Torque converter technology has evolved along with advances made inside the transmission. While manufacturers preserved the basic function of a torque converter, increased functionality was implemented to assist in fuel-saving strategies. Auto manufacturers use powertrain management techniques to maximize fuel efficiency while maintaining vehicle performance. However, the balance of performance and economy places a high demand on the torque converter assembly. This article will look at late-model GM and Ford torque converter clutch control systems and how they affect the vehicle’s driveability. We will also look at various issues concerning new strategies and components.
THE DUTY OF THE TORQUE CONVERTER CLUTCH
Without getting too basic, let’s look at torque converter function. 
We understand that one purpose of the torque converter is to allow the engine to run with the transmission in gear while the vehicle is stopped. Another job of the torque converter is multiplying the torque produced by the engine to be delivered to the transmission. As mentioned earlier, the two functions contribute to an improved driver experience; however, they are not without a downside. Since a torque converter uses fluid to transmit power, it becomes inefficient when the vehicle is in cruise mode. Torque transfer losses fall in the 10 percent range, where the fluid shear converts this to heat.
Later model vehicles add a torque converter clutch, which is used to connect the engine output directly to the transmission input. Direct coupling creates the most efficient torque transfer; however, it is not without a downside. When the torque converter clutch is transmitting engine power (on), it also transmits all vibrations and power variations. Most manufacturers use computer strategy and a damper system similar to the type used with manual clutch discs to manage vibrations. Let’s look at how these strategies and hardware come into play.
THE COMPUTER STRATEGY
In the early years of torque converter clutches, manufacturers often used pressure switches in conjunction with hydraulic shift valves to control TCC apply and release. The goal was to apply the torque converter clutch once the vehicle reached an optimal cruising speed with low to moderate torque demand, resulting in saving fuel. Manufacturers avoided low-speed lock-up to avoid complaints of sluggish acceleration and NVH (noise, vibration, and harshness) issues.
Vehicle computer systems gained more direct control over TCC scheduling as time progressed. Currently, torque converter clutch control depends on numerous vehicle operating parameters as well as driver and safety system inputs. Today’s vehicles no longer ‘wait’ for the optimal time and conditions, but they anticipate and manage torque converter clutch application by allowing for a range of slippage to full apply. Engineering data is used to determine predictable NVH conditions to avoid while commanding for as little torque converter slip as possible, maximizing powertrain efficiency without an objectional driver’s experience.
In short, the latest model units use complex strategies in their torque converter scheduling. Beginning with most 6-speed units, the lock-up management strategy includes the following:
- Possible TCC apply in all forward ranges
- Partial to full TCC apply during deceleration
- TCC command override by ABS, vehicle stability system, and ADAS
- TCC command control input from the Smart Cruise Control system
Given that the torque converter clutch is cycled on more frequently, the internal torque converter components are subject to more stress than in previous applications. Let’s look at how engineers handle this issue.
THE TCC DAMPER SYSTEM
Since late-model applications use a strategy that can call for lock-up at lower engine speeds and higher torque loads, there is a greater opportunity for NVH issues. The engineering response to these concerns is a more robust dampening system.
Looking at the GM and Ford rear-wheel drive eight and 10-speed applications (figure 1), the damper system is designed with stiffer springs and an extended throw to allow for greater shock absorption. An additional mechanical absorption device is added on GM diesel and Dynamic Fuel Management-equipped vehicles (called the Centrifugal Pendulum Accumulator, figure 2).
In conjunction with added dampening capacity, Ultra Low Viscosity (ULV) fluid is required in these applications. Do not attempt to use any fluid other than what the manufacturer recommends! Also, be wary of universal fluids that claim compatibility. Keep in mind that these formulations have been extensively tested with OEM programming, TCC clutch friction material, and surfaces. Any variation from the expected viscosity and additive package can produce shudders and other unwanted TCC apply and release issues.
TORQUE CONVERTER CHARGE OIL
Two common ways to control the application and release of the torque converter clutch are using torque converter charge oil or a dedicated hydraulic control circuit. GM and Ford use converter charge oil in their 6-speed and up applications. While this is nothing new, the strategy differs significantly from the previous four and 5-speed units.
Both manufacturers try to turn the converter clutch on and off under low RPM and moderately high torque conditions. This can be done smoothly and repeatedly, but it requires a delicate balancing act that starts with keeping the oil passing through the torque converter at a controlled rate. Converter fill depends on the following factors:
- Line pressure demand: high demand means lower TC charge.
- Cooler flow
- Bushing condition and lubrication control: worn bushings and torn lube seals mean a lower TC charge.
- Balance piston circuit health
- Internal torque converter clearances
GM and Ford both use a TCC Regulator valve with a stator shaft seal to control oil between the converter hub and the pump stator (figure 3). The stator seal helps to retain a predictable volume of oil inside the torque converter for a predictable apply and release pressure for operating the converter clutch precisely under lower engine RPM conditions. Care must be taken to prevent stator seal damage when installing the torque converter.
THE PROBLEMS
The most prevalent complaints associated with extended torque converter apply time are driveability-related. Customers are unaware of powertrain management control that comes into play to produce higher fuel efficiency and performance on demand. Therefore, their issues are associated with how the vehicle ‘feels’ or responds differently. The most common complaints are:
- Shudders, rumbles, or studders under light to moderate acceleration.
- The vehicle feels sluggish until you press on the accelerator harder.
- Busy shifting (tachometer always goes up and down until cruising speed is reached).
- Slipping during shifts.
A thorough diagnosis of the engine and related systems is necessary to determine if there is an issue outside the transmission first. Also, check for the latest PCM and TCM programming installed in the control modules.
Issues related to noises coming from the bell housing area of GM vehicles equipped with Dynamic Fuel Management and the 10L90 unit with the Centrifugal Pendulum Accumulator-type torque converter have been reported. Vehicles logging as few as 70k to 80k miles may experience problems with the rivets in the damper assembly coming loose (figure 4). Sometimes, the result can be catastrophic. Unfortunately, a failure of the damper assembly renders the torque converter unbuildable. There is currently no aftermarket remedy. You will need to purchase an OEM torque converter.
Another torque converter-related concern is with the Ford 10R80 applications. Units coming into the shop for overhaul can be left with torque converter-related issues due to misidentification of the torque converter. Unfortunately, Ford has no clear path for identifying torque converters in these applications. In most cases, it is best to have the original converter that came with the vehicle rebuilt or order from Ford using the vehicle identification number.
A common complaint with GM 8 and 10-speed rear-wheel drive units is vibration after unit overhaul or torque converter service. The complaint is usually associated with a torque converter replacement on a vehicle equipped with Dynamic Fuel Management or a diesel application. As mentioned earlier, these applications require a Centrifugal Pendulum Accumulator (CPA) torque converter. If a non-CPA type torque converter is used, noticeable vibration will be transmitted during the torque converter’s operation.
A quick way to identify units requiring a CPA-type torque converter is to observe if there is a bushing in the stator-to-input shaft area (figure 5). CPA units will not have a bushing. Note that if the stator needs to be replaced, the CPA and non-CPA stators have different dimensions! Therefore, you cannot just knock out the bushing of a non-CPA stator and place it service as a CPA replacement.
Getting better fuel economy will always be a constant quest for manufacturers. Technology will evolve to answer the challenges of increasing vehicle efficiency and change even the most fundamental systems we work with in transmissions. At ATRA, we will keep you on the cutting edge of technology to help you confidently deliver the goods to your customer, knowing you fixed their problem!
