Just disconnect the coolant hoses from the top of the heater and tie them together so that coolant still circulates through them and the 3 way bypass valve .Install a couple rubber caps on the nipples to keep debris from entering the heater assembly. The cars with the 8 speed don't heat the fluid,just the trucks do.My wifes 16 Challenger with the A8 takes a long time to get up to operating temp compared to my truck,but it will eventually settle in at about 180/185'ish after awhile.Takes it about 30 miles at 75+mph on a 75F day to finally get to 180
Doing this will bypass the "heating" of the ATF, and will not generate any codes, as the coolant circuit thru the 3-way valve will remain intact. The TBV, transmission bypass valve which is located inside a manifold behind the heater unit, will still keep ATF from flowing to and thru the trans cooler until it opens at, or close to 180 degrees. The TBV is held in place by a black plastic cap which is secured by an internal snap ring. Removing the snap ring will allow the plastic cap to be pulled out and with it, the TBV.
You can re-install the plastic cap minus the TBV and ATF will flow unrestricted to the cooler. Only issue is on 4x4 vehicles, access to the plastic cap is very tight. The entire heater/manifold assembly may have to be popped free of the transmission, the procedure done, and re-inserted. The manifold/heater assy is bolted to the trans case, and fluid flows to and from the transmission by way of two tubes, o-ringed on each end (the manifold and trans case end).
Since I've not yet done this, I have no idea just how cool the 8-speed would get. There is one 8-speed performance builder who makes a modified TBV which opens at a lower temperature, but they wont sell that separately, they only will include one with one of their complete built transmissions. Seem to me that installing a modified TBV that opens say at 140 degrees or so would be a better option than just removing it entirely.
As an added note I read somewhere, maybe this thread, that the 2019 1500's 8-speed seems to operate at temperatures at 170, never higher, which would imply that a TBV with a slightly lower opening temperature is being used.
