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21 April 2018
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Content > Engines & Driveline > AOD upgrades > Mustang Forum Australia - Mustang Tech
Info and parts to build a better AOD
Ford's "Automatic Over Drive" gearbox was used in a variety of vehicles from the early 80's through early 90's.
It's name was derived from the fact that the Over Drive (4th) gear was engaged automatically under the right conditions.
This gearbox was used around the time that fuel economy started to become a factor in the US, and it behaves accordingly in stock format. Early gear changes and lazy down shifts are common.
A fresh AOD in stock form should be good for up to 450HP if you can put up with it not being optimised for performance.
It should be noted that the AOD was put behind both 6cyl and V8 engines. The 6 cylinder AODs had less clutches in a couple of the drums. If you are going to rebuild the box, then this is not a concern as you can rebuild them to V8 specs anyway.
The AOD has a few weak links. Depending on the horsepower of your engine and your driving habits, only some of these may need to be rectified.
Note: For a comprehensive list of AOD upgrades, please see the SilverFoxTrans PDF located in our downloads section here:SilverFox AOD Tech Mods
A list of parts required to upgrade the AOD using 4R70W internals is here:
The list should be pretty good, but please do your own checking.
|Inner Input Shaft||This shaft is used for 3rd and 4th gear and connects directly to the converter body (so there is no slip). These can break under heavy throttle when engaging 3rd or 4th gear. This is the easiest part to replace before you install the gearbox, so I recommend doing it.||Do it!||300M or Chrome Moly input shaft.|
|Itermediate Input Shaft||This is the hollow input shaft. It is a weak point in the AOD over 500 HP. You can switch to a 1-piece input shaft, but you will no longer have lockup operation, so fuel economy will suffer. You will also need a converter designed for the 1-piece shaft.||Do it if you are over 500HP||1-piece input shaft.|
|Intermediate sprag snap ring||This ring has been known to come out under high rpm causing failure of the sprag.||Do it! |
If you are rebuilding the box, this is a must
|Replace the standard snap ring with a spiral ring kit|
|Over Drive Band and Reverse Drum||The OD band is clamped to the reverse drum when OD is engaged. In the stock AOD, the OD band and Reverse Drum are 1.5" wide. This is known as one of the weakest points of the AOD, and can fail when the box goes into OD under heavy throttle in performance applications.
There are a number of options to improve the OD strength.||Do it! |
Changes made depend on level of performance and driving habits
|There are a number of fixes available and I will list them from easiest to hardest to implement. |
- OD Servo. The servo is what clamps the band to the drum. There were different sizes available - C, B and A. The C was the weakest, and A was the strongest. The "A" servos are apparently hard to find, but you can get an aftermarket A+ (or jumbo) servo for around $150 which provides an improvement even over the "A" servo. If you have a B or C servo, you should definitely consider the A+. The OD servo can be replaced without gearbox removal, but the pan and valve body need to be removed.
- OD band. As mentioned, the stock OD band is 1.5". You can replace this band with a Kevlar or Carbon Fibre band for higher duty and along with the A+ servo, this will handle many applications. How often will you hit 4th gear at full throttle? This band requires the box to be dismantled, so if this is a high performance (500HP plus) application, you might want to consider a wider drum and band instead (see below).
- 2" band and drum. This is the most involved and most expensive of the options, but will all but cure this weak point completely. There are a number of components required, but it does improve a few areas, such as replacement of roller (one way) clutches with mechanical diodes from the later AOD-E/4R70W boxes.
You will need a number of components - see http://www.transmissioncenter.org/ and search for text "#1CK". You also need to check that your case can fit these components.
|Cast Steel Drums||AOD boxes were produced with cast steel drums. These have been known to break, and were replaced with stamped steel drums in later boxes.||If you abuse gearboxes, do it.||Replace the cast steel drums with stamped steel and associated clutch packs etc|
|Clutches||This is applicable to any Auto box really, but the AOD's came in various forms over their lifetime. 6 cylinder applications came with less clutches in them, but aftermarket kits allow additional clutches even in the V8 applications.||Not necessary, but if you are rebuilding anyway you might as well||Clutch packs such as the Alto Red Eagle packs can allow more clutches and steels to be installed. For example, increase from 5 to 8 clutches in the direct (3RD) clutch drum. Some 6 cylinder applications may need the drum replaced, or the snapring groove machined higher to fit more clutches.|
To really wake up your AOD, the best modification you can do is to modify or replace your valve body.
A "shift kit" may improve it, but by far the best option is to replace the valve body with one already modified by a reputable dealer.
I have personally installed three "Silverfox"
valve bodies and the improvement is amazing. A valve body replacement can have the following benefits:
- Stronger and later shifts
- Removes the "AOD (or 1-D-1) shuffle". When manually shifting an AOD transmission, shifting from position 1 to 2 will change to second gear, but will also allow the box to shift to 3rd gear on its own. To hold the gearbox in second, you need to shift 1-2, then back to 1. When you want to go in to third, you shift to position 2 again.
- TV cable is no longer sensitive to bad adjustment
- May provide a solenoid to lock out the overdrive.
- Some adjustment of WOT shift points
also make AOD valve bodies and are known to be one of the best available, but I do not have any personal experience with them.
When doing a custom AOD install, most people use the aftermarket Lokar TV cable.
We have found that the spring supplied with this cable is often not up to the task, so an additional spring/bracket is installed down at the gearbox to help return the TV lever properly.
Below is an example of the extra spring, with a bracket connected to the gearbox mount.
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