Early NAG1 transmission valve bodies, prior to about 2010, may develop a leak in one of the regulating piston’s cylinders. This can cause confusion in the transmission during shifting at peak RPM when power may just plateau momentarily. This is because these transmissions are extremely sensitive to fluid levels mainly due to their unique, and odd, method for determining transmission overheating. In the middle of the valve body is a little floating piston valve. As the transmission fluid heats up it expands raising the fluid level in the pan. If the fluid gets too hot and expands too much the floating piston valve rises up far enough to contact its valve seat and closes which signals the transmission controller that it’s overheating and the TCM goes into limp mode. So over full is very bad unless you like limping home. And....conversely, the NAG1 hates low fluid levels and can be damaged fairly quickly if the level is too low. So under full is very bad. There is no dipstick on the Challenger and the cap is sealed. Only the dealer has the proper tool to read the fluid level. If you want to try it yourself, you will have to buy a special tool made by Miller, Part #9336A. This special dipstick is marked in millimeters and to use it you need the temperature/fill level chart from the service manual (or find it online) and you need to know the exact temperature of the transmission fluid.
