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Automatic transmissions are otherworldly to many enthusiasts. The ways they manipulate crankshaft rotation on demand and eliminate the need for a clutch border on the black arts to folks who have a gear-and-cog way of thinking. Getting fluid to bend corners is easy. Getting it to turn into a rock-solid 1-2 shift is the magic mojo that makes our auto transmissions so loved.
Enthusiasts who have no fear of pulling the engine for a total rebuild often cower away from doing anything inside of those fluid-filled gearboxes. There’s just something about them. Thankfully, there are people willing to break the surface and carry us beyond the dirty side of a gasket to give a little insight into how these beasts operate.
Irvin removes the pan and 4R70W valve body. Of course, there is fluid everywhere.
Needless to say, there are a lot of components inside an automatic transmission that allow it to do what it does. As you can imagine, one of those components is prone to fail at some time. In this video, the folks at Raybestos Powertrain show us a typical issue found with Ford’s 4R70W transmission.
Ford’s 4R70W automatic overdrive transmission can be found in many of Ford’s offerings. It was used in various trucks and cars throughout the early ’90s through 2005 as a computer-controlled successor to Ford’s AOD (and AOD-E) automatic overdrive transmission. As with any mechanical component, there are items which over time, prove themselves to be the part’s Achilles’ heel. This video by Raybestos Powertrain illustrates how one small item can affect the entire vehicle’s delicate ecosystem. In this case, the vehicle would seemingly go into “neutral” during the shift into overdrive (4th gear). This is a common occurrence with this transmission. Of course, we know that only manual transmissions pass through neutral on their way to the next gear, so clearly, something is wrong!
Irvin checks the overdrive servo and C-clip. It isn't until he looks at the valve body more closely that things begin to make sense.
Raybestos Powertrain Sales & Product Development Support Engineer, Irvin Gers pulls the pan to inspect the issue more closely. Even if they don’t know what it does, most enthusiasts know an automatic transmission has a valve body. It’s that chunk of metal mated to the labyrinth-like surface in the transmission’s case, and it’s actually the brains of the operation. How it does that is fodder for another story entirely, but for now, Irvin removes the valve body and after some inspection, finds the faulty part.
There is a C-clip that holds the overdrive servo in place. While ours was still doing that job, the ends of the C-clip had broken off and one of them found its way into the valve body. C-clips can shatter into many pieces, this is a common problem. (Photos: Raybestos and F150online.com)
There is a not-so-small C-clip that holds the overdrive (4th gear) servo in place which is prone to breaking. A perfect example of an $8 part needing a multi-hundred-dollar repair (labor and sales tax included). In this case, the ends of the clips had broken off, but the body of the clip still kept the servo in place. The problem was that the small broken bits found their way into the Labyrinth and wedged the servo regulator valve.
Irvin points out the path the small piece of C-clip negotiated to wind up in the servo regulating valve.
This meant the forward clutch was released (3rd gear) but the overdrive (4th gear) band did not apply due to the jammed valve. A quick way to check if a transmission is simply not going into overdrive is to hit the “overdrive cancel” button and see if the transmission goes back into 3rd gear. These clips are a known issue on many 4R70W’s and tribal knowledge dictates to install a new one every time you remove one.
With that, Irvin reassembled the transmission.
In this video, it was only the ends of the clip that separated, and with no ears to grab onto, Irvin cleaned out the errant bits and left the remaining clip to do what it had been doing for decades. The valve body and pan were again installed and the 4R70W transmission filled with that mysterious liquid that makes tires squeal at will.
While this video shows a lot of detail directed to one area of concern for these transmissions, we’ve also included another example of YouTube gold that shows an entire teardown of a 4R70W transmission for those truly inquisitive readers.
Enjoy them both and who knows, maybe one day you’ll find yourself staring at a valve-body of your own choosing. Then would be a good time to show your friends a thing or two about these mysterious self-shifting gizmos.
