What’s more, federal emission and fuel economy standards tightened the screws making it necessary to rethink the AOD’s control system.
The Big Three automakers all had to look at mechanically-modulated automatic transmissions and examine how to fine tune shift function. GM looked at its 700R4 as did Chrysler and the 518 overdrive. Like its Detroit counterparts, Ford found it was becoming increasingly difficult to achieve pinpoint shift accuracy with mechanical modulation. The AOD was never easy to fine tune and has served as a lesson in frustration for many. If ever you’ve spent any time working with the AOD you understand what we’re talking about. You do the monkey motion with the TV cable or linkage, take it for a spin, and still can’t get the darned thing to shift right. It slips. It clunks. It shifts too early or too late. And, as an extra added attraction you get to burn up the overdrive band in the process.
Ford revisited the AOD and developed a new computer-controlled version of this box known as the AOD-E. (“E” for electronically controlled). The AOD-E was one of Ford’s first attempts at unifying engine and transmission function.
In 1991, Ford introduced the computer-controlled AOD-E, which was first installed behind Ford’s new SOHC 4.6L Modular V-8 in the Lincoln Town Car. In the years ahead the AOD-E would find home in more and more applications as use of the Modular engine expanded to more and more carlines and F-Series trucks. It did find itself behind the small-block as well in the 1990s.
Although the AOD and AOD-E/4R70W appear very similar they are in fact different. Case and valve body parts do not interchange. The only interchangeability is the geartrain inside and you must swap all of it into your AOD for it to work. Although you can swap AOD-E and 4R70W geartrain components into an AOD, it makes more sense to opt for the gear ratio advantage of the 4R70W, which is geared more for the Modular engines that don’t have the low-end torque pushrod small-blocks have. The 4R70W is little more than the AOD-E with 2.84:1 first gear, 1.55:1 second, which makes it a great candidate for the holeshot. It gets you off the line more quickly than the AOD/AOD-E. Overdrive is better too at 0.70:1, with lower cruise rpm around 2,000 rpm at 70 mph depending upon axle ratio and tire diameter.
The AOD-E and 4R70W employ lighter weight components such as an aluminum front pump versus cast iron in the AOD. Stamped steel clutch packs versus cast steel is another example of weight loss in the AOD-E and 4R70W.
1. If you have the right tools, the AOD-E/4R70W can be torn down and rebuilt on your workbench though this is discouraged. Key to success is paying very close attention to what you’re doing. Jaime Sanchez of TRC Transmissions in Chatsworth, California has experience with untold dozens of AOD-E/4R70W transmissions and is going to walk you through the process of understanding this box. Disassembly begins with removal of the AOD-E/4R70W’s front pump.
2. The AOD-E/4R70W is lightweight aluminum with a steel G-rotor package inside. The front pump also serves as the intermediate clutch cylinder (arrows), which applies pressure to clutches and plates. Examine pump rotor parts and bushing for wear and tear.
3. With the pump/intermediate clutch piston removed you have access to the intermediate clutch package. The intermediate clutches splines internally into the forward clutch.
4. This is the forward/reverse clutch, which is tied to the input shaft. What makes the AOD-E/4R70W different than the AOD is this single input shaft versus the AOD’s dual input shafts. The reason for the difference is overdrive lock-up (AOD) versus converter lock-up (AOD-E/4R70W).
5. Reverse clutches stack up like this inside the overdrive drum. If your original clutch steels look good and are void of heat damage you may reuse them. You can resurface used clutch steels and achieve a good crosshatch pattern for reuse as long as there isn’t heat damage. TCI provides new clutches and steels in each of its overhaul kits.
6. Another difference between the AOD and AOD-E/4R70W is this mechanical diode, which replaced the AOD’s conventional roller clutch. The mechanical diode, which isn’t much more than a redesigned heavy-duty roller clutch, withstands more punishment than the AOD’s roller clutch. This one has 260,000 miles on it and is ready for more.
