Oh. That sounds like the torque converter is not locking up. Chrysler has been the leader since the 1950's in developing innovations that benefit owners, and the lock-up converter is one of those. They did this for better fuel mileage starting in the mid 1970's.
What you are describing is the way all automatic transmissions worked before these lock-up torque converters came along. The engine runs 500 - 1,000 rpm faster than the transmission. That is what allows it to stay running when the truck is standing still in gear. That additional 1,000 rpm is what you are seeing on the highway, and since the engine and transmission are not locked solidly together, the engine is free to slow down a lot when you let off the accelerator pedal.
With this lock-up converter, the engine and transmission should be locked together when the engine is warmed up, you are in third or overdrive, and you are above about forty mph. That is exactly the same then as you'd see with a manual transmission and clutch.
The converter will unlock when vehicle speed drops too low, you tap the brake pedal, (because the computer thinks you might be getting ready to come to a stop), you hit wide-open-throttle, (then torque converters double engine torque over what you would have with a manual transmission), or you release the accelerator pedal, to reduce engine drag and allow you to coast more efficiently.
Now that you are an expert on what the system does, we have to figure out why it is not doing that. A defect in one part of the brake light switch can incorrectly tell the computer you are pressing the brake pedal, so it will command the converter to unlock. Usually then the cruise control will not work either. The throttle position sensor could be sending an incorrect signal voltage that is telling the computer to unlock the converter. There can be other electrical problems too with the circuit that runs the lock-up clutch inside the converter.
The place to start is by reading and recording any diagnostic fault codes. Chrysler makes doing that yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then watch the code numbers show up in the odometer display.
There are specific conditions that must be met to set a fault code. A sensor can send an incorrect voltage that is still within the acceptable range, so no fault code will be set but there can be other symptoms. When there are no fault codes, you will need a scanner to view live data to see what the Engine Computer is seeing. It will also show if the computer is commanding the converter to lock up or not. Usually the scanner will not show why it will not lock up though. You have to figure that out from the data being displayed.
Wednesday, April 18th, 2018 AT 10:02 AM