We had a Mercury sable that had a defective solenoid for torque converter lockup. The symptoms sound identical.

We would get a code related to the transmission slipping in high gear on long drives.

You can tell if the transmission is not locking up by getting up to highway speed and then slightly accelerate from say 55 to 58 mph. The tach should barely move. If the tach rises and settles back down, transmission is not locked. This could be the actuator or the torque converter.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)