Design engineers can be a strange lot. Almost all manufacturers purchase other manufacturers’ vehicles and components to evaluate what it is that their competitors are doing, as well as to glean ideas that they can implement in their own vehicles. During my time at GM, we had a building at the Warren, MI tech center that was filled with the latest vehicles from around the world. Our transmission design centers had units from numerous competitors for evaluation.
If it seems like when one manufacturer comes out with something new, it’s not long until others will follow suit, that assumption wouldn’t be wrong. One common application within the automatic transmission designs you see in your shops daily is the advent of “Clutch Balance Pistons.” Clutch balance pistons became popular with the introduction of many six-speed transmissions. Different companies use different names for the same components, clutch balance pistons, clutch dams, or compensator pistons, but they all serve the same purpose and operate similarly. (Figure 1)
The balance piston/clutch dam/ compensator piston is used primarily for three functions:
- Assist the return springs in returning the clutch piston when the clutch is cycled off.
- Since the clutch housings used on many vehicles today do not use check ball capsules or drum orifices to eliminate residual oil that can be trapped in the apply chamber, other methods must be used to eliminate the centrifugal apply of the piston due to the residual fluid that is trapped. The balance piston provides that function.
- The balance piston operation is calculated into the apply force strategy, which helps to control how aggressive the clutch piston applies clutch.
A balance piston is effectively opposing the movement of the clutch piston. In other words, it’s applying force to the clutch piston in the same direction that return spring force is pushing. By controlling the amount of pressure that is applied to the balance piston, one can control the aggressiveness of the clutch apply. Low balance piston pressure would mean the clutch piston can apply more rapidly, which would lead to a more aggressive shift. Low balance piston pressure could be caused by something as simple as a balance piston seal that is damaged, a leak somewhere prior to the piston, or by the valving that controls the pressure.
Some companies feed their balance pistons through a separate circuit such as compensator pressure, while others feed the balance pistons using lubrication system pressure. Using lubrication pressure is becoming more and more popular in today’s unit designs.
Lube pressure is now used for more than just transmission lubrication, so issues with lube pressure can now cause issues other than just burnt-up planetaries. Shift feel is dramatically affected if lube pressure is low. This means that issues such as lube bushing wear, flow, lube seal damage/wear, and cooler problems may result in shift-feel-related concerns. It also means a long time before the customer has a transmission planetary failure, they may notice some shift feel issues.
Numerous Aisin, Ford, Dodge, Mercedes, and ZF applications use lube pressure to control the balance piston position. Those units may exhibit shift feel-related concerns which can be caused by something as simple as a worn bushing. Units include, but are not limited to, the; RE5, RE7, 09G, TF60 series, TF80 series, TF81 series, AS68RC, AS69RC, Ford 6R, ZF6, ZF8, ZF9, U660, U760, 722.6, and 722.9 just to name a few. These can drive you crazy when trying to address shift feel issues if you are not aware of how those units operate.
One common practice in our industry is to replace the transmission bushings. Keep in mind that all parts are not created equal, and even if the part is new, that does not mean it is good. I have seen numerous instances where technicians have struggled to address shift feel issues only to find out that the bushings they just replaced had excessive clearance issues. When it comes to bushings, it is a buyer beware issue. Cheaper is not necessarily better in this instance. That couple of bucks you saved may cost you a lot more in the long run. (Figure 2)
So, the next time you’re faced with a shift feel issue, take a minute to pull up the hydraulic schematic of the unit you are working on. Take a look at the balance piston circuit so you can see what is feeding those pistons. Once you look at the circuit, there is a good chance you will clearly see the cause of your problem.
Until next time remember, “Life is trying things to see if they work.”