7. This is TCI’s PN 438960 Pro Super Kit for the AOD-E/4R70W. The TCI Pro Super kit consists of the parts necessary to facilitate a transmission rebuild including gaskets, seals, pan gasket (reusable type), performance frictions, steel clutches, bands, and input and output bushings. The Pro Super Kit also contains the TCI Trans-Scat valve body kit, which enables you to modify the valve body to a manual/automatic setup to deliver both a good solid shift that's suitable for towing applications and competition quality shift for drag racing. Not shown here is the overdrive band also included in the kit. It is suggested you order a complete bushing kit (PN 438602) just in case bushings show abnormal wear.
8. There are the electrical components that make an AOD-E/4R70 function. From bottom clockwise are the main wiring loom (bottom), SS1/SS2 shift valve solenoids, converter clutch solenoid, and the electronic pressure control (EPC) solenoid. Each of these solenoids provides a specific function. SS1/SS2 solenoid control shift function. The converter clutch solenoid directs hydraulic pressure to the converter clutch. The EPC controls line pressure.
9. Jaime stresses extra care when installing clutch piston seals ascertaining seals are pointed in the right direction. Seal lips must be pointed toward the pressure source. Otherwise they will leak and you will not get clutch piston function. Seals must be bathed in transmission assembly lube. Look for both inner and outer clutch piston seals.
10. Here are the new reverse clutch frictions and steels. Some transmission builders like to soak clutch frictions in Mercon IV before assembly. At the end of the day it doesn’t matter because they will become soaked when you start the engine.
11. This is the direct clutch, which splines to the output shaft. Clutches and steels are alternately dropped into the direct clutch drum along with the clutch hub. Check clutches, steels, and hub for freedom of movement.
12. Whenever you’re installing shaft seals, make sure the seal lip is generously lubricated. If you are working with a seal equipped with a garter spring, pack the garter spring pocket with assembly lube to ensure the spring does not escape during installation. Seals should be pressed into place with a seal driver. Do not beat the seal with a hammer.
13. Pump rotor is positioned with the flats and indents pointed toward the torque converter. If you install the rotor backwards, and even the most experienced technicians make this mistake, you will not be able to seat the torque converter.
14. TCI Automotive offers complete AOD-E/4R70W bushing kits. Replace bushings only when necessary. Pump and tail shaft bushings should always be replaced.
15. The planetary gear set is seated into place as shown, splined into the direct clutch. It is best to rebuild all gear train components on the bench, then, stack them into the case one at a time.
16. This has long been the AOD’s weak link; the overdrive band and drum. The drum houses the reverse clutch. When Ford introduced the AOD-E and 4R70W it opted for a wider band and drum known as the Lincoln overdrive band/drum though these wider components are standard in the 4R70W. In 260,000 miles of use there is virtually no drum or band wear.
17. When you install the overdrive band, check for clutch piston engagement as shown here. If you forget to seat the piston pin you will not have overdrive.
18. This is the EPC (Electronic Pressure Control) solenoid, which controls line pressure. You want high line pressure during acceleration. You don’t want much of it during deceleration because you don’t want to feel the downshifts. At wide-open-throttle, you want high line pressure and solid clutch and band engagement.
19. Jaime checks all valve pistons for proper function. Each valve piston is a precision fit and must be generously lubricated for smooth function.
Another significant difference between the AOD and the AOD-E/4R70W is the lock-up feature. Where the AOD has an overdrive lock-up unit, the AOD-E/4R70W goes back to a conventional locking torque converter and a single input shaft versus the twin inner/outer shaft we see on the AOD. The AOD-E/4R70 has an EPC (Electronic Pressure Control) solenoid as a means to more predictable and controllable line pressure. The manual shift control on the AOD-E and 4R70W is equipped with a sensor for improved control.
On the right hand side of the AOD-E/4R70W is a multiplex plug tied to two shift control solenoids (SS1 and SS2), a converter clutch lock-up solenoid, and the EPC solenoid. Although you will likely never see a 4R75W this updated 4R70W series transmission is equipped with an input shaft sensor as well as the output. The 4R70W is fitted only with the output shaft sensor.
The AOD-E/4R70W have even more features beside weight reduction and computer control. They’re fitted with thrust bearings, which replace the outmoded thrust bushings and reduce internal friction by a wide margin.
