Torque Converter Explaination - Part2

What is a converter lock up clutch anyway?
The short answer to this question is: Whenever the converters clutch disc is in its engaged position, a direct drive connection exists between the engine and the input shaft of the transmission. That's converter clutch lock up! We'll try to keep this explanation as simple as possible but this section contains the following topics that need to be covered to help will understand how this system operates.
  1. The driving parts and their relationship to each other.
  2. The driven parts and their relationship to each other.
  3. How driving and driven sections connect and disconnect from each other
While section one described all the parts of the converter, this section will be limited to only those six components that directly make up the converter lock up clutch system. The converter lock up clutch system is made up of six major components: the engines crankshaft, the flexplate (sometimes called a flywheel), the engine side converter front cover (#3), the clutch disc, the converter turbine and finally the transmissions input shaft/drum assembly. Three of the six components make up the driving section and three make up the driven section. The driving and driven sections are always the same parts whether the converter is working in hydraulic coupling mode (normal converter action) or locked up clutch mode. The position of the clutch disc inside the torque converter determines which mode of operation the converter is operating in. When hydraulically pushed away from the engagement surface it's in normal hydraulic coupling mode. When the clutch disc is hydraulically forced to the engagement surface, it's locked up or in direct drive mode.

The Driving Section

Most of you are already familiar with the engines crankshaft and flexplate (flywheel) so I won't spend any time trying to explain these parts. The flex plate (flywheel) is bolted directly to the end of the engines crankshaft. The transmissions torque converter is bolted directly to this flexplate creating a single driving mechanism. These three components, crankshaft, flex plate and the converter are physically connected to each other by bolts therefore always act as a single unit. They will always turn at exactly the same speed. For this discussion, we are not interested in the whole converter shell, just the forward shell portion where the converters clutch disc will interact. We have cut a converter apart at the central weld to help clarify the relationship of the parts.

Picture #1 below a side view of a fully assembled converter, engine side to the right, transmission side to the left. This converter has two pieces that make up its outer shell. These two sections are held together by a central circumferential weld. We cut this circumferential weld off using a lathe, then split this converter so we could show both sides of the converter shell in picture #2. The piece on the left in picture #2, is the pump section of the converter which engages into the transmission. The piece on the right in picture #2 is the converters front cover piece that bolts to the flexplate (note the bolt bosses). Picture #3 is a view of the inside of this front cover. Picture #3 also shows the surface where the converter clutch disc engages when in the locked position. The engines crankshaft, flexplate and the converter front cover (picture #3) make up the driving section of the converter clutch system.

If you wish an explanation of the pump section of the converter shell, see component #6 in the first part of this technical write up. This section is only interested in the front cover.

#1 #2 #3

The Driven Section

The three driven components are: the converter clutch disc, the converter turbine assembly and the transmissions input shaft/drum assembly. The driven components are always physically connected together so they act as a single unit at all times. This is done using splined bores, hubs and shafts that physically connect all three components together. These spline connections allow for slight fore and aft movement between these components.

The picture at left shows the three components that make up the driven section.

  • Left end is the Clutch Disc.
  • Middle is the Turbine assembly.
  • Right end is the transmissions input shaft/drum assembly.

These three components are connected by splined areas on each component and always act as one unit. What one component does, the other will also do. They always turn at exactly the same rpm.


The picture at the right is the same assembly as above only this view is showing the clutch disc end clearly. Note the friction material that engages with the inside surface of the front cover shown above in picture #3 above. When this clutch disc is hydraulically forced up against the inside of the converter cover (picture #3). This creates a direct mechanical connection between the driving section and this driven section.


The pictures at left are views of both sides of the turbine assembly. They clearly show the input shaft/drum assembly where it connects by splines to the turbine.


Up until this point, the pictures of the driven section have shown the input shaft/drum assembly engaged to the splines of the turbine (like the two pictures above). We have shown them this way to help clarify how they spline together. However, the input shaft/drum assembly is actually part of the transmission not the converter. When it's installed in the transmission, you can't see the drum assembly, just a small section of the splines of the input shaft (see the picture directly to the right). When the torque converter gets installed, it's placed over this shaft, where it engages the turbine. The picture at right is viewing the inside of the transmissions bell housing area.


The two pictures at the left illustrate how the clutch disc (left picture) and the turbine (right picture) spline together. This spline connection allows for the fore and aft movement of the clutch disc This movement is necessary for the clutch disc to engage or disengage from the converter cover.

The clutch disc is allowed to move because it's riding on the outside hub splines of the turbine assembly (red arrow in the middle picture above). These turbine splines mate to the inside splines of the clutch disc dampening spring assembly (red arrow in the left picture above). You now know how the driven parts connect to each other.

The converter clutch disc is designed to move slightly fore and aft, while the turbine and input shaft/drum assemblies are designed to be held in relatively fixed positions. The clutch discs movement is controlled by fluid flowing directly from the transmissions internal hydraulic pump. The direction of movement, whether for or aft, is determined by which side of the disc this hydraulic fluid is directed to. If the fluid is directed to the rear side of the disc, it will move forward slightly to engage, with force, against the inside of the converter cover. When this disc is forcefully engaged in this forward position, it "locks" the drive and driven assemblies together as one, creating a direct drive from the engine to the transmission. This action is basically the same as releasing the clutch pedal in a manual transmission equipped car. When hydraulic fluid is routed to the front side of the clutch disc, the disc moves slightly to the rear. This rearward disc movement causes the drive and driven assemblies to become physically disengaged from each other, returning the torque converter to its normal fluid coupling operation.

During fluid coupling (standard converter) operation, the driving and driven sections are physically separate by a small gap between the clutch disc and the converter cover. During this type of converter operation, power is transmitted from the driving section, through the converter to the driven section by a fluid coupling action between the converter pump and the turbine (the clutch disc is always spline connected to the input shaft but it just goes along for the ride).

During converter clutch lock up operation, the driving and driven sections are physically connected when the clutch disc is forced forward against the converter cover. During this type of operation, power is transmitted directly from the driving section to the driven section by the physical direct connection at the clutch disc and the converters front cover (then the hydraulic coupling is not used and the turbine just goes along for the ride).

When the disc is in its engaged position, the input shaft/drum assembly of the transmission will always be turning at the exact same speed as the engines crankshaft thereby eliminating any heat production inside the torque converter. When this is properly understood and used correct it can have tremendous advantages in controlling heat build up inside the torque converter!