Electronic components inside today’s transmissions have captivated us when considering parts to service and replace on a rebuild. Since computer strategy plays such an intrusive role in the operation of most units, the sensors and actuators inside the unit come into question, especially when there are shift quality-related issues that aren’t supported by damage or wear observed inside the transmission. Concern for missing a functionally deficient electronic component often blinds us to obvious, essential areas like oil passages, bushings, and seals that can be the root cause of the vehicle being in the shop.
Technology has changed the internal hydraulic architecture of most units as well. The quest for improving fuel mileage has driven engineers to produce a resourceful transmission that requires minimal horsepower to deliver maximum performance, keeping the engine within its most efficient range. In addition, hydraulic assist circuits were added to help clutch-to-clutch shifts occur ultra-fast without harshness. Here, we will look inside late-model transmissions to see new hydraulic circuits and how overlooking essential component fit and finish can create a shift quality nightmare!
Introducing Balance Oil
You can refer to balance oil as ‘dynamic accumulation;’ it alters shift quality directly with respect to driver demand. Balance oil acts on the return spring side of a clutch piston. It comes from one of two areas; lube pressure or modified mainline pressure. Typical pressure in the circuit is around 10-15 psi. Depending on the manufacturer, OE engineers manage these hydraulics differently; however, they fundamentally respond the same. As a result, balance oil pressure is directed between the balance and apply piston, creating a dynamic accumulation effect (figure 1).
The balance piston is usually partially vented toward the center and seals positively on the outer diameter. Balance oil serves three purposes:
- To assist the return springs in returning the clutch apply piston to its released position when cycling the clutch off.
- While the clutch is commanded off, balance oil keeps the clutch apply piston in the released position, preventing trapped oil from centrifugally applying the clutch.
- During shift functions, the oil pressure is used in conjunction with the shift strategy to alter the apply and release of the clutch based on demand; short, firm shifts for high demand and longer, smooth shifts for low demand.
Balance oil systems that use lube pressure operate on the same principle that governs overall line pressure versus lube balance; when line pressure is high, lube pressure is low; when line pressure is low, lube pressure is high. Systems that use a dedicated hydraulic circuit that modifies line pressure operate similarly.
Units that use balance pistons also control clutch exhaust oil control. A calibrated pressure remains in the clutch piston apply oil circuit to keep it pre-filled and ready for immediate application.
A blow-off valve in the circuit manages the pressure. The fact that the apply circuit is full even when the clutch is commanded off means the clutch apply rate is more accurately anticipated and controlled by the TCM.
Identifying the Hydraulic Controls
Every manufacturer has its name to identify the control components and hydraulic circuits (figure 2). Here, we’ll look at some of the most popular units and names identifying the circuits and control valves for your reference. Note that some manufacturers change the names of the circuits and valves when they introduce subsequent models, but the function remains primarily the same!
One of the most popular units to use balance pistons is the ZF 6-speed rear-wheel-drive series. The design includes Ford 6R series transmissions. You may hear the ZF system referred to as the Dynamic Balance Pressure system, while Ford uses Control Pressure to describe the hydraulics.
Either way, they are the same. Figure 3 shows the Delay Accumulator in the valve body assigned to maintain the balance oil pressure. On the apply oil side of the circuit, an exhaust pressure relief valve in the channel plate portion of the valve body maintains the residual pressure for rapid clutch apply (figure 4).
GM uses a Compensator Feed valve in their units (figure 5). Like the Ford/ZF applications, this is a dedicated regulator specifically for the balance oil pressure. The residual pressure on the apply side of the piston is called Back Fill Pressure. The pressure is maintained by a blow-off check ball and spring and is fed through an orifice from the Compensator Feed valve. When a shift is commanded, the regulator valve that controls the shift also simultaneously controls the Compensator Feed and Back Fill oil. With optimal control of the rate of travel of the clutch piston, shift quality is meticulously managed by the TCM, varying the modulation of the shift solenoid.
Manufacturers like Aisin Warner use lube oil for balance piston pressure. Therefore, it is especially critical to pay attention to lube-related valves, bushing conditions, and unit end play, as they all play a role in the stability of lube pressure.
When Things Aren’t Working Well
When units age, hydraulic circuits, and seals are compromised. Normal wear and tear create metallic and friction debris that circulates inside the transmission, causing valves to stick and wear, seals to harden and wear, and solenoids to mechanically malfunction. When the balance oil circuit pressure is not maintained as designed, the following shift quality issues are observed:
- Harsh shifts
- Binding
- Slide/bump shift
- Harsh coasting downshifts
- Banging forced downshifts
- Slip and flared shifts
- Burnt clutches
- Overheated fluid
If the balance oil pressure is too low, the clutch apply piston will partially apply while being commanded off, causing the clutches to burn. Conversely, if the valve sticks and causes high pressure in a regulated system, slips and flared shifts will occur.
When diagnosing shift quality issues with these units, fluid condition, vehicle mileage, and driving habits are useful qualifiers to evaluate the root cause of concerns.
Preventing Problems on the Bench
Once a transmission is on the bench, it is in ‘resolution mode.’ In other words, most testing that can be performed is over. Now, it’s time to fix the problem! When a unit with balance pistons is on the bench with known shift quality issues, you must take special care to address areas that can be the root cause.
First of all, check bushings for correct clearance. Control of lube oil is critical in later model units. The capacity of oil pumps is not as generous as they were on earlier model units. With fuel efficiency at the forefront, transmission oil pumps use as little horsepower from the engine as possible, which means the volume of oil supplied by the pump depends on the demand. There is no excess oil to waste! When there is excessive clearance between bushings and journals, the lube circuit ‘demands’ more oil. The pump and pressure regulator circuit cannot keep up with extra oil displaced, causing flared shifts, slips, and harsh shifts.
Valve bore wear is another critical area to address. Primary pressure regulator, secondary pressure regulator, torque converter switching valves, overlap valves, backfill valves, and lube regulator valves are a few to mention. However the pressure for the balance piston circuit is generated, there is a valve associated with it that must be in proper working condition. It is imperative to restore the integrity of these hydraulic circuits for proper unit operation. There are numerous aftermarket kits and components available to assist. In some cases, a new OEM valve body is a cost-effective solution. Make sure to do your homework on the initial teardown inspection.
Finally, make sure to service the balance pistons! Several manufacturers use molded pistons. It is tempting to reuse these, especially if they ‘feel’ good, but don’t take a chance. Don’t be deterred by the added cost of the rebuild. Treat the balance pistons like the apply pistons; you wouldn’t reuse lip seals, would you? Reusing balance pistons is a common cause of intermittent shift quality issues after rebuild, especially in Toyota applications.
With so many variables affecting shift quality outside the box, it is easy to become complacent when building late model transmissions. Therefore, diligence is necessary to ensure that your unit will have every opportunity to work the first time correctly off the bench. Added knowledge of how technology inside the box works will help you deliver the goods to the customer with confidence and accuracy the first time!
