My truck almost always locks the converter as soon as I hit 45 mph. Sometimes it occurs at approximately the same time as the shift to 5th gear making it seem like it shifts twice.
The torque converter is a fluid coupler. It allows you to sit still in gear and not kill the engine. Because it is a fluid coupling device it will not transfer 100% of the power coming from the engine to the transmission. There will always be some slip in the converter. The exception to this is a locking torque converter, such as the converter in our trucks (and most all GM trucks since the early 80s). There is a clutch in the converter that engages at a predetermined speed/gear/throttle position combination. Once this happens the engine is solidly connected to the transmission. When the converter locks it may feel like a shift, depending on how much throttle you are giving it, and you will notice a drop in engine RPM.
The torque converter is a fluid coupler. It allows you to sit still in gear and not kill the engine. Because it is a fluid coupling device it will not transfer 100% of the power coming from the engine to the transmission. There will always be some slip in the converter. The exception to this is a locking torque converter, such as the converter in our trucks (and most all GM trucks since the early 80s). There is a clutch in the converter that engages at a predetermined speed/gear/throttle position combination. Once this happens the engine is solidly connected to the transmission. When the converter locks it may feel like a shift, depending on how much throttle you are giving it, and you will notice a drop in engine RPM.
