The auto trans is electronically controlled.
In drive, 2 solenoids control what gear the transmission is in, the trans computer sends signals for 4 different combinations of the 2 solenoids for forward gears.1st gear = both solenoids on
2nd gear = one solenoid on, the other off
3rd gear both solenoids off
4th gear = opposite of second gear with one on/one off. When you loose electronic signals to the trans you will start out in 3rd gear when drive is selected because 3rd gear is both solenoids off.
Starting out in a high gear causes you to be deep into the torque converter stall to get the car to move. The converter is a fluid coupling that your causing to work harder and make more heat. More heat will make the fluid expand and can cause the trans to overflow out the vent tube at the top.
If you manually select 1st or 2nd gear, the shift lever moves a valve in the valve body that allows fluid to go to the selected gear even without power is getting to the shift solenoids. I would check your fuses, also the trans harness wiring to make sure it has not melted against the exhaust or been ripped out in the linkage.
Engine control fuse at the top of the driver inner fender is most common. also a fuse at the driver kick panel. Heat kills transmissions, driving without electronic control will lead to quick failure of the transmission.
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