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Do any (or all?) torque converters disengage completely from the engine while the engine is idling and the vehicle is stopped, and/or the transmission is in Park/Neutral?

If a torque converter does not disengage during these stationary/idle scenarios, how much of a load does it add to the engine?

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  • No they do not disengage, It depends on the stall speed of the converter what the load will be. Most stock converters do have a marginal load at idle and is different for every vehicle depending on what the engineer specified.
    – Moab
    Commented Sep 3, 2018 at 20:40
  • @Moab – an explanation of what the stall speed of a TC is and how it relates to the "idle load" would make a great answer!
    – feetwet
    Commented Sep 3, 2018 at 22:48

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A torque converter is a fluid based device which transfers energy from the engine to the transmission. Unless otherwise modified, it is not engaged in the manner you suggest. An explanation is available here: How does a torque converter work?

For improved efficiency, many manufacturers are providing locking torque converters. These devices engage mechanically the engine and the transmission, reducing energy losses inherent in the un-modified torque converters, but they also require that below certain speeds and gear combinations to be disengaged.

You may discover complaint posts regarding motor vehicles which chug or buck or stall after exiting a highway. These are symptoms of a locking torque converter which has failed to unlock and behave in the manner of a manual transmission with the clutch engaged while coming to a stop.

The answer to your last question is that a locking torque converter which does not disengage applies maximum load to the engine, stopping it!

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  • I am asking about an "unlocked" torque converter. If it is still engaged with the engine at idle (which would be the case unless some extra mechanism is added to "disengage" it) then the impeller is spinning in the coupling fluid at whatever the engine's idle speed is, which must consume some energy.
    – feetwet
    Commented Sep 3, 2018 at 22:50
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    Unlocked torque converters are always engaged if the transmission is in gear. As such, the transmission side of the converter is stopped and the engine side is moving fluid into the transmission side, which does, as you suggest, consume some energy. This is part of the reason that automatic transmissions are less efficient than manual versions. In Park and Neutral, the transmission side of the torque converter is permitted to spin at engine speed without sending energy to the wheels.
    – fred_dot_u
    Commented Sep 3, 2018 at 23:43

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