The background description provided herein is for the purpose of generally presenting the context of the disclosure.  Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
An automatic powertrain system may include an engine, a torque converter and a transmission.  The torque converter includes an engine side, a transmission side, and an electronically controlled capacity clutch (ECCC).  The torque converter converts engine output torque into transmission input torque.  A transmission control module is typically used to maintain a target slip on the ECCC. 
The smaller the target slip, the less slip in the torque converter clutch.  Decreasing slip improves transfer of energy between the engine side and the transmission side, which improves fuel economy.  However, the smaller the target slip, the greater the chance that the torque converter clutch will lock-up (“crash”).  Control over torque transfer to the transmission using the ECCC is lost when the torque converter clutch locks-up, since the engine side is no longer isolated from the transmission side of the torque converter.  The amount of torque transfer is directly related to pressure applied to the ECCC.  As a result, transmission torque can fluctuate due to changes in engine torque.  Changes in engine torque may include fluctuations due to, for example, changes in combustion cycle speeds. 
Also, the larger the target slip, the more slip in the torque converter clutch.  Increasing slip increases heat production in the torque converter and decreases fuel economy.  Excessive slip can damage the torque converter clutch.  Thus, a predetermined target slip is maintained using the ECCC to: provide torque transfer control; minimize heat generation in the torque converter; and satisfy fuel economy requirements.