This is a technical article to give consumers a better understanding of a few of the recently debated arguments about automatic transmissions. As always I’m going to do my best to explain these topics using grass roots terminology so everyone can understand
First off is bind up. We have one company saying bindup is the single largest cause of failure of the Chrysler 47RH/RE transmission. We have another company saying there is no such thing as bindup because Chrysler uses snap action shift valves and it is nothing to worry about. Furthermore, it can ONLY occur if the transmission is built improperly.
Who is the uneducated customer supposed to believe? Obviously, one of these companies is right and one is wrong. I’m going to provide some technical information and let the consumer decide for himself.
Let’s first talk about the snap action valve theory. A snap valve is one that instantly pops from one state to the other with no time in between. With a snap action valve there is no such thing as a partial stroke.
Here’s a picture of the 2-3 shift valve from the Chrysler service manual. On top is the valve in the unstroked position (2nd gear). The lower schematic is the valve in the stroked position (3rd gear).
Here’s a brief rundown of how the valve cycles. The 2-3 shift spring (indicated in red) holds the valve to the left. The left most oil circuit is governor pressure. This pressure is 0 when the vehicle is not moving and steadily increase with vehicle speed. Governor pressure is trying to push the valve to the right. We have a tug of war so to speak. Each force is pushing on the valve. When the spring is winning the battle, the valve stays to the left (2nd gear). When the governor pressure overpowers the spring, the shift valve moves to the right (3rd gear). Simple enough.
But the question was, is this a snap action valve?
Let me bring in an example to illustrate my point. The suspensions on a truck or car all basically work on the same principle. Whether it’s a coil sprung suspension or a leaf sprung suspension they all work the same. As you apply more and more load, the vehicle sits lower and lower. We’ve all seen this whether it was loading firewood into the bed or watching a group of grown men climb into the bed of a truck. As each person climbs in, the suspension compresses little by little. This is because springs have a progressive force to them. As you compress a spring further and further, it takes more and more force to compress it.
Well, the exact same principles apply to shift valves. As you slowly increase the governor pressure, the shift valve SLOWLY compresses the shift spring and strokes the valve little by little until enough pressure is available to fully compress the shift spring. If at some point during this slow stroking process, you stopped accelerating, what do you think would happen? Would the valve instantly jump to one extreme or the other because we want it to, or do you think it would stay right there in the middle?
Just like your suspension doesn’t sit ONLY at normal ride height or all the way down on the bump stops. It will sit at either extreme or anywhere in the middle depending on how much load is applied to it. Well, that’s the EXACT same principle that controls the shift valves.
What if I told you that the rear suspension on my truck would not sag even a quarter of an inch under 2000 pounds of load, but when I added 1 more pound to it, it would fall all the way down to the bump stops. Then when I took one pound of load off the bed, it springs all the way back up to normal ride height. If I said that, most of you would laugh in my face because you know better. You’ve been around suspensions long enough to understand it simply doesn’t work that way.
When someone tells you the Chrysler trannies use snap action valves that instantly jump from one side to the other and can NEVER be partially stroked, that is EXACTLY what they are telling you. Yet, people believe it because the average person doesn’t have any experience with valvebodies and they trust without first doing their own research.
I intentionally left out the effects of the throttle valve circuit to help simplify the explanation a bit. To be technically correct, I have to mention it. The function of the throttle circuit is to make the system act as if the spring is stronger than it is. This is what allows the transmission to shift at a higher RPM as the throttle is pressed further down. But it in NO way changes how the system works or prevents slow or partial strokes.
That about covers the snap action valve theory.
Now let’s take what we’ve learned and dig a bit deeper into bindup.
Here’s a picture of a stock front servo and cap.
When the transmission shifts into 2nd gear, pressure is applied to the region with the red arrow. This pressure forces the servo to the right extending the push rod out to apply the second gear band of the transmission. That’s simple enough. When the 2-3 shift valve strokes, pressure is applied to the region with the green arrow. This same pressure is also applying the 3rd gear clutch pack at the same time. Now maybe you can begin to see the dilemma. Once again we have a battle of pressures. The apply pressure (red arrow) is still trying to push the servo to the right, while the release pressure (3rd gear pressure), is now steadily increasing until it overpowers the apply pressure and forces the servo back to the left. With equal pressure on both sides of the servo, the release pressure will win because the piston area is larger. But at the same time, since the piston surface areas are within 25% of one another, we can apply an enormous amount of pressure to the 3rd gear clutch pack before the 2nd gear band releases. Let’s put some numbers to it to make it more clear.
The surface area of the apply side of the servo is 4.45 square inches. The surface area of the release side is 5.87 square inches. So with a stock transmission with 60 lbs of line pressure applying the servo, it will take 45 psi of pressure on the 3rd gear clutch pack to release 2nd gear. To help out there is actually a spring in between the cap and the servo which constantly pushes the servo to the left. The reason I didn’t mention it is because it has very little affect on the pressures. Taking the spring into account, it still takes 40 psi of pressure to release 2nd gear. That’s a great deal of pressure on a slipping clutch pack waiting until the 2nd gear band releases.
This is all done using parts and valvebody schematics of the stock automatic transmission. This applies to EVERY Chrysler 47RE/RH transmission ever made. It doesn’t apply to high pressure valvebodies ONLY or modified transmissions ONLY, it applies to every single one.
To avoid premature damage, we want to get through this period of overlap as quickly as possible. Anything that causes this situation to last longer needs to be eliminated. This includes, stock front servos with no oil seal around the pushrod shaft, poorly sealing front servo sealing rings, bad lip seals in the front drum, poor sealing pump stator rings, poor quality cases, improper valvebody to case seal, etc.
All of the items listed above are common leaks in the 3rd gear feed circuit. When dealing with a partially stroked shift valve, we have a limited supply of oil feeding into the circuit because the valve is only cracked open. Any leak whatsoever will prevent our required 40 psi of pressure from building to push off the 2nd gear servo because the fluid is leaking out as fast as the shift valve is letting it in.
That is bindup from a technician’s point of view. I hope that helped some of you better understand these issues.
-Chris
Linear Mode
