Torque converter lockup

Motor, May 1999 by Woods, Joe

What goes on inside those steel bagels bolted between the engine and transmission? Carefully sloshed transmission fluid and a mechanism that sometimes makes the whole thing irrelevant.

For many years, stick-shift vehicles got measurably better fuel economy and range than those with automatics. It was clear enough why: Oldstyle torque converters necessarily allowed an rpm loss between the crankshaft and the input shaft of the gearbox.When people called an automatic a "slushbox," they weren't far wrong. The slushy connection beween the drive and driven elements was entirely through the PRNDLjuice. In this article we'll look at how torque converters work, how they fail and how you can tell the difference.

Let's define our terms The torque converter is a large, donut-shaped ("toroidal") coupling between the flexplate and the transmission input shaft It bolts directly to the flexplate, which in turn bolts directly to the crankshaft. For balancing reasons, the bolt pattern between the torque converter and the flexpate often-but not always-allows only one orientation of one to the other, ensuring the right clock-position between flange and converter.

The back of the torque converter ends in a notched or slotted tube that engages the center driving element of the transmission oil pump. So when the crankshaft turns, the transmission oil pump turns, squeezing PRNDL-juice thrugh the hydraulic circuits and building pressure to work the valves, actuators and clutches.

Antique Answers & Hot Oil

When you explain to an apprentice how an ignition system works, don't you find that you usually describe first the workings now-obsolete contact point ignition system? From it, it's much easier to explain and understand how solid-state and computer-controlled spark systems work. In that same way, if we look at the first "torque converters"-fluid couplings-the rest will be clearer.

Some shop manuals for German cars still refer to the torqu converter as the "Foettinger coupling," after its inventer. These first ancestors of the torque converter replaced the clutch on WWI armored vehicles. The fluid coupling is effectively two paddlewheels in the same oil-filled drum-one part of the case, one turning freely, as you can see in the photo on page 40.

There is always rpm slip between the impeller and the turbine of a fluid coupling, but the rpm stays roughly constant, so as you raise the engine rpm you reduce the percentage lost as slip. At idle, that can be 100%; at full power, it probably drops to 2017o or so, although the slip rpm is the same or slightly greater. Of course, all the energy that doesn't go from the impeller to the turbine as mechanical torque becomes turbulence and heat. This is better than burning up clutch disks because it's easier to change oil than disks. But not a lot better.

Today's Toroidal Troubles

The most important part of the torque converter is the fluid itself. Filling all the donut-shaped empty space in the converter, the oil receives 100% of the engines output torque and delivers the power to the transmission, minus the commission it deducts as waste heat. The complex pattern of oil flow in this toroidal (donut-shaped) chamber is the key to understanding the converter. The active role of the fluid in the converter, in fact, is the reason you measure transmission fluid with the engine running and at operating temperature: You want the torque converter completely filled when you check the dipstick. Once the engine stops, the converter gradually drains about half its oil back into the sump.

Sealed inside the torque converter are several major components. The impeller has radial vanes welded in place on the rear half of the donut-shaped housing. As the crankshaft rotates the housing, the impeller's vanes spin and carry with them the transmission fluid, filling the converter and throwing the fluid outward as well as spinning it. Older converters used axial impeller vanes like the fluid couplings; newer converters' impeller vanes curve forward, in the direction of crankshaft rotation to spill the fluid slightly faster at the perimeter than the converter shell moves itself:

One of the curiosities of the modern torque converter, if you look closely, is that the vanes go the wrong way. Compare the curvature of a torn lue converters vanes to those of a water pump impeller: they culve the opposite way Doesn't this reduce the torque converter's capacity to pump oil? It sure does-but that's the point. A torque converter does not work like a basement Sump pump, to move as much fluid as possible as quickly as possible. It uses the oil only as an energy transfer coupling. Moving it around in the transmission and torque converter is friction, a necessary evil, not the point of the game. The torque converters vanes are shaped to minximize the speed and momentum of the oil at the outside edge, to maximize the inertia transmitted from the irapeller to the turbine. As it moves through the impeller, the oil moves outward and winds forward, in the direction of crankshaft rotation.