In preparation for writing a torque converter article, I searched for a catchy title to describe the unit. I found it described as a “Metal Donut” and an “Industrial Salad Spinner.” Neither sounded very technical. When developing an article, the direction and layout should be decided first. The title should be fluid and can be converted later.
In the case of a torque converter, covering the design, function, and diagnostics is an article of great length. So, what you see here is an overview of the design and function of common two- and three-path converters.
THE TWO PATH CONVERTER OIL FLOW
Most 4- to 6-speed transmissions used a two-path torque converter, with the clutch applying toward the cover. (Figure 1).
Release oil exiting from the turbine shaft forces the clutch off the cover surface. The two-path converter then has flow when the release oil has separated the clutch from the cover. The amount of flow is relative to the amount of clearance, referred to as “clutch release clearance.” Release oil exiting the transmission is then restricted by a valve or an orifice to maintain a charged pressure within the converter. After this valve or orifice, the release oil then flows to the cooler circuit.
- Conditions of LOW release oil pressure: Engine stall in gear or idle rpm cycling, trans temp rise at idle in drive, low cooler flow, harsh coast downshifts, transmission lube circuit distress.
- Conditions of excessive TC clutch release clearance: Flare on upshifts, TCC shudder on apply, early TC clutch failure.
Apply oil pressure reacts on the piston and its friction material to control the slip rate of the clutch. The clutch friction material is a seal for this applied pressure and once the material contacts the cover, flow is basically eliminated. There can be minimal leakage of apply oil over the clutch if it has a grooved lining or woven carbon material. This oil loss supports a slip rate and cooling of the surfaces. Monitoring the TCC solenoid control, the TCC slip rate via the scan tool, and cooler flow will indicate the condition of the clutch and the hydraulic controls.
- Conditions for LOW apply pressure: TCC shudder, converter overheating, TCC lining failure, low pump output, restricted filter, etc.
- Conditions of HIGH apply pressure: TC clutch piston distortion, converter failure with high contamination.
Some units, such as the Ford AX4N, CD4E, and a few imports, use a converter clutch release and apply pressure, along with a third passage from the converter for cooling. Note: this third oil circuit virtually eliminated the overheating condition of the two-path.
Before we explain the three-path converter, we should discuss the captive clutch two-path and the LuK IHC design. The captive clutch increased torque capacity with the addition of another friction element. (Figure 2). That torque increase added the complexity of holding a second clutch from rotation, independent of the piston. As in any mechanical device, the more pieces, the more points for failure. This condition has become troublesome for the torque converter rebuilder, as internal failure can destroy the core. Failure, in turn, leads to a cooler and a valve body loaded with friction and metal debris.
The torque load advantage of a multi-plate clutch is also combined with the benefit of lower operating temp at the friction surface, and the multi-plate clutch is not biased toward the cover as a single clutch tends to be. As an example, a single friction clutch with low release oil may tend to drag on the cover, whereas a multi-plate tends to move off the cover.
At this point of describing a two-path converter, we touch on the unique Schaeffler/LuK converter for the Ford 10R80 and GM 10L80. In the prior two-path, the TC clutch is applied toward the engine and the converter cover, as in illustration #1. The LuK converter clutch is very active and could apply in any gear but typically from 2nd on up. In most conditions it applies and is never released.
An interesting point of this design is that it typically functions at a lower clutch surface temperature than the two path, which applies to the cover/engine side. The three-path to be addressed runs the coolest clutch and converter temperature of all the designs in this article.
In the LuK design the TC friction is bonded onto the turbine. The turbine then applies to the impeller and toward the transmission. (Figure 3). This means there is a reversal in the oil path to the typical apply and release oil circuit. Release oil of approximately 23 psi, is all that is required to separate the turbine from the impeller. Once the clutch is released, a springfinger loading device (Circled in Figure 3), reduces over-travel. This converter design has been a challenge to the rebuilders due to reweld distortion and maintaining alignment. With the converter on the bench, looking into the hub, it appears the stator and thrust washers are loose or fallen out of place. This loose appearance is normal but do pay attention to the condition of the inner hub bushing and the stator it rides on (Figure 4).
THREE-PATH CONVERTERS IN THE 6 TO 10 SPEED TRANSMISSIONS
The three-path converter design has been adapted by many manufacturers (Figure 5). One of the first ones we became familiar with was the Mercedes 722.6. There are two main designs of the three-path converter to be addressed here. Type 1: The clutch piston area is machined into the cover, usually with a large piston diameter area. Type II: Has a smaller clutch housing attached to the cover. The three-path has many advantages over the two-path, but the main ones include lower operating temperature, full time oil flow, high clutch torque capability and more control over TCC apply.
- Concern with a three-path converter: The TC clutch apply circuit needs to be sealed well to prevent charge oil from leaking into apply. Even a minimal amount of oil in apply, under centrifugal force of rotation can start to push the piston and drag the clutch. Figure 6 illustrates some of the variables between two- and three-path converters.
CAD illustrations by Sonnax. Schaeffler/Luk for web information. Photos/Excel by Bob Warnke. Appreciation is extended to the technical group within SONNAX.