20. Jaime Sanchez reassembles the valve body after thoroughly checking function and properly placing each of the check balls. This TCI illustration shows you shift function and how it happens. If you are installing a TCI shift improvement kit, it is vital you follow instructions closely. Because this 4R70W already has a TCI shift improvement kit installed previously, we’re not going to change anything.
21. Think of these solenoids as the brains of your AOD-E/4R70W. The twin SS1/SS2 solenoids (white arrows) control shift operation. The lone solenoid in black (black arrow) is the torque converter clutch engagement solenoid. If you’re having shift control or converter engagement issues this is one of the places you should look first. The EPC solenoid (green plug on far right) controls overall transmission line pressure.
22. This is the manual shift control, which is mechanical with a roller detent to maintain the chosen selection. This bellcrank moves the manual valve to direct line pressure to give you Reverse, Drive, Drive 2 and Drive 1. Overdrive is selected electrically via an on/off switch in the shifter. Parking pawl (not pictured) locks the output shaft.
23. Ron Hazelton of TRC Transmissions advised us of accumulator spring issues with the AOD, AOD-E, and the 4R70W. During disassembly we found these broken accumulator springs, which had to be replaced. Broken springs gave us a harsh upshift but otherwise did not affect transmission function. The harsh shifts are what gave us longevity.
24. The AOD-E/4R70W is equipped with this rotary switch at the manual shift control, which controls neutral safety and back-up lights just for starters. Adjustment is simple. All you have to do is line up the reference marks and lock these bolts down.
25. This is the 2-3 accumulator piston assembly employing two springs. The 1-2 shift accumulator (not shown) has one spring.
26. This is the Trans-Scat shift improvement kit from TCI Automotive. Oh sure the Trans-Scat kit will afford you a solid connection between engine and the rear wheels. However, the Trans-Scat kit will also get you longevity, a long service life because it eliminates slippage. This particular 4R70W had a shift improvement kit installed early in its service life. It went 260,000 miles with regular fluid and filter changes. No visible wear and tear.
Although we’ve shown you the ropes of the AOD-E/4R70W this is by no means a step-by-step rebuild manual. Rebuilding the AOD-E/4R70W calls for special tools a professional transmission shop will have. You can use hardware store style C-clamps to compress clutch piston springs and it’s always a good idea to have C-clip pliers and the like. Where it gets challenging for the novice is elements only the experienced understand. Whether you knock down and rebuild an AOD-E/4R70W or hand your transmission over to a professional it’s always a good idea to understand what goes on inside and what you’re going to need to make it happen.
When the AOD-E/4R70W transmission is built properly using only the best available parts, along with a good dyno tune (yes, a dyno tune to get precise shift control), you can count on rugged dependability from your AOD-E-4R70W.
|AOD/AOD-E Gear Ratios|
|4R70W Gear Ratio|
Why Install A TCI Trans-Scat Shift Improvement Kit?
Although shift improvement kits have always had a racing demeanor they do more than just improve the connection between engine power and your Mustang’s rear wheels. They improve line pressure to clutches and bands, which eliminates slippage and overheating. When automatic transmissions exhibit a soft shift this indicates clutch and band slippage. For the street you want a firm shift, but not an upshift that will jar your teeth out. If you’re going racing you want a slam-bang shift and the continuous application of torque.
The Trans-Scat® Valve Body Kit, PN 436001, is engineered to deliver a good solid shift for towing or performance driving resulting in a harder, more positive shift and allows you to downshift the AOD-E/4R70W on demand.
Automatic transmissions are very unforgiving of torque converter installation error. Both the AOD and the AOD-E/4R70W call for strict attention to detail during torque converter installation. The AOD’s torque converter requires the engagement of two shafts, which means the shafts must be engaged twice before they actually seat. Always ensure the torque converter is seated firmly and check rotation. If the converter is completely seated you will not be able to get your fingers behind it. If there is resistance or binding remove the converter and try again. If the converter isn’t seated in the front pump properly transmission damage and failure will result.
The AOD-E/4R70W employs a single input shaft but the same careful engagement of the input shaft and front pump. Once you have the converter seated it is important to maintain the installation. We came up with these muffler hangers as a means to keep the torque converter seated during transmission installation. They cost roughly $4 apiece and are well worth the money, especially when you consider the cost of a failed front pump.