The most likely suspect is the radiator. Some cars have external oil coolers for the transmission fluid, but most have those coolers built into one tank of the radiator. Transmission fluid pressure while the engine is running can push the fluid through a leak into the coolant, which is bad. Cooling system pressure right after a hot engine is stopped can push coolant into the transmission cooler lines, which is real bad. Petroleum products in the coolant will rot rubber hoses from the inside, but that takes time. Water and antifreeze in the transmission don't lubricate moving and sealing parts like transmission fluid does.
If I'm right, you'll find an oily mud floating in the coolant reservoir.
Saturday, October 11th, 2014 AT 9:30 PM