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There are now hundreds (thousands?) of posts on this and other Nissan forums about the shudder/judder problems with their continuously variable transmission, or CVT. This posting is a very simplified explanation of what a torque converter does (or is supposed to do) in an automatic transmission. This description does not match the torque converter in the Pathfinder in every detail, but the general functions are the same. The original text can be found at AGCO Automotive Repair Service - Baton Rouge, LA - AGCO Automotive

How a torque converter lockup works
The torque converter is a coupling between the engine and transmission of the vehicle. The purpose is to allow the vehicle to come to a stop, with the engine running. To do this something has to slip, and that is the torque converter. From this perspective, it acts much like an automatic clutch.

A torque converter works by transferring power, through fluid motion. An easier way to understand might be to consider two desk fans. We connect one fan to a source of power and tum it on. We do not tum on the second fan, but place it facing the first. Motion of air, coming from the running fan, can make the blades in the second fan tum. A torque converter works similarly, but uses fluid rather than air. Fluid is much more viscous than air and transmits much more power.

They attach the torque converter housing to the engine flywheel. When the engine runs, blades inside the converter rotate through the transmission fluid. The motion of this fluid causes other blades, attached to the transmission, to tum. This drives the vehicle and at Idle, allows the engine to run with the vehicle remaining still.

Slipping is necessary in order for the vehicle to sit still with the engine running. Because some slippage continues when driving, fuel mileage suffers. Modem vehicles use a clutch inside the torque converter to prevent slipping, when we no longer need lt. This clutch 'locks-up' so the transmission torque converter rotates the same speed as the engine. This transfers more of the available power and helps fuel mileage.

When the internal clutch applies, they call this torque converter lock-up. This occurs after the vehicle is rolling and ceases when it comes to a stop. This clutch requires very specific lubricants in the transmission fluid to work property.
 

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